Lithium in the Classic Hymer (pre 95 non EBL)

Too busy for a long article? Can you just “drop in” a lithium battery into an unmodified classic Hymer? Basically yes you can, as long as you keep an eye on it, and the best way to do this is to choose a "smart" lithium battery that comes with a bluetooth app for your phone or tablet that shows you exactly what is happening to the battery in real time. Then you are in control. There is really no point in installing a "dumb" lithium battery - unless you are quite technical and have other means of monitoring and testing.

For the technical discussion .... read on.

Can you just “drop in” a lithium battery into an unmodified classic Hymer? I am afraid that there is no definitive black and white answer. I asked 3 leading lithium leisure battery manufacturers this question. One said yes, the other said their advice was to always fit a battery to battery charger, and the other didn’t reply!
So lets take each of these charging sources in turn, and try and assess their suitability for charging lithium.

The main issue is that lithium has different charging parameters to normal batteries, and the implied danger is that charging devices might get overworked, because a lithium battery can usually take all the charge presented to it. This could result, for example, in components only originally designed for charging a standard 100ah leisure battery with around 20a of charge, to deliver much more than that when a lithium battery is installed.

Alternator charging

The standard pre 95 classic, Fiat or Merc has an alternator of around 50 to 70a, and cabling between the engine battery and the leisure battery, via the split charge relay behind the fuse panel, of around 10mm2 copper, and intended to handle around 20a. Hymer fitted a 70a relay, but it is important to remember that this was over specification by Hymer so that the relay would last for ever - not to actually handle 70a.  But the bottom line here is that you are starting with a charging circuit with a design capacity of around 20a. But this isn’t self regulating - it doesn’t limit itself automatically - if asked to do more, it will, but then every component in the chain of the circuit will be working harder than before and problems can occur. In practice, if an original charging circuit is used, then it needs to be monitored for the first few weeks and months. The limiting factor is usually cable thickness, but connections can also be a problem, especially old ones that may be corroded or tired.

But some owners can and do charge lithium directly from the alternator - by upgrading cables and relays. Bt these owners are usually technically minded, and have metering and monitoring in place to ensure that every component in the chain is solid, and that the alternator isn’t being asked to work too hard. You have to keep and eye on it, and have the metering in place to monitor it.

The other worry that you read about online is that a charging system designed for lead acid batts is not suitable for lithium. This is actually not really a problem. A lead acid charging circuit will deliver around 13.8v maybe a bit more. But the limit for lithium is 14.6v, and even if this voltage was reached for some reason, the lithium bat's BMS would kick in and disconnect the charge. And if you look at the cgarge vs voltage chart for lithium batts you will see that even a charge that goes no higher than 13.8v will still result in a 95% charge. And in practice, with most vans that have lithium will also have solar, with a solar controller that has a lithium profile, solar will take care of the remaining 5%.

What many forget is that lithium batts are not dumb like lead acid, they have brains - the BMS, Battery Management System, and the job of the BMS is to prevent abuse. So the BMS will simply switch off the battery if the voltage or current falls outside of preset safety values. So in reality it is actually almost impossible to damage a lithium batt by giving it wrong voltages - it will just shut down.

But when it comes to alternator charging the simple solution for most owners is to fit a battery to battery charger - a b2b. This is a device that sits between the engine battery and the leisure battery. It has two main functions - to limit the amount of strain put on the alternator, and to present the leisure battery with a near perfect charging profile. A b2b is easy to fit - it has just 3 connections - to the engine battery, to the leisure battery, and the D+ alternator control line. (some b2bs don’t even need D+ - they are auto sensing). The only other issue for classic Hymer owners when fitting a b2b is that the original Hymer split charge relay (discussed above) needs disabling. This is easily done by removing the original relay from the fuse box. All other original cabling is left intact, which preserves the original function of 12v fridge operation while driving. Then a new cable has to be installed from the engine battery to the b2b, and from the b2b to the new lithium battery. The size of this cable is dictated by the size of the chosen b2b. And the size of the b2b is dictated not by the size of the new battery, but by the max rating of the alternator. The general sensible rule is 50% of the alternator rating - for te vast majority this will be 30a.

If fitting a b2b which needs D+ switching, then the original D+ line to the original relays in the fuse box has to be identified and tapped into, and a new wire run to the b2b.

In the case of classic Fiat vans, where the engine battery is under the front bonnet, a longer cable to the b2b is usually needed, and the D+ line can be run alongside it - the original D+ wire is usually able to be found in the area of the engine battery.

In the case of classic Mercedes, with the double battery box next to the drivers seat access is easier - the b2b can be easily connected to the engine battery and from there to wherever the new lithium battery is located. An easy way to obtain a D+ signal in the Mercedes battery box is to open the trunking that leads across the bottom edge of the door (if fitted) and locate the 12v power feed to the fridge, and carefully split the cable and tap into the fridge feed. The 12v fridge feed is relay switched by the D+ in the fuse box, so is in itself a D+ proxy - this method just makes it easier. Please note this is a tip for certain types of install, not a mandatory requirement. Most modern b2b chargers, such as Victron orion, have voltage sensing and don't need D+. 

The other factor that needs thinking about when considering alternator charging, is just how much you want. There is a trend with lithium batteries to “go large” and fit batteries of 200ah or more. So you have to think about just how much driving time you will need the next day, in addition to whatever you get from solar, to replace the power you have recently used. With most classics having around 60 to 80 amp alternators, and a sensible limit of around 50% of this, then that means no more than about 30 to 40 amps per hour of driving will be available. This is usually enough for most owners, but if you want more, then the only alternative is to fit a bigger alternator. But before you do, consider this ...

Solar keeps working all the time, and many vans with lithium also have solar, and those that don't should have! And modern solar panels are big, and cheap. In 2025 you can get a 425w black panel that is 1700 x 1100mm and will fit perfectly on the front of most Hymer roofs. And if you have such a panel, or even 2 or 300 watts of other panels further back - these panels don't stop working while you are driving. On a sunny day in summer I have regularly seen 70a of charge while driving - 40a from the alternator and another 30 from the solar! So factor in your solar ad your driving habits (winter/summer) before you go to the expense and hassle of a bigger alternator. In my experience a bigger alternator is rarely justified if thought out properly. It depends on your lifestyle and intended power use.

EHU (230v) charging

Pre 95 vans have a separate 230v charger. Most owners have replaced the original separate charger, so what charger you now have dictates what mods if any are needed. The original factory charger, if you still have it, will work with lithium, but will not be optimum. But if you do need to use it, you can.

But most owners fitting lithium will opt for a new charger. The Victron Blue Smart IP22 30a is ideal. 

When fitting a new more powerful charger for lithium, the next question will be whether to use the original Hymer cabling or not. This is particularly relevant on vans that have the charger in the back and the battery in the front, it is tempting to use the old cable because running a new one is a big job.

