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-   -   fridge on the fritz (http://www.houserepairtalk.com/f11/fridge-fritz-6640/)

calan 05-09-2009 10:26 PM

fridge on the fritz
 
I have a Frigidaire top freezer refrigerator, p/n 218972200.

A couple days ago, the freezer started thawing out, and I noticed the compressor was not starting. It would attempt to start (with a buzzing sound), and then the overload would trip. Just for giggles, I pulled the run capacitor and it started and ran...for a while. Then it tripped off again, and wouldn't start with or without the run cap...so I think that was just a fluke (it has since run and not run, both with or without the cap). If I leave the fridge unplugged for a couple of hours, it will usually run for a while and then start the same thing again. The fan is fine, no frost build up, etc. When it does run, everything appears normal; the comp output is cold, the return lines warm and temps are nice and cold.

Figuring the compressor is bad, I checked the terminals and they read about 9.0 ohms combined and appear good; with no shorts to ground.

I was thinking maybe the defrost thermostat or timer was bad, causing the heater to remain on and drawing too much current with the comp also trying to run. So I disconnected the evap heater (so that it couldn't get current even if the timer goes into defrost mode), and pulled the defrost thermostat as well (it showed open, which I believe is correct). I also pulled the timer and jumped the red and orange compressor wires at the connector to see if the compressor would run with nothing else in line with it (except the fan). Again, sometimes the compressor runs for a while, and sometimes it just tries to start and then trips the overload.

Adjusting the cold control does cause the fan to come on/shut off as it should, and advancing the timer also turns the fan (and compressor when it's running) on and off as expected.

So...I'm hoping it's a bad PTC relay. With the compressor terminals measuring the correct resistance, and the fact that the compressor will run... what are the odds that it really is the relay and not a bad compressor?

BTW - The PTC relay has long red/white wires connected directly to it that can't be removed, but the replacement I found appears to have spade terminals on it. I assume I can just cut the factory wires and splice a couple female terminals onto it for the new relay?

Thanks much...

glennjanie 05-10-2009 09:45 AM

Welcome Calan:
It may be the relay, it won't cost much to try it. It sounds more like a high pressure limit switch to me, but I doubt a refrigerator would have one. You might check the wiring diagram for high or low pressure switches and see what the relay is supposed to be doing.
You can usually find the folded up wiring diagram behind the front grille at the bottom and it should also have a 'ladder' layout.
Glenn

Nestor_Kelebay 05-10-2009 01:48 PM

Quote:

Originally Posted by calan (Post 30415)
It would attempt to start (with a buzzing sound), and then the overload would trip.

That's normally an indication that the motor doesn't have enough starting torque.

A capacitor start motor has two windings in it; a start winding and a run winding. There will be a capacitor in series with the start winding to provide a large phase difference between the current sine wave going to each winding. That results in the magnetic field of one winding developing sooner than the other. That difference in magnetic field timing between the two windings creates the illusion of a rotating magnetic field that provides the starting torque for the motor to get going.

Quote:

Just for giggles, I pulled the run capacitor and it started and ran...for a while.
If you have a "run" capacitor, that means you have a "capacitor start, capacitor run" motor. In this type of motor, you have both the start capacitor and run capacitor in parallel, and that pair will be in series with the start winding. However, BOTH windings will be energized when your motor is running. Only the start capacitor will be taken out of the circuit once the motor starts.

The run capacitor makes the motor run more efficiently, so leave it in.

Quote:

If I leave the fridge unplugged for a couple of hours, it will usually run for a while and then start the same thing again. The fan is fine, no frost build up, etc.
Probably because pressure equalizes across the compressor when the fridge is off, and that means it takes less starting torque to get the compressor running again.

Quote:

I was thinking maybe the defrost thermostat or timer was bad, causing the heater to remain on and drawing too much current with the comp also trying to run.
If that were the problem, you'd be tripping the circuit breaker to the fridge.

Quote:

and pulled the defrost thermostat as well (it showed open, which I believe is correct).
Check it when it's both cold and warm. The defrost thermostat is wired in series with the defrost heater. Once the defrost heater melts all the frost off the evaporator coils, continued heating could damage the foam parts in that area. So, the defrost thermostat breaks the circuit when the temperature gets too warm, thereby shutting off power to the defrost heater.
You should get continuity across it only when it's cold.

