Fridge thermostat repair

[Kit Jackson]

I've got a Fisher and Paykel fridge freezer model E240B but the thermostat isn't working. The motor runs continuously and eventually even the fridge compartment freezes up. I've taken the thermostat to pieces as much as I can. The adjustment knob connects to a 1K potentiometer that is working fine. Tucked behind the cooling plate are 3 sensors of some sort. One black one about 20 mm long. Is this the thermocouple? The other 2 are discs about 10 mm diameter. I thought at the start that they might be magnetic designed to stick to the back of the cooling plate but they aren't magnetic. Has anybody any idea what might be wrong. How can I test these components? What are they?

Photos here:-

Thanks.

[Thomas Prufer]

Yes, the sensor, but NTC or so, and not a thermocopuple.

3M gel connectors: all the do is connect and seal out water.

The other bits look like the connectors to the light bulb.

The interesting parts are elsewhere... probably in back.

Thomas Prufer

[Kit Jackson]

Right, so this is a thermistor and the internet tells me this is likely to have a resistance of between 100 and 10K Ohms in the temperature range we are interested in, matching well enough the 1K pot in a voltage balancing circuit. I'd guess this is very unlikely to fail then, but if it does fail it will either be a short circuit or an open circuit. If I can get some probes at it I'll try to measure its resistance but it looks like it has been potted up.

This looks like I'm barking up the wrong tree then. Lets forget them.

I'll have a look round the back near the compressor. Any clues what I'm looking for? Is this likely to be a diy job?

Thanks for your hep so far. Kit Jackson

[tabbypurr]

I wuoldn't say that a thermistor was unlikely to fail, nor that it would surely go sc or oc.

A temp sensor coming adrift from the evaporator can cause the problem, as can a faulty sensor. I'm a lot more familiar with ye olde frosting fridges & freezers, and don't know whether your stat should be attached to the evap or not.

NT

[Brian Gaff]

Yes if you can ascertain what the changes in the signal are from the device, and measure it, then you know the pot is OK my guess is that the actual electronics is faulty, for whatever reason, It could even be a connection to and from the control or sensor, but you need to find the board and see if its something you can replace. Sods law will dictate its in the most inaccessible place possible. Brian

[Brian Gaff]

Is this a frost free design or your basic jobby. Either way though I'd imagine either is at least one relay controlling the compressor on the pcb. Brian

[Kit Jackson]

This isn't looking too good. The control unit contains a circuit board. One of the connectors on the cover is marked "T SENS". There's 4 wires on the connector, presumably from the fridge and freezer parts. One pair gives a resistance of 2.85K Ohms and the other 2.95K Ohms. The ambient temperature is 20C. Is that about what might be expected? I'll put it all back together and turn the fridge on again and re-measure later to see if the values change.

There's a photo of the circuit board here but I don't fancy having to replace components on that. Is there anything I can usefully measure on the board?

Photo:

Kit Jackson

[Kit Jackson]

Follow up :-

The thermistor now reads 3.4K Ohms at 10C and the other thermistor for the freezer reads 6K Ohms. So I guess the good news is that the thermistors are OK. But I guess the bad news is that the problem is on the circuit board. Is it un-repairable?

Kit Jackson

[Brian Reay]

You said it is a fridge/freezer I believe.

At least some of these have a single compressor which is controlled by the freezer temp. The fridge as an after thought, sometimes by (for example) a flap controlling air flow from the freezer. We had one like this and the flap got stuck nearly closed. A wiggle and a clean and all was fixed.

It could be as simple as 'gunge' blocking a flap or air flow.

[tabbypurr]

You don't know whether the thermistors are ok until you know what resistance they should be. Bridging them with a resistor could at least tell you if the pcb is responding to temp changes.

NT

[Dave W]

You might be able to unsolder the two relays to test them with external power.

[The Natural Philosopher]

yes.

Probably not.