The classic Hymer fuse panel - explained and maintained

There have been quite a few issues and questions with habitation electrics recently, so let's dive into that mysterious brown panel down by the drivers left knee .... (in a LHD van).

Unlike post 95 vans, which have more complicated electrics, usually based around an Electroblok, pre 95 classics have quite straightforward habitation electrics and distribution, and at the heart of this is the fuse panel, found at the front of the van, under the dash on the left, usually close to the drivers left knee. There is also usually a big red cut-off switch mounted next to it.

(NOTE ... this article is about the fuse panel found in pre 95 coachbuilt Hymers - B544 and S555, B694, S660, and others ... but the Hymer Camp 65, 55 and 52, with metal cab and bed over, are different - there is a section on this at the end, but the whole article is still relevant, only the locations are different.)

The panel has a hinged front which pulls down, to expose a bank of 8 fuses. The type of fuses used in classic Hymers are known as barrel fuses. They were originally designed and made by Bosch, the famous German electrical company, so they are also called "Bosch fuses". Many UK sellers - Ebay etc also call them "European" fuses. Barrel fuses are not so common nowadays, having been replaced in more modern vehicles by blade fuses, but that is not to say that they are old fashioned or ineffective in any way, but it is true to say that they can be a bit more prone to failure from old age than their more modern blade counterparts. They are also slightly more awkward to insert and remove than blades.

The best fuses to buy have ceramic bodies, so search for these. Cheap Chinese ones have plastic bodies - they do work OK, but there have been occasional reports of them melting, but this is most probably down to a massive fault elsewhere, not the fuse itself. Halfords sell the thermoplastic body ones, which are fine.

Edit 2023 There has been much discussion on fuses since I wrote this, so here is more info on fuses.

Just because one or two people in 20 years had plastic barrel fuses melt does not mean to say that all plastic fuses are dangerous. The reality is that the stories came about because the fuse was overloaded anyway. A fuse is designed to blow immediately in the case of a short circuit - that is its only purpose. If that fuse is getting hot and eventually melting, which takes time, then the fuse is the wrong rating because the circuit has been overloaded. A fuse, even a Chinese one, should pass the designed current and stay cool. The rue of thumb is double. So a 16amp fuse should handle an 8 amp circuit and stay cool. If you consistently pass 16a through a 16a fuse, it will fail eventually - a ceramic fuse body will withstand the heat, but the metal fuse wire will eventually fail. A plastic body fuse may melt before the wire fails, but it is still doing its job, and I have never heard of a fire being caused by a melting plastic fuse - even the Chinese know not to use superflammable plastic types. But all failures are not the fault of the fuse, they are the fault of the designer/installer.

But it is also fair to say that some stuff from China is pretty poor, but this is common knowledge, so just why buy fuses from there in the first place?

The reality is that you need a fuse asap, and the quickest place to get a fuse in the UK is Halfords. In fact Halfords is just about the only place you can get a barrel fuse over the counter. And Halfords are a big company and they don't sell crap - or rather they do, but at least it is tested to a minimum standard - they don't want returns or lawsuits. So if it has to be now - Halfords.

But if you can wait a few days, and are a perfectionist, then the ones with ceramic bodies are the best. But as mentioned above - they are only "best" at taking abuse. Before blade fuses were introduced, every fuse in every car was a barrel fuse, also called Bosch type, or Euro type. And I have seen Ebay sellers advertise ceramics. Just search.

I can't honestly tell you what fuses are in my panel - honestly! I am sure the bottom 3 must be original - they are very low current. The rest can carry between 8 and 16 amps, but rarely do so, especially these days as most owners have converted to LED lights. I remember making a mistake a few years back and accidentally shorting a wire during a job - the fuse blew immediately - it did its job. And I just went to my box and got a new one - I have no idea if they are ceramic or plastic, all I know is that they were not sourced from China. But if I did source from China, I would just make sure that the circuit was in good order. Any fuse that melts is not the fault of the fuse, it's the fault of the circuit. Original article continues ....

In the Hymer manual the 2 most common fuse sizes are 8a and 16a - red and yellow. For some reason modern red and yellow fuses are rated at 10a and 20a - this is absolutely fine - use 10a for 8a and 20a for 16a - no problem at all, you will still get the same protection and function.

The fuses are held in place by a spring clip at one end, and a recessed hole at the other. To remove a fuse, you just have to grab it tightly and pull it out. To insert a fuse, you locate one end into the recess hole on the right and then push the other end into the spring clip on the left, where it should locate nicely in the depression in the click. The pressure of the spring clip on the left then holds the fuse securely in place and forms a good electrical connection at each end. A useful tool for inserting and removing barrel fuses is a small pair of long nose pliers, although it is perfectly possible to do it by your fingers alone.

