Bad Capacitor maybe in Central Air Conditioner ?


My a/c appears to be dying. When the thermostat turns on, the system does not always crank up immediately - sometimes not at all.. Last night it was dead so I left the breakers off overnight and when I went to turn them on this morning the system worked fine and brought the temperature down to the thermostat setting. Then once it turned off and the temperature started to rise it would not turn on again.
I replaced the thermostat batteries, and even replaced the thermostat itself.....no luck. Perhaps the capacitor is going bad...what do you think? The inside unit is 3 years old and the outside unit is somewhat older. Right now we are strapped for cash to get a pro come and take a look so I am hoping I could do something myself. Any ideas ?- could it be the capacitor ?....how can I test it.? I have a multimeter and some electronics knowledge. Thanks
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My a/c appears to be dying. When the thermostat turns on, the system does not always crank up immediately - sometimes not at all.. Last night it was dead so I left the breakers off overnight and when I went to turn them on this morning the system worked fine and brought the temperature down to the thermostat setting. Then once it turned off and the temperature started to rise it would not turn on again.
I replaced the thermostat batteries, and even replaced the thermostat itself.....no luck. Perhaps the capacitor is going bad...what do you think? The inside unit is 3 years old and the outside unit is somewhat older. Right now we are strapped for cash to get a pro come and take a look so I am hoping I could do something myself. Any ideas ?- could it be the capacitor ?....how can I test it.? I have a multimeter and some electronics knowledge. Thanks
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I think that capacitors don't get flaky, they just die completely.
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snipped-for-privacy@gmail.com wrote:

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No, while they can fail abruptly, the electrolytic types used for starting and running AC compressors and fans definitely degrade over time to a lower value, and this is a normal process. For that reason they are spec'ed a little larger than is necessary. I have more than once observed a 5 uF fan run capacitor test out to only 2 or 3 uF.
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responding to http://www.homeownershub.com/maintenance/Bad-Capacitor-maybe-in-Central-Air-Conditioner-153686-.htm centralairunits wrote:
snipped-for-privacy@gmail.com wrote:

I have the same analysis!
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royalmp2001 wrote:

Does the breaker servicing the system pop when it doesn't start?
If it doesn't it's probably not the capacitor.
If it does, it MIGHT be, but it could also be a compressor on its way to the grave.
It'difficult to properly test a capacitor with just a multimeter.
Substituting a new ocapacitor with the same ratings makes more sense. They don't cost that much.
HTH,
Jeff
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royalmp2001 wrote:

Suspecting starting cap on the motor, how come are you bugging with the thermostat? Either starting cap or cetrifugal contacts within the motor is suspected.
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wrote:

It's not dying. It just needs a repair.

I think you should stop guessing at what part needs repair and learn to do some testing.
Turn off the breaker to the compressor (not the one to the furnace). It's probably on the wall of the house next to the compressor, and instead of a breaker it might just be a piece of black plastic that you pull out altogether. Keep track of which side goes up.
Then measure the voltage between the two small wires that come into the compressor unit. When the thermostat calls for coooling, you should show 24 volts there. If you do, the thermostat is fine, if not there is some problem in the thermostat circuit (but not necessarily the thermostat.
Then measure the voltage on the coil of the contactor. The contactor has two big wires at the top coming straight from that breaker box on the wall of the house, and two big wires at the bottom going to the compressor, fan, capacitor, etc.
Then it has one connection on the left (in the middle top to bottom) and one on the right. These are connected to the coil (contactor is another word for relay, and when cooling is called for, the coil gets energized by the 24 volts and it connects the two heavy wires at the top to the two heavy wires at the bottom.
The voltage between the left and right, half way up, across the coil should be 24 volts also.
Do all this with the circuit breaker for the 240 volts open, so that there is no 240 at the AC compressor.
If there is 24 volts there, did you hear the contactor click shut? Get someeone to go into the house and turn the AC off. You should hear it click open. have him wait 20 seconds and turn the AC on. You hsould hear it click shut again.
If you don't have 24 volts, you have to find out why you had it where the small wire(s) came into the cabinet, but don't have it hear.
My last time it was the high pressure switch. It must have had high pressure and the safety switch opened. This one has a red button to push and that fixed it. It could also be the low pressure switch (which resets automatically in the AC I worked on last.) or a couple other switches also. (The wiring diagram glued inside the AC was inadequate. I had to draw my own, by tracing the wires visually.
Once you are sure you get 24 volts at the contactor, you could reconnect the 240 (making sure not to touch anything) and just watch what happens when someone inside turns the cooling on. Can you hear the contactor vibrating a bit?
If you have the 24 at the contactor and you hear it click closed, then the problem is the other half of the contactor, or the compressor or the fan (You didn't even say if you could hear or see the fan or compressor running.)

I'm more concerned about safety knowledge and habits, because 240 can kill you. Even 24 volts can be felt when one is sitting on damp grass or driveway, but I don't think it can hurt you. But don't touch it, except with the meter probes. Not you.

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mm wrote:

That is 24 Volt AC, right?
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Yeah, I had "AC" in mind, to include in the post, but forgot. Thanks.
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mm wrote:

Thanks for the vital clue. There was no 24V present at the compressor because the signal goes through a cutoff switch in the condensate drain line indoors to prevent flooding. Guess what...it was mostly clogged up so the switch would only turn on every few hours when the later level had drained off. You saved me a repair bill...THANKS. And thanks to everyone else who replied...and please pardon my ignorance.
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royalmp2001 posted for all of us...

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Tekkie "There\'s no such thing as a tool I don\'t need."

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I experienced this myself on my old AC. When the compressor is old, it needs a start capacitor to get it going. It gets worse over time. Typically new systems do not require a big start capacitor, some don't need them at all. It starts when it's cool because it has less friction when it's cool, but once it gets hot, it can't get cranked up, until it's cool again.
My suggestion is to remove the capacitor and replace with a bigger one. If it still won't start, you can run a hose over the compressor to cool it down so it will start up again. This will only keep it going for the rest of the summer. Then, you'll need to replace the unit. You'll save on efficiency and your electric bill will be reduced.
good luck
royalmp2001 wrote:

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