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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 7535
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Welcome to Just Answer!.

The humm you hear is the 'contactor' energizing properly when it gets a 24 volt signal from the thermostat to go into cooling mode.

It is most likely that the fan and compressor are not coming on because the breaker at the house breaker panel that serves the outside unit is tripped, or a breaker or fuse mounted on the outside wall next to the outside unit is tripped or a fuse blown.

First step is to check those breakers and fuses.... Next step is to diagnose the reason they tripped.

Let me know what you find and if you are handy with working on high voltage equipment and have a multi meter or not.

If not tell me which direction you are from the center of the nearest town or city and I will see what service companies I can find in your area with good ratings.

We can go from there as necessary


Customer: replied 3 years ago.

I have not checked the breaker box or ouside service breaker yet I will check them either tonight when I get home from work or tomorrow afternoon and let you know what I find. I am very familar with using a multi meter and wiring diagrams. I'm a aircraft mechanic and use one every day.

Good plan, let me know how it goes... if you find a tripped breaker or blown fuse, have someone else turn the power to the unit back on while you stand next to it listening intently.

If the compressor is bad, or the compressor run capacitor is bad you will hear the fan start and run immediately and at the same time a heavy humm, as the compressor tries to start but will not start and go off on its internal overload after 3 to 5 seconds.

If it trips the breaker instantly there is a burned out fan motor or compressor most likely.

The next tests after that is a resistance check on the three compressor terminals... if they are not burnt, make a diagram of how they are connected to S, C and R at the compressor, remove the wires, and use the ohm meter to check each post to the copper tubes connected to the compressor... if you get any reads the compressor is burned out.

If you do not get a read to ground, then check between all three combinations

C to R
C to S
R to S

R to S should be the total of the reads between C to R, and C to S.

Typical reads are 3 ohms, 5 ohms and 8 ohms... or in that range.

Let me know what you get, we can go from there. I can stay with you on this without any time limit as long as you keep rating my replies *positively.



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Customer: replied 3 years ago.

I checked all breakers found all to be good. I checked resistance through compressor found to be normal as you listed above with no shorts to ground. Found that I have 120 volts through the contactor on both yellow and black terminals when thermostat is turned on. The Condensor fan does not come on and I hear the compressor engage for about 2 seconds than disengage. So I think I have a bad dual run Capacitor so I have ordered one for now is there anything else I should check?

Hello again, Good work! Thanks.


However It is important that you check **between** the two legs of power, thats Line 1 and Line2 as marked on the contactor when checking voltage.

If you only check each leg to ground it is possible to get a bad read...(because of electrical feed backs internal to the machine). You are looking for a read somewhere between 220 and 240 volts depending on what the name plate on the unit says.

If you get only 110 volts then there is a problem and we have to investigate that further.

I recommend you do that check before buying a new capacitor.... the odds are about 50% that the capacitor is merely bad, 25% that the compressor is locked up internally, and 25% there is a problem with power to the unit. (one leg can be dead and *still read 110 volts to ground).

Let me know what you find, I will keep the question open on an unlimited basis as long as you are able to rate my advices so far positively.!



Customer: replied 3 years ago.

I have 238 volts between the two legs coming out of contactor.

Hello again, that narrows down the problem to a bad run capacitor as you suspect, 90% chance of that, or a stuck compressor. 10%

Replacing the run capacitor is the next step or you can check its capacitance with a multi meter that has microfarads on it..

Be careful handlling the capacitor.


Be sure the power is off, triple check that, then use a 10 ohm resistor to short the center terminal to each of the out side terminals... these hold a 370v to 440v residual charge. New ones have been know to have a significant residual charge as well.


I turn my face away from a capacitor when first powering it up, they are known to exploded on occasion.


Thanks for the positive rating.


Customer: replied 3 years ago.

The capacitor arrives on Friday I'm crossing my fingers that it fixes the problem. If not I assume that if the compressor is locked up there is no fix for that and it would require a new compressor or new unit is that assumption correct?

Hello again, if the unit is over 5 years old I do not recommend replacing its compressor.


Make sure the new capacitor has the same microfarad and voltage ratings as the old one and you have the fan and compressor sides wired properly.

Chances are in the 90% range that the capacitor will work... cross your fingers. Stay in touch as needed.


Customer: replied 3 years ago.

The capacitor fixed the problem thanks very much for sharing your knowledge.

You are entirely welcome. Good work on your end!

It will be a good idea to go over all of the wire connections with a screw driver and tighten them up...they get quite loose over the years, then burn.


Customer: replied 3 years ago.

ok thanks again


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