At each end of each fuse you can see that there is a screw terminal which attaches the wires to each end of the fuse holder. These wires are just visible down each side, and they disappear behind the panel. 

The fuse panel isn't actually just a panel - it's a box, and the box extends for about 30cm behind the panel. There are 4 screws that hold the panel/box in place, and if you remove these, you can then pull the panel forward, exposing the box behind.

This box contains several components that are critical to the correct operation of the habitation electrics. These are the relays, and the shunt, which will be explained later.

At this point it is important to understand the distinction between "habitation electrics" and "vehicle electrics". Habitation electrics are everything in the living area of the van - the lights, the Truma stuff and the water pump etc. Vehicle electrics are everything to do with the base chassis ie the Mercedes, Fiat or other chassis onto which Hymer built the rest of the van. The vehicle electrics are completely separate from the habitation electrics. The vehicle has its own separate wiring and it's own separate fuse box. There is no common wiring between the two, they are completely different systems. There is only one single electrical connection between the vehicle electrics and the habitation electrics, and that is a single wire that brings 12v power from the vehicle charging system, to the habitation system. 

Many owners get confused about this, so it is important to grasp. If you have an electrical fault with the vehicle, like a tail-light, or headlight, or wiper, or any one of a hundred vehicle related faults, then you have to be looking at the vehicle system, not the Hymer system. Similarly, if you have a problem with the Hymer habitation electrics, there is no point in looking at the vehicle fuse box or wiring for a fault. So know the difference between the two, and know where your base chassis fuse box is. In Mercedes chassis it is always in the passenger footwell. In Fiats and others it varies, but is also usually in the general area of the passenger seat also.

So back to the Hymer fuse box.

The biggest and most common problem is old age and corrosion. When your classic Hymer came out of the factory 30 years ago, all those bits of metal in the fuse panel were all shiny and brand new. But over all those years, as with any metal, the surfaces of metals gets corroded. The same goes for the fuses - the strip of metal in the fuse can also get corroded. Eventually the corrosion builds up at the ends, and starts to interfere with the electrical connection. Electricity can not pass through corrosion - it needs to pass through a connection that is bare metal to bare metal. Unfortunately this is a very gradual process - it just doesn't happen overnight (although it does sometimes!). As the corrosion builds up, the electricity finds it harder to flow. This is called resistance. As resistance builds up, more electricity is needed to overcome the resistance, and ultimately one of two things will happen. Either the connection will break down completely, and the electricity will cease to flow, or the fuse will blow.

The same thing can happen elsewhere in the van. If any of the connections in the van get corroded and start to draw too much power because of corrosive resistance, then eventually a fuse will blow. This is what fuses are for - they are doing their job.

So that is the theory - how does it work in practice? Well it is really quite easy. All you have to do is clean the fuse panel. This is a job that should be done in every classic Hymer every 5 years or so. All you have to do is to remove each fuse and inspect the ends. Corrosion can either be quite invisible, or obvious. All you need to do is to rub the end of each fuse with something abrasive. This can be a bit of sandpaper, or a nail file, or the file bit of a swiss army knife - it's just common sense. You may even decide to just replace the fuses with new ones - after all they have been there for many years, and they are very cheap.

Next, you need to clean the ends of the fuse holder. At the spring clip end, you can use a cotton bud dipped in solvent. Or you can carefully use any pointed instrument. You don't have to bring it back to a new polish, all you have to do is break the surface of corrosion. Just a few scrapes is all you need so that you can see a bit of shiny metal. At the other end, the bit with the hole, this is a bit more awkward, but usually there is less corrosion here, so quite often a wipe is all that is required. Just use common sense. The main requirement is to get metal on metal.

Don't worry about the electricity - unlike 220v in your house, which can kill you, 12v is harmless, so you can touch it with your bare hands. 

Once this is done, for all the 8 fuses, you are good to go for another few years.

If you are trying to fix a fault in an emergency, it is worth remembering that if you do have a corroded fuse connection that is causing part of the van to go dark, simply removing the fuse and replacing it is usually enough to cut through any corrosion and remake the circuit. If in doubt, and several lights or appliances are not working, remove and put back all the top 3 fuses. It could be a simple fix. It is a very common problem in old vans.

Another very common issue is that a fuse can look OK, but the ends are corroded so that no power is flowing - so don't be fooled by this. Remove each fuse and replace it, even if it looks OK.

