Thetford N3112 fridge not working on 12v

[newebr]

Hi, I need some help and advice. My N3112 on my Swift Fairway 584 SE (2014 model) will not work on 12v when towing. I have followed the diagnostics given by Thetford and get fault code 6 on the led display. I have then followed the flow chart, and the conclusion was faulty power board. Ordered and fitted a replacement, but still the same problem.
Have checked all wiring on car and van, no issues.
With the fridge turned off I have 13.98v at the multi-plug at the back of the fridge. Turn the fridge on, select battery power, all I get is flashing lights, and the voltage at the multi-plug is down to 8.97v!
The fridge was pre-chilled overnight using mains power.
To eliminate any caravan wiring I ran a new set of cables from the input plug on the 12v fuse box at the front of the van, directly to the multiple at the back of the fridge, and it made no difference.
Fault code 6 basically says the D+ voltage is out of range, I.e. below 10v or above 15v.
Well mine is definitely below 10v, but only when the fridge is turned on.
If I disconnect the 12v heater element, then the voltage returns to normal.
So is it the heater element that is faulty? I've checked it's resistance, and that seems to be in spec, 1.1 ohms.
Any help or suggestions much appreciated.

19/7/25 UPDATE
Managed to do bit more investigation inbetween rain showers!
Borrowed a 12v power pack from a neighbour. Connected this directly to the back of the fridge, so that the board was powered from the caravan battery, and the 12v+ and earth from the car were replaced by the power pack. Fridge worked every time!! So can eliminate any issues with the fridge itself.
Checked all connections on car socket, all as expected. Fridge earth 0 ohms.
Next decided to connect the power pack directly to the fridge 12v+ and earth on the caravan 13pin plug. Took the plug apart, and removed specified cables, and connected directly to these.
Powered up and turned fridge on, just got the flashing lights, so not working. Voltage at caravan 13pin plug was 11.4v, but at the multi-plug at the back of the fridge it was down to 9.2v, well out of operational range.
Now got to investigate this further.
Will keep you posted.

 

[Martin_E]

It sounds like the supply to the tow socket isn’t heavy enough, once under load all the juice is zapped.

You could try putting 12v directly to the socket at the fridge to see if the fridge then works as it should, if it does then it would be a supply issue

 

[Hutch]

I found that the fridge earth return went through the cars electrics, when I put in heavy earth cable direct from the 13 pin car socket, to the car chassis , every thing worked fine.

 

[IanB1485]

A N112 fridge takes about 10A at 12V and does put cables and fuses and connectors under a lot of stress.
Doing a quick calculation, the difference in voltage = 13.98V - 8.97V =5.01 V at 10A equals 0.5 ohm resistance , so to find where this is occuring in the supply chain ,put the heater under load and measure the voltage drop at various points , often corroded or loose contacts , particularly at the fuses can be responsible. I have a set of probes with needle points so can pierce into wires along the various points to help determine where the voltage is being lost.

 

[newebr]

It sounds like the supply to the tow socket isn’t heavy enough, once under load all the juice is zapped.

You could try putting 12v directly to the socket at the fridge to see if the fridge then works as it should, if it does then it would be a supply issue

Thanks for the suggestion. Have tried a separate 12v feed, made no difference. Obviously more investigation is required. It's worked fine for 3.5 years, same car ( 2019 Sportage 2.0 diesel), so something has failed/changed somewhere.

 

[newebr]

A N112 fridge takes about 10A at 12V and does put cables and fuses and connectors under a lot of stress.
Doing a quick calculation, the difference in voltage = 13.98V - 8.97V =5.01 V at 10A equals 0.5 ohm resistance , so to find where this is occuring in the supply chain ,put the heater under load and measure the voltage drop at various points , often corroded or loose contacts , particularly at the fuses can be responsible. I have a set of probes with needle points so can pierce into wires along the various points to help determine where the voltage is being lost.

Thanks for the suggestions. I'll do some more investigating.