You can use the old cable - it will handle 30a, but you have to take into account that the original charger was only 10a, and the 12v cable from the old charger to the front of the van goes to the fuse box, and then via a 16a fuse, and then to the battery. Not only will the fuse blow if you fit a 30a charger in the back, but the length of cable, with old and possibly corroded connections along the way, will lead to an unacceptable voltage drop - ie won't charge the battery optimally.

So what do you do in this situation? And remember we are only talking about vans with the charger in the back and battery in the front. You have 2 options. The first is to move the charger to the front and connect directly to the battery. This means you will have to supply the charger with a new 230v supply. How easy this is depends on your van layout and your DIY skills. The second is to use the original cable, but disconnect it from the fuse box and run it directly to the battery. This takes out the problem of the original fuse and the old connections in the fuse box, and it also shortens the cable run considerably. I have performed this mod on many vans and it works well. But there are different tricks to identify the cable and do this, depending on the van. Anybody wanting more info on this can contact me direct.

On vans with the charger already in the front, things are much simpler, but you still have the problem of the original charger cable going via the fuse box. So in these cases it is usually best to simply run a new cable, and leave the old charger cable unused and safely insulated and tucked away. But it can be used if you are willing to put a bit of extra work in - it can be disconnected from the fuse box and re-routed directly to the battery. Again I have hints on this if needed.

Solar charging

This is probably the easiest of all. Solar power is free from the sun, and is the only charge source that is available “off grid”, so is ideal for wild camping. Solar panels and MPPT charging are now so cheap, and powerful, that if you are fitting lithium batteries it makes sense to fit as much solar power as your roof will easily take. On most vans this will be between 200 and 400w. This means that on a summers day, you can reasonably expect to replace between 80 amp hours (200w panel) and 160 amp hours (400w panel) on a sunny summer day, based on 6 hours of sunshine. But in winter the figures can be as low as 20% of this because the sun is much lower and the days shorter in winter. This is no problem if you don’t use your van in winter, but for full timers it is a major factor in your off grid capability. Full timers who want to be off grid as much as possible need to fit as much solar as they can. This goes for any battery system, not just lithium. A common misconception is that a 200w solar panel is a constant - it is not! In December it is a 40w panel! And that is assuming that the sun shines in December.

Technically, modern solar MPPT controllers usually have a lithium profile, but you can still use those that do not. If the controller has user definable settings, just set everything to 14.2v. If this isn’t possible, then just leave it set to the lead acid profile, it won’t do any harm.

So those are the main points to consider about charging lithium.

The result of all or some of this work is that the new lithium battery will have some or all charge sources (EHU, engine, solar) connected directly to it. But power still needs to get from the battery to the habitation circuits - so the original Hymer cables are left in place - black positive and brown negative. These feed direct to the fuse box, and then to the van.

If you are fitting high power devices such as an inverter, then this should be connected directly to the battery via suitable fusing. More complex installations may use busbars or connection posts.

Types of lithium battery - what to buy.

The next subject to discuss is "what type of lithium battery"? All lithium batteries for the leisure vehicle industry are lithium iron phosphate - known as lifepo4. These are the safest type.

Prices are now so low that lithium batteries are hardly more expensive than lead acid. The choice nowadays is do you opt for "dumb" lithium or "smart" lithium. Smart means the battery has a bluetooth BMS that talks to an app or display. In the motorhome context, the choice is a no brainer - always buy smart. Having an app in your hand which tells you exactly what is going on in real time (see screen grab below) is the killer app of modern battery technology. It is daft to fit a dumb lithium battery to a motorhome. But for those that have fitted dumb lithium, all is not lost, because you can make a dumb system smart by adding a Victron Smart Shunt (or cheaper chinese version) which will then offer you the same valuable info as a smart system. But note that if you shop around, you can now buy smart lithium 12v batteries for roughly the same price as dumb ones - Epever and Fogstar from Bimble are 2 solid recommendations. Note that when browsing 12v lithium leisure batteries online, if it doesn't specifically say "Bluetooth BMS", then it will be dumb. There is a huge amount of old stock flying around, and also a lot of cheap chinese lithium leisure batteries. Chinese batteries from either AliExpress, or UK/EU suppliers and importers are everywhere now. There is great debate about quality. There are Youtubers who cut open cheap chinese batteries and dissect them and analyse at length. I have even done it myself. My conclusion - and I am by no means the last word on this - it is just based on experience, is that there is nothing wrong with cheap no name chinese lifepo4 batteries - as long as they are smart bluetooth, and you have to be careful with the ad small print. There are lots of dumb batteries around - used in golf carts and fishing boats (trolling motors) and many other applications that don't need smart - but for a motorhome owner, smart is essential, because having all that info in your hand is a game changer for van life - it puts you in control - no more unwelcome surprises of the lights going dim half way through the weekend.

To my mind, the choice comes down to whether you want UK support and warranty - if you do, you have Bimble, Fogstar and several others, and you will pay a bit extra. Renogy and Ecoworthy also deserve a mention - their support is not quite as good, but it does exist if you push. But the unknown suppliers on AliExpress, you will get little or no support.

But little or no support isn't as bad as it sounds because these batteries are pretty reliable. The reason for this is that unlike lead acid batteries, which are easy to abuse, and fail as a result, it is actually very difficult to damage a lithium battery because they have a computer brain - the BMS - protecting it. If you abuse it, it just shuts down. 

So basically cheap lithium smart batteries are fine for motorhome use. By all means pay a bit extra, but if budget is tight, you are usually OK.

But there is one area where it pays to go big on quality and that is high power. Big lithium at a reasonable price means that things that were only dreamed about in the past are now possible - I am talking about high power - microwaves, airfryers, hair dryers, coffee machines, toasters, even kettles, these can all be powered with a big lithium battery through an inverter. Putting aside that the installation has to be solid and well installed, and that any power you use for these luxuries has to be replaced somehow .... but just concentrating on the battery .... if you power a 1500w device that will be drawing over 100amps from the battery. That is a lot of power, so now quality does become a factor. Big power needs lots of copper, and generates heat, so build quality is important. So if you are planning big power then probably best to buy a battery at the top end of the scale. Reputable suppliers will publish detailed specs, and 200amp continuous discharge is easy to find.

But having said all that, as long as you have done things right in terms of cable spec, there is nothing wrong with a 20 minute airfryer session from a cheaper battery. Even the no name suppliers don't want warranty returns and bad online reviews.

many owners have no idea about power  - they are used to just plugging anything in at home and it works - for all intents and purposes power at home is limitless. But it is completely different in a van. It takes a LOT of 12v power to convert to 230v.

So here is a list of common appliances to guide you.

