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Our air conditioner recently stopped blowing out cold air. When I went to check on it, I noticed that the outside unit wasn't coming on. We had had problems with ants in the contacter in the past, and so I went to take off the lid and check it out. I noticed that the circuit breaker marked "AC" was tripped. Like a good DIYer, I turned it all the way off and back on. It immediately tripped again. In an effort to isolate the problem, I went to the outside unit and pulled the safety connection to disconnect the main power of the outside unit. When I went back and reset the circuit breaker as before, the breaker didn't trip. Reconnecting the outside safety switch caused the breaker to immediately trip again. Later, we tried turning off the air conditioning in the house, resetting the circuit breaker, and turning the AC back on. The circuit breaker didn't trip as soon as it was reset, but when the AC was turned back on inside, it immediately tripped as before.

My question is what should I check in this situation? This seems like it can't be an uncommon problem, so I'm sure HVAC professionals encounter this frequently and have a checklist of things to try. What are the possible causes, remedies, and safety precautions I should consider in this situation?

Edit:

Answers to some comments: No, I didn't see any obvious damage to the wiring near the outside unit. I did look pretty carefully, since I had already taken the panel off to check for ants in the contacter. I didn't try to spin the fan with my hand, and I'm not in a place that I can try this right now. I will try it later today; perhaps in the meantime that could be a step in the diagnosis of the problem if anyone gives an answer between now and then? Likely, future readers of this question will encounter the same problem and some will have a fan that spins by hand and some will have one that is stuck.

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Can you spin the condenser fan with your hand? Do you see any obvious shorts or damage to any wiring? –  Tester101 May 21 '13 at 15:07
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If the condenser fan does not spin, you can stop the diagnosis. You need a new motor, yours is locked. When a motor is not spinning (locked rotor), resistance is very low (nearly 0), making current very high. If this situation is not corrected (i.e. the rotor does not start to spin), the current in the circuit will quickly surpass the level at which the breaker should trip. I'll try to write a proper answer later when I have time, unless somebody beats me to it. –  Tester101 May 21 '13 at 18:05
    
The fan spins freely, and I tested the capacitor and it appears to be good. –  David Hollman May 21 '13 at 23:16
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There are two motors, one is the fan motor, the other is the condenser motor. –  Brad Gilbert May 22 '13 at 21:38
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@BradGilbert There is a condenser fan motor, and a compressor motor. However, the compressor motor is part of the compressor assembly, and typically can't be accessed easily. –  Tester101 May 31 '13 at 20:05
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1 Answer 1

up vote 5 down vote accepted

Disconnect the power

Start by turning off the breaker, and pulling the serviceman disconnect, which will typically look something like this.

enter image description here

This will insure no electricity is flowing to the condenser unit while you're working.

Open the unit

Next you'll want to disassemble the unit, to allow access to the electrical parts. This will vary from unit to unit, so check the owners manual for the procedure for your unit. Once you have the unit opened up, make sure to discharge the capacitors.

enter image description here

These things store enough power to kill you, so you don't want them to discharge accidentally.

Resistance is not futile

Once the power is completely removed from the unit, it's safe to start poking around (electrically speaking, don't go busting the refrigerant lines). Start by tracing the wires from the condenser fan motor, back to where they connect in the electrical box. There should be 3 or 4 wires. In my unit, I had Black, White, Brown, and Brown with a White stripe (your model may vary). To determine if the motor is good, you'll measure the resistance across each coil. To do this, you'll have to disconnect the wires, so the motor is no longer part of the circuit (make note of where the wires connected).

Typically you'll have 3 wires, start, run, and common (we'll ignore my 4th wire in this answer). Set your multimeter to measure Ohms, and start measuring. You're going to measure the resistance between each combination of two wires to determine what each wire is, and if the motor is still good. Let's start with Black and White...

Black -> White = 15.9
Black -> Brown = 35.4
Brown -> White = 51.2

Knowing that...

Common -> Run = Lowest resistance
Common -> Start = Medium resistance
Start -> Run = Highest resistance

We can determine that...

Black = Common
White = Run
Brown = Start

If we also know that the two lower readings should always add up to the larger reading, we can safely say this motor is still good. If you measure 0 or infinity between any pair, that means you have a shorted or an open winding and the motor should be replaced.

Repeat the same procedure for the compressor motor.

Shorts on the ground

The other thing you'll want to check for, is shorts to ground. Set your multimeter up to test impedance. Put one probe on the equipment grounding conductor of the feeder, and the use the other to find a solid ground on the motor. You may have to scratch some of the paint off, especially on the compressor. Once you've found a solid ground, measure from each motor wire to your ground spot. If the meter beeps or give a low resistance reading, you have a short to ground. As with the resistance test above, the motor should be isolated from the circuit when doing this test (once a solid ground is located).

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Fantastic! Thank you. Unfortunately, the problem was the compressor motor, which had to be replaced :-( –  David Hollman Jun 3 '13 at 14:24
    
I just wanted to thank you again for a fantastic answer. I ended up calling an AC repair man, and he followed the exact same steps you outlined. Thanks again! I hope others find this helpful in the future. –  David Hollman Jun 5 '13 at 15:25
    
@tester101 What's the best way to discharge a capacitor? Screwdriver, Jumper cable, 100K resistor in series with a cable? –  HerrBag Aug 16 '13 at 11:42
    
@HerrBag a resistor or test light are the safest, but most folks have more fun with a screwdriver. Next time the HVAC tech is around, dig through his tool pouch. You're sure to find a screwdriver with little nicks in the blade. –  Tester101 Aug 16 '13 at 12:02
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