Anyone know about fridge thermostats?

[waddycall]

We have a ten year old hotpoint RLFM151 it’s been running really cold lately. I changed the thermostat a while back but it’s still doing the same thing. It runs continuously and gets down to 0 degrees but doesn’t shut off. I’ve ordered another new thermostat which arrived today.
I wanted to test the new thermostat before fitting it so I dunked the capillary in iced water which was down to 4 degrees expecting the thermostat to click but it didn’t at any thermostat setting. I checked continuity with a meter.
I’ve looked up the danfoss part number and it has a warm cut in temperature of 2.5C and a warm cut out around -5C
The part number on the thermostat is the right one for the fridge.
I’d assumed the thermostat sensed the internal fridge temperature but am I wrong? Does it instead sense the suction gas temp coming out of the evaporator or the temperature at some point within the evaporator?
I’m wondering if the fridge is low on gas or has another issue reducing mass flow e.g. high condensing temp but the condenser is clean and I’ve pulled the fridge out from the wall.
It’s probably time for a new one but I’m not giving up without a fight. Seems a shame to have to manufacture a whole new fridge for what is probably a nothing issue.

 

[waddycall]

Thinking this through it makes sense for the thermostat to not sense the internal temp of the fridge as that would make it short cycle every time the doors opened. Maybe the compressors on the way out

 

[jsf55]

My neighbours did that as i heard it through the wall in one of our quiter rooms, it would momenterily cut out and start back up and run for an age, they didn't know when i said about it, but said everything was freezing up in there, no idea of the fault tho !

 

[voipio]

Many fridge freezers, especially older, or simpler models, have their thermostat sensor-bulb / capillary-end positioned either directly onto, or very close to, the evaporator (the bit that gets cold). Could there originally have been a mounting clip that has broken? In normal use, the sensor reaches sub-zero temperatures whilst the compressor is operating, so a warm cut out temperature of -5C would seem quite reasonable. The sensor needs to be bathed in the cold air coming off the evaporator. Anything that prevents that, could cause the operation that you're seeing.

Does the evaporator plate become iced up in use, when the compressor is running, and does that ice melt before the start of the next compressor cycle? It's worth checking that the door seals are making good contact all the way round, when the door is closed. Are there any crumbs, or other food contamination of the seal surfaces, both on the door and on the body of the fridge. Check for splits in the door seal material. Also, check the condenser on the outside at the back of the fridge. Does it run warm, when the compressor is running? If so, that's a good sign. If the condenser is covered in fluff, or is too close to the wall, that could have quite an impact on the fridge performance. Reading the original post, you've already checked that.

 

[waddycall]

Thanks Voipio, door seals seem good - checked with paper and tough to open the door if it’s been closed recently. The back of the fridge gets iced up and stays that way. There are no compressor cycles I’m aware of - it’s just on.
The thermostat is fitted from the front with the capillary fed down a tube kind of like doing an endoscopy. I had wondered if the capillary was taking a wrong turn somewhere and had wanted to get a closer look but it seems that access to the evaporator would pretty much wreck the fridge.

 

[waddycall]

Found a bit more info from Danfoss which confirms that the thermostat should contact the evaporator. Seems there’s no way into the evaporator without destroying the foam insulation that fills the void between the inner and outer skins of the fridge. I’ll turn it off and let all the ice melt, fit the new thermostat and hope for the best. If that doesn’t work it’s new fridge time.

 

[knighty]

You could fit a different type of thermostat?

Pickup a digital on on ebay for £15 ish... it'll let you dial in exactly what cut in/out temps you want

Just put the sensor inside the fridge.

It'll cut in every time you open the door but that's not a big deal?

 

[not done it yet]

If the temperature is far lower than expected, it is doubtful that the problem lies with the compressor. Usual failure is it stops or runs continuously because of a lack of refrigerant gas.

 

[waddycall]

So second new thermostat fitted and wiring checked. Door taped shut all the way round with duct tape. Compressors been running for 2 hours since I taped the door shut. The temp in the fridge at that point was 0 degrees. Condenser is warm. I reckon the deferential between the fridge temp and the evaporator temp isn’t high enough for the thermostat to cut out. Or the tube the capillary is fed down has moved and the capillary isn’t positioned correctly.

 

[waddycall]

Might borrow a long thermocouple and thermometer from work tomorrow and see how cold it is down the end of the tube