Low power, no problem under 50 watts- phones, pads, any small battery appliance -shavers tec.

Low-ish power - 50 to 200w - TVs, laptops, ebike chargers.

Medium power - 200w to 800w - small coffee machines, small kitchen appliances such as whisks.

High power - 800w to 1500w. Small airfryers, small kettles, microwaves, big coffee machines, hairdryers. Basically anything that generates heat - check the label for the watts, divide by 12, that is how many amps at 12v you need.

Air conditioning is a special case and a separate conversation.

Finally .... do you really need it?

There is also one more big thing to think about - and that is - “do you really need it”??!!

There is nothing special about lithium - it is exactly the same volts and amps as any other battery. So you have to really identify the need.

The three main advantages of lithium are power density, long life and better information. Power density means you can get more power out of the same size - so a rough example would be if you only have space for 1 traditional 100ah leisure battery, and you don't want or can't relocate the battery, then you would get roughly double the power from the same space.

Long life is exactly as it sounds - they last longer. In regular use a standard leisure battery is only good for a few hundred cycles of charge and discharge which in average use means they need replacing every 2, 3 or 4 years. Lithium batts are good for thousands of cycles. The jury is still out because it's new technology but it is generally accepted that a decent lithium battery setup should last decades. So if you use your van a lot and intend to carry on doing so for years, then the financial deal alone can justify it.

But the third reason, better information, is for me, the best reason by far - better monitoring and information. The latest lithium batts come with bluetooth monitoring, and a phone app that displays accurate battery information. This tells you exactly how much power is left in the battery, how much charge you are getting (from all sources), and how much power you are using. Basically it is an accurate battery fuel gauge. After a few days of use you soon get a feel for your battery system, and can plan your activities accordingly. This is particularly useful for those who like to be off-grid as much as possible, or for those who may need to run devices that potentially use a lot of power, such as coffee machines, ebike chargers etc. You soon get to know exactly how much power these devices use, and how much charge you need to replace that power used - from solar etc. It is incredibly useful and gives you complete control, with no nasty surprises. With a standard lead acid battery - it is quite difficult (but not impossible) to get this sort of accurate information. Having used these Bluetooth lithium battery systems for over a year now, for me, it is by far the best reason to do the upgrade.

But none of that is to say that lead acid batteries are old hat or obsolete - they are not. A lead acid battery with a good solar system can easily provide enough power for normal summer holiday use. One of the best reasons to upgrade is when your existing system simply isn't giving you the power you need.

But now that lithium is only slightly more expensive than lead acid, it should be considered. And for the hobbyist and technical enthusiast, a lithium upgrade is a fascinating project.

Lifepo4 charge discharge characteristics

Epever 100ah smart bluetooth Lifepo4 battery

A typical BMS phone app, showing exactly what is being used, being received and how much is left.

TESTING CONTINUITY WITH THE MULTIMETER

One of the most useful tools in the box for sorting out wiring and cabling problems  is the continuity feature of your multimeter. On most meters it is marked with a small arrow. When selected, if you touch the probes together, the meter should make a sound - usually a beep - this means "short circuit".

Not all meters have this beep function. Most modern ones do, and if you are going to buy a meter, make sure yours does. On meters without the beep, then you have to use the "Ohm" setting, or "Diode" setting which will display zero on the meter display when you touch the probes together. 

So to test a connection, you touch one end of the wire with one probe, and the other with the other probe, and if you get a beep, then that proves there is "continuity" along that wire.

So if you have a bunch of wires at 2 ends, and no idea which is which, you use this feature to identify which is which. Attach one probe to one wire at one end, and then touch the other probe to each of the others until you get a beep.

But you have to remember that one end of the wire being tested is free - ie open circuit. You can't use this technique on connected wires, because there may be a path through another part of the circuit that will give you a false reading.

If the meter leads are too short, then you simply extend them using a long piece of wire, suitably connected - can be as crude as wrapping and taping. You check that you have a good connection by simply touching the probes together.

This technique is also useful for identifying grounds. DC current flows from pos to neg, with neg being chassis ground. If the ground neg wire or connection is not a solid path, then the circuit will not complete. So when testing and troubleshooting, you have to be sure that the neg has a good path to ground. The way to do this with a meter (set to continuity) is to attach one probe of the meter to the negative terminal of the battery - because that is 100% guaranteed to be the neg ... and then the other probe to the neg wire in question. Again, it is usually necessary to extend the meter probes. A favourite tool of the auto electrician is a 20 foot long meter extension wire with a crocodile clip that connects to the battery negative. Then neg grounds can be tested anywhere in the vehicle - especially rear light clusters - which are just about as far away from the battery as can be.

This is hopefully a simple explanation for motorhome owners - there are lots of Youtubes that explain multimeter continuity testing in more detail.

Inherited a van with an old LPG tank?

This article was written in reply to a new owner who had bought a 1988 B544 Hymer with an big underslung LPG tank, and wanted info. Most tanks are 40m 50 or 60L, so if you have a smaller tank - adjust the estimates accordingly - and remember - they are just estimates.

If the van is LHD then is likely an import, and the tank could have been fitted any time since. They were never a factory option - always fitted by dealers or gas companies - here or in Germany (usually).

Now what you have to understand about gas is that the equipment doesn't last forever, and at the very least should be checked over every 10 years or so. That is not to say there will be any danger - lots of people love to shout doom and gloom whenever the word gas is mentioned - I am not one of them. I know of plenty of original gas tank systems that are over 30 years old and still going strong. If you have reasonable common sense you can inspect the system visually and assess it yourself. If it fills with gas at the pump, and then after standing still on level ground with no wind, you can't detect the slightest whiff of gas from anywhere, and the appliances appear to work fine, then you have taken reasonable precautions and are most of the way there. But the general common sense attitude is that you should have any gas system checked over properly at least every 10 years. if you have children, it's more or less a duty!

The most important component is the regulator. Regs do have a use by date - and should be replaced every 10 years. Old regs can and do fail, and it is a regular question on this technical group. An LPG tank reg is the same as a bottle reg, but is usually bulkhead mounted, or mounted out of sight near the tank. Remember that old german vans pre 95 almost always have 50mb gas systems - not the 30/37mb that is the modern standard. Don't let any gas guy tell you otherwise.

if you do feel it prudent to have it checked over, then this group does have a recommended gas guy - Charlie Lister at Autogas 2000 Ltd up in Yorkshire. He will fit a new reg and check things over, and is a mine of info on all things gas. There are other firms dotted around the country, but Charlie knows Hymers and motorhomes in general and has been an invaluable source of advice over the years - so worth the trip.

About the gauge - the glass ones fitted on the tank are hard to read, but generally reliable. Study it carefully so that you understand it - you really have to read the dial to get to know what is empty and full position. There are usually accessory LED level gauges available nowadays, and these are convenient.