Quote:

Adjusting the cold control does cause the fan to come on/shut off as it should, and advancing the timer also turns the fan (and compressor when it's running) on and off as expected.
Right. The defrost timer simply alternates where power is going to. Most of the time it's going to the cold control, which in turn, sends power to the evaporator fan and compressor, or not, depending on whether the fridge is warm or cold. Consequently, when the cold control sends power on, both the evaporator fan and compressor go on together, and shut off together. For 20 minutes or so every 20 hours or so, the defrost timer diverts power to the defrost heater to melt the frost off the evaporator coils.

Quote:

So...I'm hoping it's a bad PTC relay. With the compressor terminals measuring the correct resistance, and the fact that the compressor will run... what are the odds that it really is the relay and not a bad compressor?
It really is the relay. Or, at least, from what you're saying, it sounds like the relay.

That "Positive Temperature Coefficient" relay is just a thingy that gets hot when current flows through it, and greatly increases it's electrical resistance when it gets hot.

The old "current" relays would be wired in series with the motor run winding. When the motor first started, the current through the run winding would also go through a coil in the start relay, and that energized coil would act as an electromagnet to complete a circuit through the capacitor and start winding. As the motor came up to speed, the current through it's run winding would subside, and so the coil in the relay would lose it's magnetism and would disconnect the start winding and capacitor.

You have a "capacitor start, capacitor run" motor, so the relay is only taking the start capacitor out of the circuit. (The motor runs with both windings energized.)

The problem is that PTC relays are problematic. They heat up every time the motor starts, and the heat gets to them so they stop working right. And, without the start capacitor in the circuit with the start winding when the motor is trying to start, it just won't have enough starting torque, and it'll just overheat and kick itself out on thermal protection just as your describing.

Here, read this:

relay ptc [Archive] - Refrigeration-Engineer.com forums

Quote:

BTW - The PTC relay has long red/white wires connected directly to it that can't be removed, but the replacement I found appears to have spade terminals on it. I assume I can just cut the factory wires and splice a couple female terminals onto it for the new relay?
That's what I'd do.

calan 05-10-2009 02:59 PM

Excellent info...thanks!

My only concern is that most people say that the PTC relay rarely fails on this fridge, but the compressor is a very common failure point.

Maybe I could beat the odds for once this time :)

Nestor_Kelebay 05-10-2009 04:22 PM

Quote:

Originally Posted by calan (Post 30427)
My only concern is that most people say that the PTC relay rarely fails on this fridge, but the compressor is a very common failure point.

Everything you're describing suggests to me that the compressor itself is OK, it's just that the start capacitor isn't in there when the motor is trying to start. And, it's that PTC relay that cuts the start capacitor out.

I would replace the PTC.

calan 05-11-2009 02:49 PM

PTC relay is on the way...I've got my fingers crossed that this will be a $30 repair vs. a $500 one. :)

Nestor_Kelebay 05-11-2009 09:23 PM

In the web page I linked to, they said you could check the PTC relay by connecting a light bulb to it to see if it flashed on an off.

Quote:

So, how would you test a PTC? Connect a 100W light bulb in series with it (mains -> PTC -> light bulb -> mains return). The bulb should flash once when power is applied.
Just to be sure, maybe try checking the old PTC as described.

Also, have you / can you remove and check the start capacitor on your fridge? A bad start capacitor that only works intermittently would cause the same problems as a PTC that only worked intermittently. The electronics instructor at any local trade school should have a good quality digital multimeter than can check capacitors.

Finally, check the terminals on your compressor. The resistance across two pair of terminals should equal the resistance across the third pair. With three terminals, there will be three pair of terminals (1-2), (2-3) and (3-1). If there are no shorts in the windings, then the resistance across two pairs of terminals will add up to the resistance across the third pair.

I don't want you to lose the fridge cuz you didn't gamble $30 on a new PTC. But, I don't want that $30 to be wasted on a part you don't need either. It DOES seem to me that the problem is with the start capacitor not being in the circuit when it needs to be for the motor to start, tho. Still, I think we need to make these other checks before ordering the new PTC relay.

Would it be a problem just to phone the parts place and ask them to just hold off while you do these additional checks?


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