Finally, if you are doing a full service, take a good look at where the wires are attached to the screw connections. If the wires are still looking shiny then all well and good, but if you see any green gunge, then that is corrosion. It isn't a serious problem, a bit of corrosion doesn't do any harm, but best to give it a spray of WD40 and clean it off with a stiff brush or whatever you have to hand. Then, with a screwdriver, give each of the screws a quarter turn anticlockwise, as if unscrewing it, then tighten it back up again. This will break through any corrosion deep inside the connection, and make the connection 100% again. Problems with these connections are rare, but while you are at it, it is such a simple thing to do, so daft not to.

So now you know how to refurbish your fuse panel, let's talk about what the fuses actually do. If your Hymer still has its documentation, then somewhere there will be a fuse diagram, but many classics are either missing their manuals, or the manuals are in German. However, in most classics with the standard fuse panel, the layout is always the same.

The top 3 fuses are the most important ones. All power to the habitation electrics go through the top 3 fuses (with a couple of exceptions depending on what model van). If you look closely, you can see that they are all connected together by a strip of copper on the right hand side. Which fuse is connected to what in the van, depends on your layout and van model, but if something has stopped working somewhere in the van - ie the lights, or a Truma, or the pump, then it will most likely be powered through one of the top 3 fuses. There are other single fuses and junction boxes in the back of the van, depending on your model, so the fault may be with one of those, but generally, all power goes to the van through these three.

It is worth remembering that if one of the top 3 fuses has a problem, then more than one thing in the van will not be working because each fuse feeds power to a group of things, not just one. If just one single thing is not working, but everything else is OK, then it is unlikely to be one of the top 3 fuses, and more likely to be a problem closer to the thing that is not working - a loose connection, or something - every van is different.

The next fuse down is number 4. This fuse is for the battery charger. When the van is hooked up, the charger operates and supplies 12v to the leisure battery. This power comes through fuse 4. It is the only function of fuse 4. 

Fuse 5 is the fridge 12v fuse. When the engine is running, 12v is fed to the fridge via fuse 5. This is the only function of fuse 5.

Fuses 6 7 and 8 are the fuses for the panel meters - the two meters that show battery voltage, and power - the famous Strom meter. These fuses only affect the operation of the panel meters, and do not supply any power to anything else in the van.

However there are many different van models, and there are some differences, but in general the top 3 fuses are the main fuses. Many vans have additional fuses - for example there is often a fuse in the bathroom. On some models, power for certain appliances is taken directly from the leisure battery, via a separate fuse. An example of this is the S700, where power for the front Truma heater is taken directly from the battery box. You need to be familiar with your van to know if you have any extra fuses hidden away anywhere.

Finally, let's explore the box behind the panel. This can get a bit technical, but it's worth knowing what goes on in there. The first thing to know is how to get the box out - it can be a struggle. After undoing the 4 front screws, the box should pull forward, but very often it only comes about half way, and then seems to get stuck. This is because there are a couple of bits of metal sticking out of the frame that hit the side of the panel hole. What you have to do is wiggle the box up and down as you pull forward, so that the bits of metal get over the edges of the hole. It's difficult to explain in words! But the whole box will come out, as shown in the picture. 

There are 3 components in the box - 2 relays and a shunt. the relays are small black (or occasionally silver) boxes - one bigger than the other. The shunt is a horseshoe shaped piece of copper located at the back of the box - see photo.

The bigger relay is relay 1. Relays are basically just remote controlled switches. This relay is connected to the vehicle electrics - ie the starter battery, on one side, and the leisure battery on the other. The relay only operates when the engine is running, so that power can flow from the vehicle alternator to the leisure battery. When the engine is off, then the relay is off, and the leisure battery is isolated from the vehicle battery. this means that you can run down your leisure battery, and the van will still start. Relay 1 is critical to the operation of the habitation electrics, but thankfully it is a very reliable component and rarely fails. But if you are getting no charge to the leisure battery while driving, and you have checked everything else, then relay 1 has to be checked.

Relay 1 is not Hymer specific - it is a very common automotive relay and available almost everywhere. The original relay is 70 amp - these might not be easily available over the counter at your local car parts shop - they are usually anything between 20 and 50 amp - but you can use a lower rating to get you going again - Hymer as usual used a high rating quality relay that would last for years, but in reality more than 20 amps flowing through the relay is quite rare. A 50 amp one will do permanently, and any lower figure can be used temporarily - especially in a standard van with just the usual single leisure battery.

To troubleshoot the main relay - the easiest check is to put your finger on it and feel for the click when you either switch on the ignition (Fiat) or start the engine (Merc). The relay can only be broken 2 ways - either permanently open or closed. If stuck open, then you will not get any charge through to the LB from the VB while driving - you can check this with a meter - the VB should be up over 13v but the LB will be unchanged at around 12v. With a working relay, both batt voltages should be the same with engine on. If the relay is broken closed (rare but can happen), then the LB and VB batts will be the same voltage with engine off, and both will run down together because they are now in parallel due to a permanently closed relay. 

