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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8517
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Have an issue with A/C tripping the circuit breaker. I

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Have an issue with A/C tripping the circuit breaker.

I am an Electronics Technician, 25 yrs. Used to be a construction electrician before that. But no A/C experience, and motors are kind of a weak point for me.

I narrowed the problem to the outside unit. When I switch on the disconnect switch, the motor turns a little bit with a kind of growling noise, then the breaker in the house trips. What is interesting is the A/C is turned off at the thermostat, so the fan should not be coming on.

I opened up the unit and don't see anything imediately obvious like bug problems, cut and shorted wires etc. I removed the feeder wires, turned on the breaker and disconnect switch and all ok, so I am sure the problem is with the outside unit.

Model number is XXXXX Serial number is XXXXX This is a Heat Pump, but is not being used as such.

I did observe the contactor switch when I threw the disconnect on, and it did not engage. So why is the motor trying to turn on?

Looking for technical advice as to what I should look at or measure next.
Hello.Sorry you are having problems. My name is XXXXX XXXXX I will do my best to help.Please wait to leave rating until we are finished.Take a close look in the contactors. You may have to remove cover it they have one and see if perhaps the points are burned together.
Customer: replied 4 years ago.

Looked at the contacts and they appear darkened but not fused together. I can press the spring and it moves freely. Contacts measure open at rest and shorted when I press them, so all seems ok there. That was my first suspicion as well, because as I said, the A/C is set to off at the thermostat, and the ontactor does not activate, so this fan should not be coming on.

I am new to JustAnswer. For my $30, are you going to stay with me until I get this working?

I have to leave shortly. Ifwe have not got it figured out, I will open to othe r experts. Please ait to leave rating until you have received a satisfactory answer.
Lets look at the defrost board. Disconnect the low voltage wires from the disconnect board(place where they can not touch anything) and see if breaker still trips. If so, replace defrost board.
Customer: replied 4 years ago.

Have schematic in front of me.


The only circuit board is labeled "Circuit Board," not "Defrost Board," but it does contain the Defrost Relay and Circuitry, so I will assume this is the right one. Wiring from this board leads to the Defrost Thermostat and the Reversing Valve Solenoid among other things.


I will disconnect the low voltage wires, apply power, and see what happens. I see high voltage lines connected to the defrost relay, I will not be disconnecting those as per your instructions.


Will report back shortly...

THat is correct. You should have a wire from the fan coming from this board. What we are doing is seeing if this board is allowing voltage to go to the fan.It should not be. The defrost board sends voltage to the fan when call for cool or heat and disbales fan when in defrost mode so coil will defrost.
Customer: replied 4 years ago.

Breaker still trips.


Point of confusion on your comment. You said "What we are doing is seeing if this board is allowing voltage to go to the fan.It should not be."


The Defrost Relay is depicted as a normally closed relay. I measured 0.0 ohms across the contacts. That tells me that it IS allowing voltage to go to the fan, unless the logic circuit tells it otherwise, which it won't because we disconnected all the low voltage.


Am I correct?

I am going to have to leave, so I am going to open this to other experts.Please wait to leave rating until you are confident you have issue solved.
Customer: replied 4 years ago.

Will stand by...


Inside temperature 79 degrees...

Welcome !.

If the breaker marked 'heat pump' or 'ac' in your house breaker panel is tripping *instantly when you try to turn on the AC... there is a *dead short in one of the components.

If the breaker is tripping after 1 to 5 seconds, then one of the components is burned out or locked up.

You can determine which one by use of a clamp around amp meter... set the amp scale on 0-100 amps... and check the amperage on each of the wires leading to the fan motors, and compressor..etc... until you find which of those is drawing the heavy amperage.

If none are drawing more amperage than the breaker is rated for, then check the amperage at the breaker... if none of the wires are drawing more than the breaker rating, and the breaker trips....replace the breaker.

Let me know what you find.



Customer: replied 4 years ago.

Don't have a clamp on, wish I did. My handheld is rated 20A Max Fused. The breaker that is tripping is 30A, so I'm thinking if I put my handheld in series, the meter's fuse will go before the breaker.

Besides, I think it trips to fast to take a measurement.


Note that there are three breakers feeding my A/C, which I thought was kind of weird. Two double pole 60's and a double pole 30. It's the 30A breaker that is tripping. I traced it out and it goes to a 30A breaker in the A/C unit in the house, and to the condensor outside. Disconnecting the condensor stopped the tripping.


When I first started troubleshooting, the fan would turn for about two seconds with a wierd growling/grinding kind of noise, then the breaker would trip. Now it trips instantly.


Powered off, the fan spins freely with no grinding or other weird noises.


Hello again, it is most likely a burned out fan motor... you can use a mirror and flash light to look into the exposed end bell of the motor.. if the windings are good they will shinny copper... if burnt they will be completely burnt looking

The growl sound was not caused by bad bearings, but by partially burnt motor windings unable to run the motor.

