I have a 2.5 ton unit, supposedly that is not running. No name on the equip, didnt take the top off yet to get to the compressor to see what make it is.

Turn the tstat to cool and the compressor contactor pulls in and brings on the outdoor fan , the furnace also comes on but the problem is in the outside unit.

Outdoor fan comes on but compressor does nothing. checked contactor and it is good but just in case I pulled the disconect and wired the compressor terminals from the contactor to the incoming wire therby bypassing the contactor, reinserted disconnect and fan comes on again but no compressor. I also checked amps on common compressor wire and it O, nothing.

Checked compressor capacitor, it is a 35 uf and it reads 35 uf.

I removed the compressor start run and common wires and checked all for ground, not grounded. I checked the internal overload and it is not open.

I then checked the resistance of the compressor wires, I read 1.1 from common to run, 3.2 from common to start and 4.3 from start to run. Now If these resistance reading for each winding are correct and the c to r and c to s add up to what I get from r to s then the windings should be OK shouldnt they. I always heard but never understood that to check for shorted windings in a motor you measure c to r and c to s and they should add up to same as r to s. What I dont know is what the windings should be reading, I have seen anything from .5 ohms up to 10 ohms on motor windings but dont know what is right for this compressor motor.

Any replies or comments as to what is wrong or could be wrong would be appreciated.

Also if someone wants to delve into it.......

If my common to run winding is say... 1 ohm but it is shorted, and my c to start winding is 3 ohms but ok, wont they both add up to 4 ohms, which according to what I have been told would show both windings ok.

Will a shorted winding when added to the other good winding not add up like 1 plus 3 equals 4.

thanks

 

It is possible to have partial voltage that would run the fan but not power up the compressor. Sounds to me like the compressor and OD fan are OK. You need to see whether there is 230 volts at the compressor terminals R and C when the power is supplied. If the voltage is not adequate, test at the contactor in and out terminals and at the power source. Move upstream until you get full voltage. The problem will be downstream form there.

If you had an ammeter you could also check the current draw on R, C and S wires under powered up conditions.

 

I went around both sides of the contactor and I have full voltage to the compressor leads, didnt actually take the top off and look at the compressor terminals but what I dont understand is that I have what looks like good winding resistance and if I am reading what appears to be good winding resiatance thru the leads and I have voltage on the wires going to the comp, which I do then I should be getting some sort of reaction, I did have a clamp on ammeter on the common compressor lead at all times when running the unit and it never read anything, nothing..........
I will check it again tomorrow, I was in a bit of a hurry today but I just cant fiqure it out unless there is something bizarre going on at the comp terminals themselves or in the compressor itself.

thanks for replying

 

Turn the power off and remove the cover on the terminal box and inspect the connections inside. I agree that the test from the contactor SHOULD be enough, but in this case it is not. Something bizarre is going on.