The smaller relay is relay 2. This also only operates when the engine is running, and supplies 12v to the fridge while driving, via fuse 5. Troubleshoot this by feeling for the click. Be aware that it is very rare for the fridge relay to break - usual causes for no 12v to the fridge while driving are either the fuse, dirty fuse connections, corroded 12v connections at the back of the switch or a faulty red 12v switch on the fridge itself. All these have to be checked before you go to the hassle of pulling out the fuse box to get at the relay. If you have 12v at fuse 5 (in most vans) then the relay is working.

The shunt - this gets a bit technical, but the shunt is a device through which all 12v power in the habitation electrics flows. The shunt senses how much power is flowing, and in what direction, and then sends this information to the power Strom meter in the main panel, via fuse 6, so you can see how much power you are using, or receiving, on the meter. In the red you are using leisure battery power, in the green you are receiving it - either from the engine, or from the charger, (or from solar. However because solar systems are not factory fitted, not all solar panel installations register on the Strom meter, because whoever installed them didn't connect them through the shunt.)

There is very little that can wrong with the shunt - it is a thick copper wire. However, there have been instances where the shunt has completely corroded away and disintegrated, If this happens, all power to the van is lost, and it can be a real head scratcher, so it is worth mentioning. A corroded shunt is caused by road spray from the engine compartment getting into the back of the box. Normally there is some protection to prevent this, but if this protection has come loose, perhaps from accident damage or some other engine work, then a lot of water and damp can get into the back of the box and corrode away a shunt over 20 odd years. It is very rare, but has been known to happen.

Another reason the shunt can be damaged, even completely destroyed, is if somebody previously has connected the leisure battery the wrong way round. This can result in enormous current through the shunt which will melt it. This is rare, but I have seen it twice in 20 years.

The plastic cover to the fuse box - that hinges down, is actually the same component as the cover on the hookup cover on the side of the van. These are easily available on Ebay and from caravan/motorhome shops. See photo below.

The big red cut off switch. This does what it says on the tin. It is there mainly for safety - if you have a fire or a funny electrical smell then this is the first thing to do. It is also used for winter storage. There are two things to remember about this switch - it is a bit awkward to use, and can occasionally be operated inadvertently. Many a red face has been caused by this red switch. Secondly - it can very very rarely, fail completely. I have only heard about it once in 20 years. So if there is zero power to the van, and the shunt is OK, and all else has been checked, then the switch has to be checked as well. This can be done with a multimeter.

So that is just about everything I know about the classic Hymer fuse panel and box - if I have missed anything, please comment below.

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The shunt

A home made shunt repair using the original eyes and 2mm copper wire. To do this you will need a good soldering iron. And please note that the dimensions are critical - it is unlikely the new one will be exactly the same as the old one, so the meter may not indicate accurately (if the meter still works). You would have to calibrate with a multimeter.

Typical fuse layout

Typical circuit diagram

The relays - can also be black

Replacement cover - same as the EHU cover.

EXTRA NOTES FOR HYMER CAMP OWNERS 

This is a Hymer Camp - with metal cab.

The Hymer Camp does not have the fuse panel as described above - Camp electrics are under the front passenger or drivers seat. Under the seat you will find the leisure battery, the EHU charger, the fuses and the relays. Operation of these is exactly the same as described above. There are only 4 fuses in a Camp. 

There is no shunt in a Camp, and so no Strom (amps) meter.

Some Camps have a main on off switch on the panel - usually marked in German 12v ein aus - 12v on off. This has the same function as the big red switch in the other Hymers - it isolates the leisure battery - ie switches habitation power on and off. The actual relay is the big black box next to the charger. It s a latching relay made by Schaudt, and is remotely controlled by the panel switch. They are not known to be unreliable, but on the other hand some have been known to fail. If you have no hab power and you know the battery is good, then it is easy to test the relay - it has 2 thick wires in and out that carry the battery power between the LB+ and the distribution connector block, and 2 small wires that go to the switch. Short out the 2 thin wires and the relay should operate, if it does, then the panel switch is faulty. if it doesn't, then short out the 2 thick wires. if power comes back on then the relay is broken. In which case temporarily fix the problem by disconnecting the 2 thick wires and connecting them securely together and taping up. This will get the power back on, but you lose the ability to switch on and off from the panel.

Engine battery connection is a red cable that comes from the engine battery positive, via a fuse in the engine compartment, to the bigger relay, via the connection block.

Camp under seat electric box - battery removed in this photo

The charger is often coloured blue or silver.

Camp circuit diag - all pre 95 Camp models are the same

Camp fuses