Your tank looks quite big - around 60L. The normal 11kg German bottles that fit the gas locker are 11kg, and there is 2kg in a litre. So 2 bottles is around 44L. The germans fitted them for 2 main reasons - 1. Long periods between refills, minimum hassle, max time on site or off grid. 2. Crossing European borders. There are several different bottle systems in different countries, which makes gas bottles an expensive inconvenience for long euro road trips, especially in winter.

So LPG tanks are the system of choice for long termers and road trippers. If you are planning multi country Euro trips, then you have a good asset. A new LPG tank system is around £1000 these days. I swear by mine - had it for 8 years now, been all over Europe and it has paid for itself twice over. All you need is a set of LPG pump adaptors and you are good for any country - and LPG is much cheaper than bottle gas. There are a few idiosyncrasies - ie no LPG in Morocco, and different tax rates in Italy, but I've always managed. LPG availability is also a factor. There has been much discussion in recent years about LPG slowly disappearing from UK forecourts, and this is true, mainly because of the electric car boom, but LPG as a fuel will be around for decades yet, and the only inconvenience I have found is that you have to plan your fill ups - there are non forecourt sources in most big towns and cities, and there are apps that tell you where they are. On the continent there is no problem.

Consumption wise, in summer, fridge and cooking, a 60L tank will last you for months - at least 3 or 4. In winter, when you have the heating on, then a tank will last you at least a full month using the heating for 12 hours a day. I have a 40L tank and in cold weather with the heating on 24/7 - ie non stop, I get 1 to 2 weeks. In summer it's every few months. It is heating that uses gas - fridge and cooking are low consumption.

At the other end of the scale - if you had it checked over and a professional condemns it - corrosion perhaps - then you can have it removed and revert to gas bottles for not much money - the plumbing will still be there and the installer will remove the old tank and install new fittings in the gas locker. This would probably be the best option if you don't want to shell out for a new tank - ie if you only plan to stay in the UK and not use the van very often, and only in summer. But I only add this for info - it is highly likely that your system will check out OK - most do.

No need to spend big bucks to get wifi internet in the van with a router and why roof mounted antennas and MiFis are a waste of time

EDIT 2024. I have been getting some stick online from various "experts" who are adamant that the only method worth considering is to have a roof mounted antenna, and various other gizmos, usually costing £300 or more in total. As I explain below, you simply don't need to go to this expense. The number one technical requirement to get a decent mobile signal is the size of the antenna. Once you have a big antenna, as found in a full size router, as opposed to the tiny antennas built into the body of a MiFi unit or a normal phone, you get 90% of the advantage. It is antenna size that is the major factor. Phone antennas are small simply because they have to be. Mifis are the same - any Mifi is a complete waste of time in a motorhome because size and portability and battery power are not a factor - Mifis are for travellers who stay in hotels. I will go further .... the Netgear Mifis, Nighthawk etc, are ridiculously overpriced, and are all easily outperformed by a "normal" 12v full size router such as the B525, placed by a window.

The reasons for the arguments are purely commercial. There is no profit in a secondhand fifty quid Chinese router - it is as simple as that. So companies such as Solwise and Motorhome WiFi dress it up as a technical subject and sell it to you as a package. Nothing wrong with that, good luck to them. But I am not involved commercially, I don't sell anything, I am a motorhome blogger with no axe to grind, and I just happen to have an interest in radio technology, having held an amateur radio licence for 30 years - and to get one of those you have to pass an exam, and 30 years ago it was quite a tough one - I even had to learn morse code.

The title of this blog says it all - there is simply NO NEED to spend over £300 for a router based motorhome wifi system. or to put it another way, the £50 option performs as well as the £300 option.

I now have a standing bet - a bottle of champagne - the proper stuff - for the van that parks next to mine and shows a working internet connection from the same network that is only available to him and not to me.

Here is the original blog .....

You don't need anything fancy to get the best chance of internet in the van over 4G. It's the size of the antenna that counts, not the location. The antennas in a proper full size router are 10 times bigger than the tiny antennas in a phone - that's why a router will pull in a signal long after the phone (hotspot) says no service.

The advantage of paying extra for a roof mounted antenna, plus the cables and adapters is negligible, and I have proved this by experimentation, based on my training in antenna theory as a licensed radio amateur. Antenna performance is like earthquakes - the scale of power is logarithmic. This means that for noticeable gains the performance has to double each time.

So the biggest gains are made by having a much bigger antenna. But after that, the positioning of the big antenna is less important, so the difference between on the roof, and just a meter or so below, by a window, or on the dash, is hardly noticeable.

EDIT see below about the insides of a roof MIMO antenna.

So all I use now is a Huawei B525 12v mobile router, which can be had for around £50. It lives on the front dash, or inside a cupboard if it is too sunny. The only time I have to try and reposition it is on the rare occasions I get no signal, or just 1 bar - in these situations it rarely improves. There has to be a minimum signal strength to support a decent data rate - so if there is no signal at all, then that's that.

In my early days of experimenting I used to put the router on the roof temporarily, in a plastic box. But I soon learned that no signal in the van (by a window) was no different to being on the roof.

If you are slightly technically minded, here is the methodology - the B525 has a page which displays signal strength and quality of signal parameters - QOS.

This a screen shot of the numbers with the router on its internal antenna by a window. With my roof top antenna the numbers were barely changed. The reason I did the experiment was that my rooftop antenna terminated in the wardrobe, so the router had to be there. But then I couldn't see the lights on the router, which was a pain. So I relocated the router inside the van where I can see it, and for the last few years have monitored it quite carefully - hardly any difference. I have never had a situation where I had no signal, and reconnecting to the roof antenna gave a signal. If there is no signal, there is no signal. 

The Huawei has proved very reliable and easy to use - when I change country and get a new sim, in it goes and 9 times out of 10 it connects immediately.

The takeaway for full timers is that you don't need to spend more than the cost of a Huawei B525 - about £50. There is a whole family of 12v mobile routers - others are available, but I like Huawei - mainly because of the software and firmware.

There is no point at all for the average motorhome owner to pay silly money for any of these expensive "motorhome wifi" kits. All you need is a 12v router costing around £50. There is also no need for any sort of power conditioning, as often suggested. They work fine on any voltage that can happen in a van - from 11v to over 14v. Now that I am on lithium batteries my resting voltage is 14.4v for days at a time (when plugged in) and I have not had a single failure of anything 12v in the van.

With regard to SIM cards - that is a big subject, as is EU roaming. In the UK I have found the EE network to be best, but EE is expensive. Three are the cheapest - you can get unlimited data on a 1 mont contract for under £20 ... but Three is the worst network -they have too many customers on at the same time and sppeds go down in the evening - depending on where you are. I recommend 1pmobile - it is one of very few subnetworks of EE, and you can get PAYG data from them on the EE network, cheaper than EE. Their deal for 200gb for £20 is very good, and no contract. I have never used O2 or Voda, but they are average from what I read. 

