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. This 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.

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.