The freezer was being used for a laboratory experiment a couple years ago and would easily cool down to about -24 C and sit at that temperature with little variation until deactivated. Recently though the freezer has begun to fluctuate temperature, going through warming and cooling cycles once every 1.5 hrs. When set at the coldest setting the freezer cycles between -31 and -19 C(although the manufacturer has told me that the fridge is only rated for -23.3 C).
I’m really not sure why this behaviour is happening and would welcome any suggestions or thoughts about what could be wrong. If you need any more information just let me know and I’ll post it for you ASAP.
First I would check that the condenser coils and any venting are clean.
Next if the unit has a condenser/compressor fan check that it is running when the compressor is on and that it’s blade is clean.
If when the temperature variation happens, the compressor seems to be cycling normally other than being off for longer than it should be causing the temperature to rise then I would replace the thermostat.
It is not uncommon for these to fail in this way (stick open).
One way to partly confirm this is if you can catch it when the temperature is rising and the compressor is off. Then move the thermostat up/down a bit (not to off). If the compressor fires up then the odds are very high that the thermostat is the culprit.
The back of the freezer is completely sealed so there are no condenser coils to clean and only the compressor is accessible. It seems to be running normally while the temperature is dropping but then reaches a certain point and shuts off.
I tried adjusting the thermostat when the temperature was rising (I moved it one setting colder) and instantly the compressor kicked on. I’ll try it again on the next cycle (one setting warmer) and see if it does the same thing.
When I had the thermostat/controller out of the fridge yesterday there was something rattling inside when I shook it. I’m not sure if this means anything as I’m not sure how they work mechanically.
Before I ordered the new controller I was looking at the old one and noticed what looked like a set screw underneath the temperature setting dial. Would this have any effect, what are these designed to do here?
Usually they are a calibration screw.
Especially if painted red.
There is really no way to calibrate it reliable so is basically a factory adjust.
In any case it would not fix a fluctuating (intermittent) thermostat.
Replaced the thermostat with the new one and it didn’t end up having any effect on the behaviour. I had to call Marvel’s recommended service people to come. Turns out that I have an “inefficient compressor” that’s drawing too much current and working too hard so it shuts off. Then it takes a long time before it can turn back on again and the fridge warms up.
Any idea what a new compressor should cost to order / install?
Sorry to hear it was not the thermostat.
Sounds like it was popping an internal thermal protect or it has a different overload as a normal overload (clixon) usually takes 5 minutes or less to reset.
Well here I am again, a couple weeks down the line with a new compressor installed in the freezer. However, the problem continues to persist, the freezer cools down below what it’s rate for, shuts off, and warms back up until the compressor kicks back on.
I thought that maybe I didn’t understand the behaviour of the freezer correctly but looking back at old data from experiments it was used for the temperature used to stay perfectly constant (within a half degree at most). The freezer has been moved since those experiments were done, would that have any effect on performance? I can’t think of any other real changes. Any more thoughts or suggestions would be great. I’m not sure the recommended repair company from Marvel knows the freezer that well.
I am not replying to your new post as I am out of ideas so with luck someone else will reply to it with a fresh perspective.
Often when other regulars see a post has been replied to they will not read it especially if the forum is busy.
I will keep my fingers crossed that this unit will be fixed.
But not being able to find any tech info on it makes it difficult to troubleshoot especially since you have already replaced the thermostat and the compressor.
Either it has some part that is not usually in a freezer or I missed something simple. I hope it is missed something simple.