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.
Part 2 More Diagnostic Testing
So far I have established that the fridge mechanism is working normally on gas and 240 volts, the 12 volt heating element has not failed and the two 20 amps fuses in the 12 volt supply to the fridge are good. I have read a lot of forum posts on the internet but not found a solution. My engineer’s sense tells me that the most likely place to find the problem is in the control box or the fridge or the wiring leading to the fridge, I need to investigate both.
Is 12 volts getting to the Thetford Motorhome fridge control panel?
The fuses are not the problem, so is there a problem in the wiring between the fuse box and the fridge itself?
12 volt and 240 volt wires leading to the fridge
Here is a photo showing the wires that lead to the fridge, the gas pipe is also visible at the left-hand side.
There are two plugs, a white one for the 12 volt supply and a black plug for the 240 volt supply. Before doing this I checked the motorhome’s hookup cable was not connected.
After reading so many forum posts I already knew that there are 5 wires for the 12 volts supply to the fridge; but not which wire is which, so before going any further I needed to identify them.
Thetford make available a large selection of documents on their website for the Thetford 3000 series fridges, access it here (opens in a new tab). The installation manual is what’s needed because it explains how to connect the 12 volts supply to the fridge. While on the Thetford website I also downloaded a manual that tells you how to put the Thetford N3142 fridge into Service Mode where you can access Fault Codes. A full list of error codes is provided in the document.
12 volts supply to Thetford N3142 fridge
The wires are . . .
Red 12 volts +
White 12 volts –
purple 12 volts +
black. 12 volts –
orange 12 volts + when the engine is running
The photo shows the red and white wires are thicker, they are the 20 amps supply for the heating element. Purple and black are the 12 volts supply to the electronics of the Thetford N3142 fridge and the orange is a control signal that tells the electronics of the fridge that the engine is running.
Using a digital multimeter I checked the voltages and found all of them to be what I expected, including the orange wire which had zero volts when the engine stopped and the appropriate voltage when running.
Diagnostic reasoning so far
Summarising what I knew to this point, the Thetford fridge fuses are good, the 12 volt heating element has not failed and all the 12 volt supply voltages are present and what I expect as far as the actual voltage. That means there is only one place for the fault to be, and that’s in the electronic control box, so that’s where I have to go next.
More internet research followed and after a great deal of reading through forum posts.
Opening the electronic control box of the Thetford N3142 fridge
Disclaimer: If you attempt this you will certainly invalidate any warranty on your fridge. It is not for the faint hearted. Do not attempt unless you really are competent, instead take your motorhome to a competent repairer. Before doing any of this I double checked that the motorhome engine was stopped and the 240 volt hookup cable was NOT connected.
Thetford 3-way fridge mechanism and wiring
The control electronics for the fridge are inside the black box at the top of the photo. To open it I needed a Torx screwdriver. I removed the cover-retaining screw and carefully lifted the black cover upwards then outwards. Inside was the printed circuit board that controls the fridge.
Inside the Thetford N3142 control box
Here you can see that I have marked all the connectors and taken a photo so I know how to replace them later.
I carefully removed all the connectors paying attention to the different types, some pull off, some have locking tabs that need to be eased open.
Removing the connectors
The control circuit board comes out towards you to reveal it’s top surface, where there are three more Torx screws, only the right-hand one needs to be removed to allow the board to come off. The underside of the board is shown below.
There on the board is yet another 20 amp fuse! The fuse itself is blackened and nearby plastic is deformed by heat from the fuse. Clearly the fuse has been very hot over a long period of time. Here are more photos to show the problem fuse holder.
The overheating of the fuse and its holder is probably due to electrical resistance between the fuse and fuse holder. Discolouration of the circuit board is clearly visible.
Solder joint damaged by overheating
The overheating is clear when the circuit board is viewed from this side. The copper tracks are sufficient for the current as there is no other evidence of heating. The solder needs to be reflowed to ensure good conduction.
Solder joints after reflow
Here the joints have been reflowed to ensure good connections.
There must be a good reason why Thetford chose to put this fuse on the printed circuit board, my preference would be to put the 12 volts on a separate circuit using wire, but I’m an old-fashioned engineer from a different era!
Having reflowed the solder joints and replaced the 20 amps fuse, ensuring good electrical connection between the fuse and holder by slightly tightening the fuse holder by squeezing it with pliers it was time to put everything back in place. Finally a test to ensure that the fridge worked on 12 volts with the engine running. Success, the fridge changes over as it should.
Conclusion and advice
This has been a very long and difficult process on the Thetford N3142 3-way fridge. The fridge is now working on 12 volts when the engine is running. I am happy because I enjoy the challenge of fault finding.
My advice to anyone with the same problem is NOT to attempt what is shown here but to have a new control board fitted by a competent person.