MIFI - I am not a fan of the little mifi boxes - they do need external antennas - and expensive adapters for their tiny antenna sockets - so what's the point. I would go so far as to say that a mifi box is a waste of money in a motorhome - you might as well use a phone hotspot. The only practical use for a mifi is for  travellers who live from a suitcase in hotel rooms etc. Mifis are also battery powered from USB, and tey don't like being plugged in 24/7. But if you already have one, and you are happy enough, then it ain't broke so don't fix it - not everybody is a speed crazy internetaholic like me!

Update for 2023. I have been experimenting with 5G, and it has either been not available, or just as fast as 4G and only occasionally super high speed - ie over 100mbs. The bottom line is that 5G is mainly in cities and big towns, and while I am sure it will eventually replace 4G, just as 4G has replaced 3G, for now, I just can't see the point, and 5G 12v mobile routers are still expensive - well over £100. After 3 months I sold it for £20 less than I bought it for and went back to 4G. 5G is nothing special - 4G is and always has been fast enough to deliver an internet TV service - Netflix etc to a motorhome. Only working professionals in technical need of over 100mbs need to consider 5G in a van. Most new phones are now 5G - mine is, but unless I am in a town I never see it connected. But when I have been in a city I have noticed it can be amazingly fast - but in reality the extra speed makes no difference to daily use. All most van owners need is streaming TV netflix etc, and 4G is more than fast enough for that. I streamed Netflix and Iplayer for years on 3G before 4G arrived.

EDIT 2024/2. I have had a dome MIMO antenna on my roof since 2015, but I have not used it for many years since I bought a B525. Recently I was on the roof and noticed it was damaged, probably by a tree. This is what I found inside. I know it looks weird but I do have some training in antenna design, so the use of aluminium sheet is not unusual - the shape gives more bandwidth, but bandwidth does not equate to signal strength - only multiples of a wavelength can do that. But the build quality is awful, using foam and glue - totally unsuitable for the vibration experienced on a van roof. I bet that the internals collapsed long before the case was broken. Electrically - ie in terms of radio reception, the antennas inside this dome are no different to the same size antennas inside a B525 or similar sized router - which explans the very little difference in performance. But that is not to say all roof mounted antennas are bad - only this particular type. See photo below.

These are typical QOS numbers from the admin page of a B525.

Big antennas like these are the secret of of good mobile reception.

Inside an old broken MIMO roof antenna

How a 600ah lithium battery setup has changed my motorhome full-timing life.

But let me be honest from the beginning - very few people will actually need 600ah. I was actually planning 300ah, which was a meaningful upgrade from my original 200ah lead acid setup. But at the last minute a friend who was advising me said that for what it will cost - all other installation costs being the same, an extra £600 to double the capacity was, on paper, a bargain. Furthermore he said that he had found, with that much power on tap, that there were many more intangible benefits that would crawl out of the lithium woodwork. And he was right!

Background - my wife and I full time in our S700 in all seasons. We are also heavy power users. TV and laptop are on most days, most of the time. Our daily power use varies but averages about 75ah a day. Our original fully charged 220ah  lead acid battery, backed up with 500w of solar, would, in winter, just manage a full weekend off grid, with no engine charging or EHU. In summer we had power to spare, and could stay off grid almost indefinitely. But for full timers, it is winter performance that matters.

In winter, our routine was always different to our summer routine. In winter we needed to drive more, and plug in more, which meant using camp sites or visiting friends more often, or even occasionally running a generator. We never liked using it, but occasionally - maybe twice a year, it was a life saver.

Of course there is also the point to be made that you can always use less power - this is a valid point, and we can and do switch things off if we want to stay put for longer and eke out the battery, but it's no fun.

This last system, we ran for about 5 years, and it taught us a lot about energy use in a motorhome. We discovered 3 things. 1. accurate monitoring is essential - you need to know what you have and what you have left - same a the van fuel gauge. 2. No matter what, every amp you take out, has to be put back in. 3. Solar power is by far the most important part of any off-grid system.

1. Monitoring I have blogged about elesewhere. It's important. But I would add that in the context of lithium batteries, you usually get it for free - in an app, and this has proved to be a real benefit.

2. This sounds self evident, but is really important! In summer, a decent solar system will put in the next day all you took out the night before. But in winter, solar panels give a fraction of what they do in summer, so you have to monitor very carefully and plan your activities to put back in power from other sources.

3. Solar power is the only free source of power, and the only one you can be truly off-grid. So in a nutshell, you need to fit as much as your roof will hold.

The reason I make these 3 points is that having a huge battery is not the simple answer to all these problems. But what it does do, is give you options.

One example of this is that with 600ah on tap - whereas before we would have to move after 3 days - now we can stay for much longer. If we use 200ah over a weekend, we still have 400ah left. However we soon learned that if we do stay in one place and use say 90% of a 600ah capacity, then we really need a proper recharge. Winter sun would take weeks to recharge that much, so the reality has proved that if we do need 500ah of recharge, the only meaningful option is to plug in. So our EHU charger has to be capable of completely refilling a 600ah batter in an overnight charger - this basically means 10 to 12 hours at 50a. So another rule we have learned about big lithium, is that your EHU charge source needs to be big enough to recharge in less that 12 hours - ie overnight.

The same goes for alternator charging. A lithium battery will suck in every amp that you throw at it, so a 60a alternator will deliver exactly that, for long periods of time, and will eventually overheat. There are various ways to improve this - we chose to install a B2B - battery to battery charger. This is a unit that sits between the engine battery and the lithium battery, and limits the power that can be transferred. In our case this is 60a - half the capacity of our 120a upgraded alternator. A side effect of this is that with a 60a b2b, 2 hours of driving actually gives us 120a. This is quite different to our old lead acid battery, which would suck in high power for a short while, and then settle down to a fraction of that - so a 2 hour drive would give something like 30a of charge, even though 60a was theoretically available. This is quite technical, and down to the difference in charging characteristics of lead acid and lithium, but this is a noticeable example.

This also a good example of how fitting a lithium battery means that you also have to think about, and if necessary modify, your charging equipment. Lithium batts have different charging characteristics to lead acid batteries. In our case this proved to be a good thing, because as already mentioned in the past, especially in winter, we had to plan our movements carefully, especially with regard to the weather, and driving distance - now we have a decent B2B, we can drive less for the same power, or get more power from the same drive. So when we are just touring around, distance between stops can be less, saving diesel. It also means that with a 50a EHU charger, we can take advantage of even short stops where EHU is available - ie at friends etc.

