cross-posted from: https://slrpnk.net/post/26897688
Either the relay is broken or the thermostat. I know this because I could force the fridge to run by hotwiring 220V across the common and run pins then briefly jumping the start pin to the run pin (thus sending 220V through the start winding for a split second).
The thermostat assembly has too many wires to know what to measure resistance on. But I figure that the thermostat can be black-box tested simply by checking whether it sends 220v to the relay switch. There are THREE wires involved (in addition to ground). With the relay removed (thus compressor disconnected), I plugged the fridge into mains. Got these measurements:
common pin to run: 220v
common pin to start pin: 20vI’m tempted to conclude that the start winding /can/ accept 220v (proven by hotwiring), but 20v is also sufficient to cause enough motor movement to get it started. Does that sound reasonable? Can I conclude that the thermostat works and the relay is toast?
It’s hard to be confident without a service manual, particularly when the relay¹ has continuity. The common pin goes through a cylinder marked “Klixon MRP342LZ-32Q1”, which as 0.5 Ω. The start and run pins go through what seems to be a ceramic disc having 45 Ω. Is that reasonable?
I am ½ tempted to attach the relay, bypass the thermostat, and wire the relay to mains with start-run jumped persistently. Is that crazy? I assume the relay would internally disconnect the start winding. The fridge has no fans AFAICT, so no need for anything else to have power in this test.
¹ The relay I have has part № “A 3B7 8100” printed on it, which I cannot find online. But it looks exactly like this one.
Update
I hot-wired the relay. It was non-trivial because the relay has /3/ inputs. After studying it closely, I realised that one of the inputs (apparently for the start winding) is phony. The spade connector that attaches to it ultimately goes nowhere. When connecting the remaining 2 pins to load and neutral for the house mains, the fridge started and ran just fine.
Thus I have to conclude that the thermostat went bad. And replacements are not sold. However, Amazon has what’s claimed to be a universal fridge thermostat. It’s cheap, but I boycott Amazon. I think I will wait until someone throws out their fridge. Perhaps I can just take any thermostat and make it work.
Those PTC start relays die all the time. These are rated a 200k starts, approx 11 years at 2 starts an hour.
https://www.sensata.com/sites/default/files/a/sensata-8ea-series-ptc-motor-starters-datasheet.pdf
A proper application datasheet- everything you could ever want to know about them
Is there a step-down transformer anywhere? That relay isn’t big enough to have such a transformer or switching power supply (I don’t think).
It would be strange (to me) to build everything for 220, including the t-stat, but not the start winding. That would then require either a transformer or switching power supply for that one thing.
The thermostat should be easy enough to test - you know where the supply is, and which wires energize this relay. Test voltage at the relay when the thermostat closes.
If something is dropping 220 to 20v, surely it’s got to be heating up?
Edit: That start relay is designed to control current, not voltage. It initially allows full current and then drops the current available to the start winding. It seems more likely this relay has failed than the t-stat, as the t-stat also controls the defrost cycle and any fans.
Is there a step-down transformer anywhere?
I doubt it… unless that’s something we would expect to find in a fridge? I see no big coils. There are also no fans on this fridge. No start capacitor either. The YT videos I saw do not correspond well with this. There is no freezer→fridge damper either. There are coils in both.
The thermostat should be easy enough to test - you know where the supply is, and which wires energize this relay. Test voltage at the relay when the thermostat closes.
I cannot easily remove the thermostat… hard-wired in, otherwise I would try putting it in a working freezer then check that it closes. But OTOH that test is perhaps not useful since the problem is not continuous running. The problem is not running at all.
I can’t access that wiringwork page, but I would expect current and voltage to both be zero when the thermostat is happy. In any case, I appreciate the feedback.
I wouldn’t expect a transformer anywhere, I was just shooting in the dark. That would make no sense to me, but I’ve seen crazy stuff in appliances, and I certainly don’t know it all…
A better test may be to forcibly energize the relay so it closes. If it closes and the motor starts, the problem is in the thermostat. If it closes but the motor doesn’t start, the relay is the problem.
A better test may be to forcibly energize the relay so it closes.
By this, I assume you mean to hotwire the relay. How do I do that?
I was actually planning on that. But then when I saw that the thermostat was sending 20V to what seems to be ultimately the start winding pin, it triggered me to first start this thread.
I am confused because I would expect there to just be two wires going to the relay. I believe it’s the relay’s job to break that into 3 pins. The relay has a connection that clearly goes to the common pin, which is the load. There is also a connection marked “N” for neutral, which goes to the run pin. The voltage across those two pins (coming from the thermostat) is 230v. So far, that’s all expected.
But then there is a 3rd wire from the thermostat going to (what I think is) a line that ultimately leads to the start pin. There is 20V across that and the load. So how do I hotwire that? I would obviously connect the load to and the neutral wires to their respective inputs, but I don’t know if it’s safe to jump the neutral to that 3rd input (which I think is the start pin). There is like ~54 ohms between the start and run inputs on the PTC relay.
I suppose it would be safe to just connect 2 wires to the PTC relay and connect nothing to the start input, just to see what happens. But it expects /something/ on the 3rd connector so I don’t suppose it will start the fridge.