Diagnosing home AC unit

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Mar 2, 2004
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2,962
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Kentucky
Came home today to a blazing hot house and indeed have no cool air coming from the vents. Not sure how the wife didn't notice, she's been home all day. Looking for some help to troubleshoot this.

Blower works fine, turning on/off with thermostat. Outside the house at the unit, I've tested the contactor, it clicks on and off in response to the thermostat input, and I get good continuity across the terminals when the contactor is engaged.

The condenser fan is working fine, no issues there. When the system is powered on (and thermostat set to cool) there is voltage to the compressor, but I don't get any indication that it's running-- no noise from it whatsoever. But the compressor itself is warm (almost hot) so seems to be receiving power.

I have a single capacitor on the unit, it has three terminals, yellow brown and blue wires IIRC. I'm not sure if it is a start capacitor for the compressor, condenser fan, or both. Not really sure how to test the capacitor.

Refrigerant has good pressure.

Any ideas where to go from here?
 
Joined
Mar 2, 2013
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The Midwest
How do you know the refrigerant has good pressure? I'm curious if you did a quick tap on a fitting to determine that. But test the caps.
 
Joined
Jun 4, 2005
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Location
North Florida
Came home today to a blazing hot house and indeed have no cool air coming from the vents. Not sure how the wife didn't notice, she's been home all day. Looking for some help to troubleshoot this.

Blower works fine, turning on/off with thermostat. Outside the house at the unit, I've tested the contactor, it clicks on and off in response to the thermostat input, and I get good continuity across the terminals when the contactor is engaged.

The condenser fan is working fine, no issues there. When the system is powered on (and thermostat set to cool) there is voltage to the compressor, but I don't get any indication that it's running-- no noise from it whatsoever. But the compressor itself is warm (almost hot) so seems to be receiving power.

I have a single capacitor on the unit, it has three terminals, yellow brown and blue wires IIRC. I'm not sure if it is a start capacitor for the compressor, condenser fan, or both. Not really sure how to test the capacitor.

Refrigerant has good pressure.

Any ideas where to go from here?
The last time I had those symptoms it was the capacitor but I don't know how to test it. The A/C tech diagnosed mine. The part was less than a $100.
 
Joined
Dec 26, 2006
Messages
649
Location
Phladelphia
Just went through testing the capacitor when my blower fan went out last week. You need a multi-meter that has a uF reading. looking on home depot, looks like the Klein MM600 has that ability to go up to 1000uf which should be good.

Youtube also has a bunch of videos, but not sure what multimeter you have access to.

edit: capacitor was super cheap to replace (like $10) even though it was my fan that was broken.

another tip is to take pictures before you disconnect anything so you know how it was before.
 

JC1

Joined
Nov 29, 2008
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6,431
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Oshawa, Ontario Canada
The capacitor on my previous AC unit went out. You can take a long screw driver and stick it between the top protective grate and use it to push the fan blades on the outside to get the AC started. Try that first.
 
Joined
Sep 1, 2015
Messages
566
Location
TX
If the compressor was hot, it has an internal overload that will keep it from running.
You can check with a ohm meter to see.

FYI, you can do a very inaccurate check of a capacitor with an ohm meter also.

On a 3 pole capacitor, ohm from the H (compressor) terminal to the C (common) terminal. F (fan) is the other terminal
You should be able to see the ohms drop. Reverse your leads and the ohms should jump up and bleed back down.

Capacitors in the past were rarely the problem
Failures are very common now.

You can run cool water on the compressor to get the internal overload to reset quicker.... or throw a bag of ice on top.

My guess from here, either you had a power failure that caused the compressor to heat up or its the capacitor.

Being low on Freon can cause one to heat up also.... but you probably would have noticed it not working well before now.
 
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92saturnsl2

Thread starter
Joined
Mar 2, 2004
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Kentucky
I'm pretty sure I've narrowed it down to the capacitor. I have a meter that reads capacitance, though given the price of it, probably not the most accurate in the world. It seems to indicate that the compressor (H) side of the capacitor is a completely open circuit. It reads 2.5 uF on the fan side (the value should be 5 uF, but the fan IS working), but I get nothing on the compressor (H to common terminal). If I change the meter to resistance, I get an open circuit from H to C, whereas with the meter connected to the fan side, resistance jumps up and down.

Should I be worried about the compressor overheating or burning up due to it being on all day without a working capacitor? The compressor was plenty warm when I went out there poking around, but it wasn't scorching hot or anything.

Anyone know if these capacitors are available at Home Depot / Lowes / Menards, anything open later in the evening? My Googling doesn't turn anything up. I suspect I'll have to wait for an HVAC supply store to open up in the morning, but maybe someone knows something I don't. The capacitor I need is a 45/5uF 440V, which seems to be plenty common.

This AC unit is only about 2 years old, strange that this part died so soon.
 
Joined
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Ohio
Being two years old, it should be under parts and labor warranty from your dealer. I would go that route. Just don't tell them you were in the unit.
If you prefer to DIY it,
I would change the capacitor AND contactor. I just replaced mine as a preventive measure. Everything still worked and I wanted it to stay that way. My contactor contacts were badly burned after 20 years and I last changed the capacitor ten years ago. Both items should run you about thirty dollars. You can easily find the items on ebay.
 
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Joined
Aug 5, 2002
Messages
18,853
Location
Silicon Valley
If I were you and have a "low end" meter that can test capacitor (despite not "precise") is to take the capacitor out, test it not on the motor, and compare it to a potential replacement (part house, or wherever you can get for your AC model, and see if they are close and within spec.

If it is hot on the motor then it can be either the motor or the cap, how much is the cap?
 
Joined
Mar 30, 2014
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NJ
Right place, cap is $15 or less. Get 2.
Yes, very weird. I paid like $60 for a cap and then happened into a parts store more targeted towards professionals and saw them for $15. I'm stocked up. LOL. I've gone through several of them. First time the HVAC pro diagnosed it and used a variable capacitor since they don't have to carry a bunch of different ones on the truck. Second time the cap blew up. Wasn't much doubt what was wrong. I was ready.
 
Joined
Aug 5, 2002
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18,853
Location
Silicon Valley
After shutting off the breaker.
Breaker doesn't do much when the energy is stored as charges inside the capacitor (it is like a big battery), if you have a large enough resistor (say k Ohms range), use that to short it instead of the screw driver, hold it with a plier with rubber coating on the handle and wear gloves for extra insulation.
 
Joined
Feb 14, 2021
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here's me doing mine last weekend cost 12 dollars.
 

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Joined
May 5, 2013
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527
Location
Peace valley, Missouri
Compressor that is very hot is off on a internal overload. Pull disconnect top grill put a water hose on top of the compressor running a stream water over the compressor for 30 minutes. After turn water off wait 5 minutes is compressor gets warm run water for another 15 minutes.This should cool it off and reset the overload. Just because the compressor will feel cool before 30 minutes it the internal temp that you can't feel. If compressor starts are the condenser coils clean? Might want to pull the top of unit see if the coil is made up of two coils if so coils separate at opposite piping end and can be spread apart to clean about 80%. Condenser fan could also be failing when this happens compressor still will run but sound differentially, it stops pumping and bypasses gas and run till it overheats.
 
Joined
Jul 15, 2020
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Raleigh Durham NC
Like most have said change the capacitor, mine went out last summer I had a spare on hand. Replaced and ordered a new spare to have when it may happen again. I have 2 heat pumps and a spare capacitor for both on hand along with a spare contactor. Make sure you get one with the same voltage ratings.
 
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