But by far and away the biggest advantage for us has been simply the fact of having all that power on tap. We no longer have to move because we have to - we move because we want to. We can stay a week or more instead of just a weekend, and if we do, and use almost all of that 600ah - then we know that all we need is an overnight charge, and we are good to go again. In pure financial terms, with campsites and CLs costing around £15 these days, in winter we are saving around £60 a month. 

There have also been some minor advantages, but satisfying nevertheless. We now run our fridge on 220v while driving - this keeps it cooler. And in summer we run it on 220v quite often, even though it is a 3-way fridge and takes 10amps. This is saving us gas - LPG, and we have already noticed that we are using less gas. I have always defended 3-way fridges because they are the ultimate in versatility, and they are. Our 3-way is quite new, so no intentions of throwing it out for a compressor fridge, but if the situation arose, I would now have no problem handling a compressor fridge, even in winter.

Another advantage is the use of high power items. My old system was easily capable of delivering the 100a or so needed to run a toaster or coffee machine for a few minutes, and on paper, because they were only on for a few minutes - eg 100a for 5 mins = 8.3ah - not a lot. But over the months I soon noticed that regular high power use was changing the characteristics of my lead acid battery. I looked into the technicalities of this, and learned that the capacity (and long life) of a lead acid battery depends entirely on the type of discharge it is asked to perform. It will last longer with only light discharge, and shorter with heavy discharge. And because I have accurate monitoring, and am a bit of a geek, it didn't take me long to realise that these morning coffee and toaster sessions were affecting the battery noticeably. 10 minutes of breakfast use was taking much longer to recharge from solar than the numbers would suggest. So after a year or so, we stopped, and went back to gas for toast and coffee. No such problem with lithium - they are quite linear. Doesn't matter if you take out 100a in 1 hour or 1a in 100 hours - the result is the same. So although the basic rule of put back in what you take out still applies, there is no worry about affecting the life of the battery.

Don't forget what I said above - these are minor advantages, but I have enjoyed discovering them and experimenting. 

So do you need 600ah?? Probably not is the honest answer. But several of us full timers have, and nobody yet is regretting the extra cost. 300ah is probably a meaningful upgrade for most van livers upgrading from 100 or 200ah lead acid systems. My system is now 6 months old and I have yet to go below 300ah, but I know it's there, and I expect to be using it for the rest of my van life - 10 to 20 years - which is another good reason to go lithium - long life.

There are 2 ways to get lithium. DIY or ready made. I chose the DIY route. The DIY route involves buying the individual 3.2v Lifepo4 lithium battery cells, and connecting them together, in a case or frame of your own construction, to the power and voltage you want. So in my case I used 8 x 3.2v 300ah cells, in a 4 series 2 parallel configuration, giving 600ah at 13v. The mechanics and connections was a relatively easy DIY job. A lithium battery consisting of multiple cells needs a circuit board called a BMS - Battery Management System. This is a board that sits between the cells and the outside world. It has multiple functions - it balances the cells, and protects them from over and under discharge and voltage. It is an essential part of the system. Good BMS also have bluetooth built in and come with an app. That is all the technical detail I will go into at this point. But anybody with a multimeter, who knows how to use it, has hooked up batteries and stuff before, and is willing to read online from the many excellent DIY websites and forums - is capable.

The non DIY route is to buy from an established battery manufacturer. These companies do what I have just described above, but in a much more user friendly way - ie they put the 4 cells and the BMS inside a box that looks and feels like a traditional 12v battery. These can usually, but not always, be straight swap replacements for old 12v lead acid batteries - but you should check with the supplier. You also get UK based support backup and warranty. A typical example in the UK is Roamer batteries, who consistently get good reviews. But the cost difference is about 50 to 100% over DIY, because you have to pay their profit margin, which is well deserved!

Lastly - order cells directly from China at your peril. BMSs are usually OK, but ordering small quantities of lithium cells can be a nightmare from China, using Alibaba etc. Just take my word for it, and don't be tempted by what appear to be really cheap prices in dollars - that is only the beginning of it. Contact me more more details.

  

MERCEDES OM602 FAN BELT FIT

 This engine is found in most Merc S class classics - late 80s to 1995. I am not sure if the same arrangement is found on earlier or later engines.

I have now had to be recovered in my S700 4 times in 23 years, and 3 of those were for broken fanbelts. 1 was random, 2 a coincidence but 3 is definitely a pattern. It is probably a service item and should be changed every 50k or so - I don't know, I don't have that info. But the reason it rarely gets done is that it is an absolute pig of a job. I know this to be true because I have been witness to the grunting and swearing that has emanated from the garages I have used, plus the posts I have seen on other groups.

So if you have a Mercedes, take note of this post, which will be in the index, for future reference.

All the difficulty stems from releasing the tensioning roller in order to slip on the new belt - at first glance there seems to be no way of doing it.

The first thing I discovered is that most of the online information, and in particular the Russek manual, is either wrong, or at the best, ambiguous. The diagram in the Russek manual is accurate in the general layout, but lacking in detail. After the event, I managed to find much more accurate diags - see below.

The procedure is as follows. Identify where is the tensioning spring, which runs parallel with the hydraulic damping cylinder. Here you find a nut (3 - yellow arrow) which is located on a bolt, which appears to be attaching the top loop of the spring to the bracket. This nut has to be taken off - but if you try, the whole thing just spins. What you can't see, and what isn't explained clearly is that there is a hex bolt head at the other end of the bolt (6 - red arrow) - but where is it? It isn't obvious. The bolt is actually a 3 inch long bolt that goes into a hole in the top corner of the engine block - the hole is about 3 inches deep, and the head of the bolt is back there - you have to reach your fingers in to feel it, and you can just get a spanner on it. One you do, then the front nut removes easily.

Then you have to insert a bar/lever into the recess in the top of the spring bracket - this is depicted in the diag with arrows showing the direction to push. This bar has to be provided by you. By moving this bar to the left or right you can move the spring bracket, release the spring tension, and then using a drift (anything handy) you can then push the bolt back through the bracket and into its hole - you can also help it out with your fingers round the back - but don't take it out completely - just enough so that it no longer fixes the top spring bracket in place. Once this is released, the spring will lose tension, and the tension pulley will fall down a couple of inches, finally allowing enough slack to get the belt around all the various pulleys. It is still a tough job, but it will go on.

The blue arrow indicates the actual pivot point of the bracket - for info only, it has no function in the job.

It is also worth mentioning that there are 2 ways to approach the job - access is really tight, and that is half the problem - you need a good set of tools to get at that nut and bolt. But if you take the radiator off, the job is a whole lot easier. Taking the rad off is a 20 minute job. If you are doing this job yourself, and are not pushed for time, and maybe doing other work, consider removing the rad.

