old Ruud AC Unit outside compressor/fan won't kick on

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old Ruud AC Unit outside compressor/fan won't kick on

Postby acrewofone » Sun Jul 29, 2012 4:51 pm

25 year old Ruud Central Air, outside unit is compressor/condensor and fan, inside unit is evaporater inside furnace plenum.

Two weeks ago, the outside unit would not turn on when thermostat was set to AC, inside unit was running. Opened up the outside unit access cover and saw the contactor was totally rusted/gummed up and covered with bugs (it is the old exposed contact type). Bought another 2 pole contactor assembly from local HVAC Supply, and installed it (basic 24volt, 30AMP, 2 pole unit). Outside unit still will not turn on, but you can push down on the contactor button and everything starts to run and put out cold air.

Metered no 24volts AC on the contactors coil connections, but happened to pull off one connection and now read 24volts AC coming from the transformer in the inside unit, going to the contactor. So I only get 24volts AC when the connection is disconneted from the contactor, as soon as I reconnnect, no 24volts AC, and no coil engagement.

Thought maybe I got a bad contactor or the wrong rating, so I found a contactor online that was exactly the same as the original, and installed that. Same result, no 24volts AC on contactor, and no coil engagement. But if I pull off one connnection (either one), I read 24volts AC coming through the wires.

(Since it is now almost 100 degrees outside, I have taped down the contactor button and everything is running and putting out nice cool air, but since the outside unit has been jury rigged to run all the time, I am shutting it off at night by tripping the breakers, inside unit is cycling on and off by itself when it reaches the set temperature).

I am at a loss as to what the problem is, the transformer puts out 24volts AC going out to the contactor, but only with no load on it. Maybe it got weakened when everything was rusted and gummed or shorted up? (or am I wrong in thinking I should be able to read 24volts AC on the contactor with everything hooked up and set to run)?
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- old Ruud AC Unit outside compressor/fan won't kick on

Postby Freon » Mon Jul 30, 2012 11:03 am

Check the contactor coil for continuity. I'm sure the contactor is good but this is a simple check. Be sure ALL power is off. Next you can try and feel for the break in the 24 volt contactor supply wire or simply cut it back a foot or 2 to a point where it has not been flexing. I have had this problem. The insulation is tough enough the wire holds together even though the copper conductor is broken. One way to test the broken wire theory is remove the thermostat cover. Connect the 24 volt control line to the contactor. Call for cooling. Now measuring the voltage at the contactor you should read zero volts, as before. Leaving everything as it is, now measure the voltage at the thermostat between the Y and C (common) terminals. If you have 24 volts there then go to the air handler and measure the voltage at the control board between the Y and C terminals. If there's 24 volts then you know the break is between the air handler control board and the contactor.

The last possibility is a bad transformer. But since everything works when you force the contactor closed, I'm not sure the transformer is bad
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