Look that over and let me know what you find.... you may have to use a vacuum cleaner to suck the lint out of the motor so you can see the windings.

We can go from there.

Customer: replied 4 years ago.

Can't see any windings, the unit is completely sealed. I guess anything other than that would have surprised me, being exposed to the elements the way it is.

I disconnected the leads to the fan motor and measured them. Here's what I got:


Yellow to Black = 43 ohms

Yellow to Brown = 76 ohms

Brown to Black = 33 ohms


I would expect those numbers to all be equal, but again motors are a weak point for me.


The schematic depicts this motor as a circle with two windings in a V shape with Black being common. Not sure if that means anything.


Hello again,

The motor windings vary in length and wire diameter, so that your reads will vary.

Regarding the colors, those vary widiely between motors and manufacturers and between model numbers and years... not standard except that brown is usually for the capacitor circuit on *fan motors.

You will need a clamp around amp meter for further diagnosis... $60 at a big hardware store for a workable but not durable one.

You can also disconnect the wire from the fan motor that goes directly to the main contactor... then turn the unit on for 7 seconds, if the compressor starts and runs easily..and the breaker does not trip... the problem is likely with the fan motor.

Let me know, we can go from there.

Customer: replied 4 years ago.

OK. Hooked all the low voltage back up. Disconnected the yellow wire from the fan to the contactor. Threw the disconnect switch and heard a strange sound for about 1 second, then the breaker tripped.

Sounds like the compressor can't run.

What I don't understand is, why is it trying to? The thermostat in the house is set to OFF.

Hello again.

It sounds like we have two problems here.

The first thing to check is for a burned out compressor.

Kill all the power to the outside unit, triple check that with your voltage meter.

The make a diagram of how the wires connect to the compressor..

Then disconnect all 3 wires to S, C and R terminals on the compressor.

Use an ohm meter to check between each terminal and the copper tubin... if you get any read, the compressor is burned out.

If you get no reads on those tests check the following:

Ohms from C to R
Ohms from C to S
Ohms from R to S

The R to S read should equal the total of the other two reads.

Let me know what you get, we can go from there

Customer: replied 4 years ago.

Compressor is kind of hard to get to, so I found the C, R, and S designations on the schematic and disconnected the wires at the other end. Two from the contactor and one from the capacitor.


All wires measured short to the copper tubing. One measured about two ohms, the others were dead short.


I take it from your comments that the compressor is shot. (Bizarre, I just got through changing the compressor in my car!)


What I still don't understand is why it's trying to do anything when the thermostat is set to OFF?


Any idea what this will cost me, ballpark figure?

Anything else I can do myself at this point to save a few bucks?


Hello again, your test is most likely good, and the compressor is most likely burned out. If you can find the refrigerant charging ports and let a tiny bit of gas out, and onto an acid test strip you can get at the drug store...and it tests acid it is burnt for sure.


It is not always a good idea to replace a burned out compressor if the unit is more than 5 years old... tell me how old the unit is we can go from there.

The thermostat issue can be checked by checking voltage between the two coil wires that operate the contactor... if you have 24 volts there when the thermostat is off there is a problem with the thermostat, or a shorted wire on the way to the thermostat or elsewhere.

Let me know what you find, we can go from there.


Customer: replied 4 years ago.

No voltage on the contactor coil. Contactor never activated when applying power.


Unit was used when we bought the house 5 years ago.


Will my heater still work?



Hello again, your heater will work as long as there is 24v power at the transformer, unless there is something else wrong...

The AC compressor problem and the furnace problem are not related.

The compressor will not try to run and trip the breaker unless the contactor is engaged..or.... the contacts are burned and have the contactor stuck closed... there can be unusual feed backs that give a grounded compressor reading checking it the way you did..about a 1% chance of that.

You may want to try checking the compressor for ground again with the 240v wires that go from the contactor to the compressor disconnected...

Let me know how that goes, meantime try the furnace see if it comes on.


Customer: replied 4 years ago.

Ok, removed the electrical connection from the compressor itself. Readings are as follows to copper tubing:

S=1.9 ohms

R=0.9 ohms

C=0.6 ohms


Heat works fine. Now it's 82 in here!

Hello again. Thanks! but its bad news... the compressor is burned out.

It will not be a good idea to replace the compressor because the unit is so old, and the compressor is burnt... a burnt compressor contaminates the entire system.

If you replace just the condensing unit you will have to get one that uses the same refrigerant, most likely R-22..and fit a 'suction line filter' into the suction line if the oil is clear.

If the oil is black or heavy with sludge, replace the cooling coil attached to the furnace, the refrigerant tubing.... and the condensing unit.

If that is not in the budget now, a $99 chinese window air conditioner will cool the bedroom, or two $250 chinese window air conditioners will more or less cool the entire house.

I hope that helps.

I will put up the rating box now so that you can rate my advice positively ... that way I can hold the question open on an unlimited time basis.


Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8517
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
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