Once it is on, you have to re-tension the spring and then lock it back in place, under tension. To do this, you have to push your bar, which is still in its hole, well over to the left, which will pivot the top spring bracket anticlockwise, until the point where the holes in the bracket and the engine block align, and you can push the bolt forward, through the holes, and the spring is then tensioned and locked in place. Replace the nut, and that's it, job done. When you know what you are doing, and you have just the right sized bar, it's a 5 minute job. If you don't, then it's a frustrating episode with much swearing and skinned muscles.

The confusion arises from the fact that removing the nut is counter intuitive because you think that is what is holding the spring in place. It is and it isn't - the spring is held in place by a collar in the bracket, and the bolt slides through the middle to lock it in place. the actual pivot point of the racket is further down.

I just had to pay a French garage 2 hours labour because they had no clue, despite me presenting them with photos from my collection, and the Russek manual. In the end what saved us was a telephone call to my own mechanic friend in N Wales.

None of this will make the slightest sense to anybody! It doesn't apply to Fiat family owners, and it is of no interest to Merc owners, until it happens to you - at which point, this detailed description should be useful to you or your garage.

I am also now reasonably convinced that in motorhome use, Merc fanbelts have roughly a 5 to 8 year lifespan - probably nearer 5, especially if you have modified electrics, like I do - ie big batteries. In motorhome use, the alternator works harder and hotter and so does the belt. The belt can get damaged slightly if the engine is turned off when the alternator is hot, and the belt can get slightly "cooked" at that point. The alternator pulley is the smallest on the belt, and therefore is working hardest. My intention now is to make sure I have a new belt either every time other work is carried out at the front, or every 4 or 5 years.

Fridge not working on 12v or leisure battery not being charged while driving - how to troubleshoot. The D+ line explained.

This article is about pre 95 vans - with the old style fuse panel, and no Electroblock (EBL). However if you do have a later van with EBL, there is useful info here so it's worth a read. See note at end about EBL.

Fridge not working on 12v while driving is quite a common fault - usually a bad connection caused by old age and corrosion. It is closely related to another fault - leisure battery not charging while driving. 

How it works.

There is a red cable that leads from the positive of the engine battery (EB) to the Hymer fuse box. On most vans there is a big inline blade fuse of 20 or 30amps and although this is not famous for blowing, it can happen, so it needs to be located and checked before anything else.

This red cable then leads to the 2 relays in the fuse box. These relays only operate when the engine is on, so that the flow of power from the engine battery and alternator only works when the engine is on. The relays are operated by a wire from the alternator known as the D+ wire. When the engine is running, the alternator is spinning, the D+ wire goes up to 12v, and this operates the coils in the relays, which close the relay contacts, allowing the power to flow through the relays to both the leisure battery (LB) and the fridge. When the engine is switched off, the 12v on the D+ wire goes off, the relay coil relaxes, the contacts open, and the power is disconnected. This ensures that the EB is disconnected from the LB and fridge when the engine is not on, ensuring that the EB can not go flat, even if the LB does. It is a fundamental feature of all motorhome electrics - also called "split charge relay" - the charge is split between the EB and LB, but only when the engine is on.

The D+ wire which operates the relays, also operates the ignition light on the dashboard - in a normal, non motorhome commercial van, this is part of the original Fiat or Merc wiring.  In order to make the D+ wire operate the additional relays for motorhome use, it was necessary for Hymer to "tap into" this wire. To do this they added an additional yellow D+ wire extension of their own, connected to the original D+ wire, and leading to the Hymer fuse box where the relays are located. Where Hymer tapped into this wire - see photos - can corrode with age - a flickering red ignition light is also an indication.

This is a general description - the principle is always the same, but there are differences between Fiats and Mercs, and different models and different ages. But all vans share the 2 common basics - the thicker red wire from the EB+, and the thin yellow D+ wire to the relays. All you have to do is locate them in your particular van.

In Fiat family vans, the EB is located under the bonnet. The red battery cable has to run right across the engine compartment to the other side, where the Hymer fuse box is located, usually by the drivers left knee (on a LHD van).

In Mercs, both the EB and the LB are located in the battery box by the side of the drivers seat.

In both vans, the wiring colours are always the same - red cable leads from the EB+, to the fuse box, through the relays, and back to the LB+ in a black cable. There is also a brown cable to the LB- negative. Do not get the cable colours mixed up! Brown is negative in Hymer 12v electrics.

Troubleshooting

The first test is easy - you need to know if it is all working or not. Disconnect the black cable from the LB+. With the multimeter set to DC volts, place the black meter probe on the negative post of the battery - ie ground - and hold the red meter probe on the end of the black cable. There should be around 13v there with the engine running, and nothing with the engine off. If you have 13v here with engine on, then everything in the charging, relay and D+ side is working, and the problem is elsewhere. If there is no 13v here with engine on, then see below.

If you are troubleshooting fridge not working on 12v, then the test for that is to locate the fridge fuse this is the first fuse below the fuses that are all connected by the copper bar - the top 3 or 4 fuses - it varies from van to van. But the next one down is the fridge fuse - so it's either fuse 4 or 5. Again, with the black probe on the battery negative, and the red probe on fuse 4/5, you should see 12v with engine on, nothing with engine off. If you have no 13v here with engine on, but you have 13v with engine on on the main black wire to LB+, then that means it can only be the fridge relay - the smaller one of the two.

If you have no 13v at these two points with engine on, then the problem lies either with the main red cable, the D+ wire, or the relays. The relays are the least likely cause, they rarely fail.

So the next thing to do is to expose the inside of the fuse box, to do this undo the 4 screws in the surround, and work the box forward - it can be awkward, there are a couple of screws protruding that you can't see which try and prevent the box coming fully out, but if you work the box up and down, it will come, and there is enough slack in the cables at the back - because that's how it went in in the factory.

Once you have the box out, and can see the relays, then the next test is also simple. Get your ear close to the relays, and rest your fingers gently on the relays, then start the engine - there should be an audible click and vibration of the relays operating - relays are switches, and they make an audible click. If you hear and feel this, then that proves the relays are working. If the relays are working, then the problem then must be in the main red cable feed from the EB+. This is rarely a problem in Mercs, because the cable run is short and inside the van, but in Fiats it has to run right across the engine compartment, through the bulkhead and into the back of the fuse box. So you need to locate this cable and check it. The usual culprit is an inline fuse which is either blown, or corroded. Failure of this fuse, its holder, and connections, are quite common in Fiats, rare in Mercs.

For those that don't know, this is what an inline spade fuse looks like - this is a generic pic from the internet, your will look similar, not exact, and may or may not have a cover.

But if the relays are clicking, but you have no 12v on the black wire to the LB+, or to the fridge, then the fault has to be between the EB+ and the relay.

