Anyone know how to test a run / start capacitor for a refrigerator compessor ? My fridge starts up about 50% of the time. The rest of the time it hums as the compressor tries to start, then kicks off the thermal fuse. When the thermal fuse kicks back in it tries again with about a 50 % sucess rate. Appliance repair forums indicate it could be the capacitor but I can't really understand how to test it. An ohm meter across the terminals causes a jump to infinity, then a sharp drop to zero. Reversing the leads produces the same result. I think it should be dropping slowly to zero, not imediately. Anyone familiar with this ?
I'd buy another & replace it but it's $ 60.00 to try & shotgun this repair. Thanks.

Wow... $60! Geez, I am quite surprised at that price. I replaced mine in an old home A/C unit and it was something like $10.

This may help for testing:
http://www.applianceaid.com/micro4.html

Charge it up.....place your tongue across the leads.

If you can still speak, it needs replacing.



<edit>..My smart-ass replies aside, I spent a few years fixing slush freezers, and the compresor setup is about the same as a fridge.

From what you are describing, I would swap out the capacitor with a known good one...and just for test purposes, just about any similr-sized cap can be used (hit your local used appliance shop and they will have a million laying around, or even borrow one off your chest freezer for testing).

Also check your start relay, a bad one can work "sometimes"...and to rule out the thermal overload, just remove it from the circuit.

But from the description, I would say the 2 most likely culprits are the cap, or the compressor itself.

[This message has been edited by MidEngineManiac (edited 06-17-2010).]

quote Originally posted by red88gt: An ohm meter across the terminals causes a jump to infinity, then a sharp drop to zero.

Using an analog ohmmeter on a healthy capacitor, you should see the opposite of what you reported ... a sharp initial drop to near zero, increasing resistance as the capacitor charges, and ending up at or near infinity.

There are three failure modes for capacitors: 1) open, 2) shorted, and 3) significant change in capacitance. Only case 2 can reliably be diagnosed with a simple ohmmeter; the resistance will drop to a low value and stay there.

This is one case where capacitor replacement is probably the most reliable diagnostic. That said, $60 seems awfully high for a small-appliance capacitor. You might want to shop around. It doesn't have to be an exact replacement; it has to be a "non-polarized" capacitor, and then all you need to match are the capacitance and working voltage rating.

If you call around you may find an electric motor repair shop that has a capacitor tester and can test it for you. If you do that, though, you should feel obligated to a new capacitor (if needed) from them rather than from someplace else.

[This message has been edited by Marvin McInnis (edited 06-17-2010).]

The wealth of knowlege on this forum never ceases to amaze me. Thanks guys but I was wrong its a run capacitor. I spoke with someone who seems very knowlegeable on these & he doubts very much the capacitor is bad. This compressor has a PTC relay (potential something something) that is part of the start circuit also. He pretty much indicated to me that his techs really don't have any high tech tools to diagnose these problems & shotgun it (that really makes me feel good paying $90 for a service call). He suggested I try a new relay / overload kit ($50) & if that didn't work the compressor startup windings were probally beginning to fail. They could be checked with a amp meter at run & start but even then unless the readings at start were extremely high we were shooting in the dark. With him it was mostly a knowlege of product life expectancy, modern compressors were good for 12 years give or take. MODERN compressors, 12 years ? Tell that to my 1953 Fridgeadare beer fridge in the garage ! I did find comparable capacitors at a local electic motor repair place for $ 11 so I might still try that.

Too bad you're so far away, I have the cap. tester you need right here. You have any "Mom & Pop" TV repair places local? They should be able to test it for you.

Mike

quote Originally posted by red88gt: The wealth of knowlege on this forum never ceases to amaze me. Thanks guys but I was wrong its a run capacitor. I spoke with someone who seems very knowlegeable on these & he doubts very much the capacitor is bad. This compressor has a PTC relay (potential something something) that is part of the start circuit also. He pretty much indicated to me that his techs really don't have any high tech tools to diagnose these problems & shotgun it (that really makes me feel good paying $90 for a service call). He suggested I try a new relay / overload kit ($50) & if that didn't work the compressor startup windings were probally beginning to fail. They could be checked with a amp meter at run & start but even then unless the readings at start were extremely high we were shooting in the dark. With him it was mostly a knowlege of product life expectancy, modern compressors were good for 12 years give or take. MODERN compressors, 12 years ? Tell that to my 1953 Fridgeadare beer fridge in the garage ! I did find comparable capacitors at a local electic motor repair place for $ 11 so I might still try that.

And when he says extremely high, he means it !!..We used clamp meters to test the draw on start, and I've seen properly-working 13A machines momentarily hit 85A on start-up. Its only for a split second, so not long enough to heat anything up, or pop a breaker--but they will hit that high in normal operation.

You may be able to visually check the caps for leaks.It sounds like the relay to me.Check where the relay plugs onto the compressor to make sure it isn't burned at the connections.Take one of the leads off the start cap.(that will be the one that has the biggest numbers (microfarads) and just let it touch the same capacitor terminal you pulled it from.Plug the unit in and if it hums pull the loose terminal off the start cap.It will spark and if it runs,it needs the relay,You have just done what the relay is supposed to do .If it still hums then have the caps checked Do all the tests after making sure the unit is unplugged and touching across the cap. terminals with a screwdriver.Good Luck !