Fridge freezing on lowest setting

[PhilipHa]

No, I ended up 'burrowing into the works to get to that difficult connector block' because I didn't realise until I had split the connector block that I couldn't manage to remove the bullet ends of the old thermostat from the block. However, yes you are right if you decided in advance to not try to split the connector block you could have just cut the old thermostat wiring in half, splice in the new thermostat to the connector block end of the old thermostat wiring without having to burrow and split the connector block, and it should still work.

Sorry no pictures of the new connection, as I cut the wiring close to the connector block in anticipation of removing the bullet ends from the connector block and so the Wago connectors are now buried near the connector block at the back of the fridge which I had to do to reconnect the block. I have now put everything back together, so would have to split the connector block back together to provide you with a photo which as per the comments above is a little fiddly!

 

[1508ian]

Understood! I'm thinking that cutting the old wiring nearer to the thermostat end and bringing the connection to the front would make it easier to replace it future if it is likely to fail again in a couple of years...

 

[Rob & Sandra]

There is also an informative YouTube clip on how to replace the sensor. It’s in German but you get the gist on how it can be done. (Also a few other useful clips from this guy.

 

[knoxwells]

As per title, have searched through the forums and sure I saw a post about this but cant seem to find it.

When on setting 1 the temperature is all but freezing, this hasnt always been the case and was wondering if anyone has had similar issue and been able to reset it somehow?

Thanks.

We had exactly the same problem. We bought a replacement sensor from the Mercedes dealer (£73) but since noticed you can by them slightly cheaper online for a Westfalia fridge.
To change the sensor, we followed a YouTube video which is in German, but you get the gist.

You have to remove the center draw to access the screws that hold the panel on in front of the fridge, move the rear seats forward to then remove the panel.
Disconnecting the electrical connector is tricky as it is underneath at the back.
Once disconnected, you have to remove the pins in connector with a tubular removal tool (£5 from Amazon).
You can then remove the sensor head from inside the fridge and thread the sensor out.
Thread the new sensor back in and put back together, we used the same mastic to seal the sensor back in place.
Fridge now works fine.

 

[Rob & Sandra]

Understood! I'm thinking that cutting the old wiring nearer to the thermostat end and bringing the connection to the front would make it easier to replace it future if it is likely to fail again in a couple of years...

Yep. My thoughts too. A couple of small WAGO in line connectors.

 

[viseng]

Have just replaced the sensor having had the same issue that many owners report - that the fridge freezes even on setting 1. As a retired electronics engineer, this puzzled me, as a simple NTC sensor should not fail this frequently as the environment in which it is installed is quite benign; only a manufacturing or design fault would explain it. From this point it all becomes somewhat geeky, so only proceed if you are prepared to tolerate some very technical mumbo-jumbo.

I disassembled the failed sensor head to investigate. As new, the sensor used will measure approximately 450 to 500ohms at room temperature, increasing to over 3000 ohms at freezer temperatures. My failed device was less than half the expected resistance, yet still operated to a point, which confirms to me that some form of gradual deterioration has occurred, typically from loss of integrity of the encapsulation around the element and/or chemical attack from solder flux used during soldering..

The device is embedded in a sealant within the plastic tube, so from a basic construction aspect, it cannot be faulted, and moisture ingress can be dismissed. However, I was horrified to discover that the sensor bead itself was within a mm or two of the solder joints connecting the extremely fine wires from the device to the substantial wires of the connecting cable (mains lead). This should never happen. The sensor technology is naturally highly sensitive to temperature, and will hit a limit around 240 deg C beyond which damage can occur. Fine, long wires are used to support thermal isolation or "thermal shunting" using a clip-on tool to sink the excess heat from the soldering operation, typically 360C for soldering onto wires. If this is not done, the bead can reach over 300C when its so close to the soldering iron tip. In addition, its almost impossible to clean off acid residue when the wires are so short, opening up the possibility of penetration into the device encapsulation. You can draw your own conclusion.

Should this happen again, I will be making my own for substantially less!

 

[Rob & Sandra]

We too have been through the hoops.. may background is also in Electronics and control engineering so thanks good to see the resistance range. Be good to see plot verses temperature. (As you say it’s NTC rather than simple RTD)

I tried a simple ‘generic’ sensor from eBay theoretically applicable to Dometic fridge at £6 but no success.

As the fridge is a DOMETIC unit and the fact that Mercedes had zero idea about the sensor. I spoke with Dometic UK and sent photos of unit ID plate and subsequently bought the part directly from them. They wouldn’t confirm what the sensor was or any standard as I’m sure they want to protect IP. The connections are accessible so I’ll do some plots of resistance..

As the unit had failed to an ON/OFF mode I bought a simple Bluetooth tensor placed in fridge to keep a check and just monitored on phone turning on and off to try to maintain a cool fridge.

With the new one fitted and seemingly working OK and I can plot temperatures against the 1- MAX settings on the control module (ours is older separate 2018 unit) and see how quality on control I’d suggest it’s just a simple on/off control replay driving the compressor..

We are away this coming week so will do a few checks.

 

[Rob & Sandra]

We too have been through the hoops.. may background is also in Electronics and control engineering so thanks good to see the resistance range. Be good to see plot verses temperature. (As you say it’s NTC rather than simple RTD)

I tried a simple ‘generic’ sensor from eBay theoretically applicable to Dometic fridge at £6 but no success.

As the fridge is a DOMETIC unit and the fact that Mercedes had zero idea about the sensor. I spoke with Dometic UK and sent photos of unit ID plate and subsequently bought the part directly from them. They wouldn’t confirm what the sensor was or any standard as I’m sure they want to protect IP. The connections are accessible so I’ll do some plots of resistance..

As the unit had failed to an ON/OFF mode I bought a simple Bluetooth tensor placed in fridge to keep a check and just monitored on phone turning on and off to try to maintain a cool fridge.

With the new one fitted and seemingly working OK and I can plot temperatures against the 1- MAX settings on the control module (ours is older separate 2018 unit) and see how quality on control I’d suggest it’s just a simple on/off control replay driving the compressor..

We are away this coming week so will do a few checks.

View attachment 4024

 

[Rob & Sandra]

Quick update. Today from lunch time.
Lower colder trace shows fridge temp set to 3. Higher values set to 2.

 

[Ian M]

You need a new temp sensor which can be obtained from Campervantastic

It is a fiddly job but just need to be Careful, you put the connecting wire through the before connecting anything - also there is a good video on Youtube on the how to