If the relays do not click, then most likely you have a D+ problem, which is a break or bad connection in the thin yellow D+ wire. If you look closely in the box, you will see the yellow wire going to the socket of the big relay, and then an extension, also yellow, going to the socket of the small relay. If there is no 12v on the D+, the relays won't operate - they won't click.

In this photo you can see the yellow D+ wire going to the base of each relay.

As mentioned above, Hymer had to install an extension to the original D+ wire, running to the fuse box. On many vans they used a blue plastic connector called a scotchlock - also called Scotcklok, or snap connector. This is a connector that allows you to easily connect a new wire to an existing wire, without cutting.  

It is very difficult in just words to describe how these look, and work, so the best thing to do is to watch this Youtube. It shows exactly how they work.

https://www.youtube.com/watch?v=9U0N_BFHyaY&ab_channel=CURT

A typical example of the Scotclock D+ tap connection. Most pre 95 Fiat family vans are similar, but you have to root around to find it, and it might not be identical to this photo.

Unfortunately Hymer didn't know back in the 80s and 90s that these would corrode after 20 years and cause problems - but they do. So if you have no D+, you have to trace the yellow wire and look for this connection, because most likely it will be that - it's quite a common fault. The only problem is that with so many vans over so many years, not all vans have one, or it might have already been "fixed" by a previous owner. There is also a possibility that the fault is where the D+ wire is actually connected to the back of the alternator. I can't be more specific - it varies from van to van, but if your relays ain't clicking - it's that pesky yellow wire - somewhere!

If you do discover that the Scotchlock is your problem - then what do you do? The quick and dirty way is to snip all the wires - 3 ends - strip them back, twist together, and then seal with tape. This will get you going again, but will eventually fail after more years, depending on how well you did it, and how wet and damp it gets. Basically you can use any method that reliably re-connects the wires. Choc block connectors, Wago connectors, and 3m have upgraded the original to make it better - the new ones are round and better sealed. there is no one and only way - it's simple electrics - you just have to connect all 3 ends as best you can so that the power flows again.

As already mentioned - the relays are usually reliable and not faulty. But it is not unknown for them to fail. If you are sure that the feeds to the relay are present, but not getting through, then you have to remove the relay and test or replace. But you must be sure - relay failure is quite rare - it's usually always something else. Fortunately these are standard automotive relays, available everywhere. So you either borrow or buy a new relay, change it, and see if the problem is fixed. 

The relays can be hard to remove - work them out slowly by rocking side to side.

Or you can test the relay with a meter. To do this, you need to know how to use your meter to test continuity. All relays have the same pin numbers, which are marked on the relay - 85 86 87 and 30. Pins 87 and 30 are the relay solenoid coil. When 12v is applied to either of these pins, the other pin is permanently connected to ground through the socket, then 12v flows through the coil, which then creates magnetism in the coil, which then forces the other two pins 85 and 86 to be connected. So 85 and 86 should be open circuit when the relay is "off" and connected together with the relay "on". So how do you test this with the relay in your hand? The easy way is to get a PP9 battery - the small rectangular 9v battery that you can buy anywhere - and touch the battery terminals to pins 87 and 30 - the relay should click. If it doesn't, the relay is broken. If you know how to use the meter to test continuity - it's too long winded to explain this here because there are so many different meters - then pins 85 and 86 should be open circuit when off, and closed circuit when on. But 9 times out of 10, if you don't get a click with the battery, you don't need to go to the trouble of testing continuity with the meter - just replace it.

As with most things - there is a Youtube - this short Youtube shows exactly what I am trying explain with words!

https://www.youtube.com/watch?v=UBpQJ1DRa9A&ab_channel=Wisdom

The big relay is the main split charge relay that allows engine power to the LB - the original is a 70 amp relay. This is quite a rare size - it doesn't really need to be replaced with a 70 amp relay, it's a typical case of Hymer belt and braces - they used a big relay value so that it lasts a long time because it isn't working so hard. In normal use the relay rarely passes more than 20 amps. So if you can't get a 70 amp, any value above 40 amp will do. The smaller fridge relay is usually 20 amp and the fridge takes around 10 amps on 12v.

The original relays usually had metal covers - modern ones are usually black plastic - either will do.

So now we have explained how the power is connected from the EB to the LB and fridge, through the relays, and how the relays operate. 

So the only thing left to explain is what to do if you have a working relay system, and you have 12/13v on fuse 4/5 with the engine running, but the fridge is still not working. Well in this case, the problem must be with the fridge. The 2 commonest reasons for this are corroded connections at the fridge end of the 12v supply wire from the fuse box, or the actual fridge switch itself. Just look at this photo - that's how bad corrosion can be. Remove the bottom external vent to the fridge, and check behind there, looking for a connection like this, using the flat plastic covered spade connectors. That is how most classic Hymer fridges are connected to the 12v supply from the fuse box. In this photo the 12v from the fuse box will arrive here, but go no further!

Don't forget - Hymer 12v electrics - blue is positive, brown is negative.

The next most likely cause is the fridge 12v switch itself. These switches don't get much use and spend long periods in storage, unused. Over the years the contacts inside the switch can get corroded. So one trick is to work the switch many times - on and off until your finger hurts. You might get lucky and the fridge switch comes back to life. If you are lucky, sometimes you can pull the switch forward and there is just enough slack in the wires to get at the back of the switch for further tests, or even replacement - but it varies from van to van, as I have often frustratingly found out.

Unfortunately if none of this works, further checks and tests mean getting access to the fridge, and unfortunately this means the fridge has to come out so you can get at things. 

The 12v element in the fridge is quite famous for not lasting more than about 30 years. But to get at it - you need to take out the fridge. The element is a simple heater element - if it breaks, it will show open circuit with a meter. There are Youtubes and other online explainers on this.

EBL vans. 

If you have a later van with an EBL, then here are some notes to help you. On these vans, the relays are inside the EBL, and the main red EB+ feed and D+ yellow wire go into the EBL and the 12v feed from the EBL to the fridge is also there and easy to test. Although you can't get at the relays without going inside the EBL, you can troubleshoot, because the connections are on the front of the EBL, and you can shove the needle of your meter probe into the back of the plugs to take readings. There are different EBLs, but the diags and pin numbers are all online so all you have to do is find them. The D+ is quite easy because it is yellow. So the first thing to test on an EBL is that there is 12v on D+ with engine on. You should also be able to hear the relays if you are lucky. 

But all the above description about the feed from the EB and the way the D+ is tapped into, also apply to EBL vans, as do the comments about working on the fridge itself - the connections, switch and element etc.


HYMER CAMP NOTES

If you have a Hymer Camp, then all the above still applies, but you won't have the Hymer fuse box described above. In a Camp, the relays and habitation fuses are under the passenger seat, and the red cable main feed from the engine battery, and the D+ wire, lead here first, to the relays.