Thetford Motorhome Fridge Not working on 12 volts Problem Solved

Disclaimer :: Do not attempt this repair unless you are competent. Without taking the correct precautions you could be exposed to dangerous voltages and may cause damage to your fridge unit. 

If you just want the solution without all the reading and photos here it is. There is a ‘hidden’ fuse on the underside of the control circuit board that had been overheated for a long time. Eventually this fuse ‘blew’.  My advice to anyone with the same problem is to pay for your fridge to be fixed by a competent person. What is shown here is for reference and technical interest and is not intended as a DIY repair manual.

Part 1 The Problem

I recently noticed that our Thetford fridge was not working from 12 volts when we were driving – I knew because the battery icon on the control panel was not showing. As we are back in the UK and it was October there was not urgency to fix this problem. However, fixing problems is something I am always eager to get on with. In our case the fridge still worked properly on 240 volts from the mains or gas. That’s important because it tells me that the fridge’s cooling circuit is not damaged and is not the cause of the problem.

Reading more about motorhome fridges not working on 12 volts

A search on the internet yielded dozens of results, mostly forum posts from people who had the same problem, their motorhome fridge was not working from 12 volts.

Fridges and freezers work by removing heat from the inside of an insulated cabinet and transferring that heat to the surrounding air. In motorhomes and caravans the fridge can usually be powered by 12 volts, 240 volts from the mains or gas. That is very flexible, it allows the fridge to be used almost anywhere, these fridges are often referred to as 3-way fridges. They work differently to the ‘compressor’ fridges that are used in most houses. The big difference between 3-way fridges and compressor fridges is how they produce the cold. 3-way fridges use a chemical mixture that circulates around a sealed system, this has to be heated in one place to drive the cycle. A compressor fridge has a small compressor that compresses a gas that then circulates around a sealed circuit. Compressor fridges use less energy to run, and are less expensive to buy so they are more popular in houses. 3-way fridges are silent, compressor fridges make a humming  noise when their compressor is running.

The following video is very helpful because it shows how much work is involved in removing a fridge and changing one of the heating elements. If you watch this video carefully you can see all the different parts of the fridge mechanism, that will be helpful when reading the rest of this article. Notice how much difficulty this experienced technician has when removing the fridge.

Initial fault finding Fridge not working on 12 volts

Most of the information about fridges not working on 12 volts suggested checking the motorhome fuses first. When the fridge is working from 12 volts the 12 volts is supplied by the motorhome engine. A 3-way fridge needs quite a lot of power to run it, at lease 200 watts, which is too much to draw from the battery of the motorhome and why the fridge only works on 12 volts while the engine is running. 200 watts is around 17 amps of electrical current – a lot! – so a big fuse is needed.

On our Hymer fuse box the fridge is labelled Kühlschrank (Hymers  are German motorhomes), the fuse is 20 amps and when tested with a meter was ok. Some forum posts mentioned a second fuse located by the engine battery, another 20 amps fuse. This second 20 amp fuse was slightly damaged by heat, the yellow plastic housing was deformed due to the fuse itself getting hot, the fuse itself was fine.

That’s not surprising really because with at least 17 amps flowing to the fridge the fuse could easily heat up. A test meter showed no problem with the fuse, but I changed that fuse anyway.

So after about a day of research on the internet and checking and changing fuses I was no further forward except knowing that the 12 volt 20 amp fuses were good. While I was checking fuses I did test every fuse on the Hymer’s electrical panel, and also checked every fuse in the vehicle’s fuse box that is located near the steering wheel.

More internet research regarding the fridge not working on 12 volts

Having eliminated the fuses my first thought was that the fridge’s 12 volt heating element had failed. A brief word of explanation is needed here. In 3-way fridges heat has to be supplied to drive the fridge, the heat can come from the gas flame, a 240 volt heating element or a 12 volt heating element. I knew the fridge worked on gas and 240 volts so that left only the 12 volt heating not working.

Now it was time to dig deeper into the innards of the Thetford fridge.

Thetford 3-way fridge mechanism and wiring

On the side of most motorhomes there are two grilles, one above the other. These are where air circulates for the fridge, removing the lower grille reveals the ‘works’ of the fridge. This photo shows the metal housing of the gas burner on the right, and the black plastic housing of the control electronics of the fridge.

Looking at the electronics box there are 4 thick wires that come out, two of these wires go to the 12 v heating element and two to the 240 volt heating element.

My suspicion was that the 12 volt element had failed so I wanted to test the element to find out. I did some more research on spare parts for the fridge, a model N3142 Thetford fridge. I found a fault finding guide that told me that the resistance of the 12 volt element should be 0.61 to 0.78 ohms and the resistance of the 240 volt element 192 to 248 ohms.

Testing the 12 volt heating element of the Thetford N3142 fridge

Before removing the cover from the electronics box I double-checked that the 240 volt hookup cable was not plugged into the vehicle. See the disclaimer at the top of this article before you even think about doing any of this.

Special tool required here, a Torx screwdriver to remove the cover screw.

Cover removed from electronics box

Notice two things: First that I have a photo of the wiring etc before going any further so that I know exactly how it needs to be put back. I can not emphasise 

Second that I have marked the positions of the wires that go to the two heating elements.

The terminals marked 1 to 4 were difficult to remove, they are large push-on terminals and there is limited space to get hold of them with long-nose pliers.

After removing the wires from the printed circuit board I measured the resistance of the two elements, 0.4 ohms and 217 ohms. (I suspect that my meter was not displaying the correct value of resistance here.) The 12 volt element’s resistance is not within the 0.61 to 0.78 ohms of a new element, but it tells me immediately that the element has NOT failed. If the 12 volt element had failed then its resistance would be infinite and it would have shown as OL on my meter. That is what I set out to determine.

Where does that leave me now? I expected to find that the 12 v heating element had failed, but it hasn’t, that means more thinking, more research and no doubt more work to get to the cause of what is preventing the fridge working on 12 volts.

At this point I was ready to take the motorhome to a specialist and ask them to fix the fridge. That’s what I recommend if you have a similar problem.

Or was I? We used to have someone at work called Fletcher, an electronics specialist, one of the senior engineers was talking to me about an extremely convoluted fault that Fletcher had diagnosed, and said Fletcher was like a terrier. You point him at the problem and leave him alone knowing that he will not give up until the cause is found. In this situation I find myself being like Fletcher.

Continue reading (next page of this article).