Air Conditioning2464 views

Why don't I have 24 volts at the contactor on the outside AC unit?

Hello all! My AC stopped cooling recently. The blower fan inside works. The fan on the outside unit works, but the compressor motor in the unit would fail to start. After about 15 minutes, the compressor motor would eventually start up so I diagnosed it as a bad capacitor. The new cap was 8 bucks so I ordered it. I thought my problem was fixed.
However, as I poked around in the access panel I noticed that the 24 volt line was frayed as it passed through the sheet metal. There was no wire clamp to secure it and it had worn through over the years. As I moved the conduit, the 24 volt wire grounded out to the AC unit body and small sparks flew and the contactor rapidly engaged. I carefully removed the old crummy wire and ran a proper wire through. I shut off the power and made the connections. However... now the contactor won't click/AC unit won't turn on. Upon testing the leads, there is no 24 volts coming through. I checked the *only* fuse on the circuit board for continuity. It's good. What did I fry?

Asked July 28, 2016

2 answers

  • Votes0

    Did you check the emergency shut off that’s adjacent to the AC unit? This shut off is a safety device and may have shut off if power was disrupted. This would cut off power to unit.

    Contractor · September 9, 2018

  • Votes0

    The 24 volts comes from a low voltage transformer. Please see the pictures of the two transformers below. The transformer has a primary (high voltage side) and a secondary (low voltage side). The high voltage side takes 110 to 240 volts AC and transforms the voltage to 24 to 28 volts AC. If the contactor is not getting the 24 volts to the contactor coil then it could be a time delay relay (if the unit is equipped), thermostat problem, transformer, safety control (low on refrigerate) or wiring problem. I would like to suggest that you turn the power off to the unit and check all your connections to make sure they are good and tight. You can turn the power back on and set the thermostat so the system is calling for cooling (AC ON). You could test the contactor with a volt meter to see if you are getting 24 to 28 volts AC to the contactor 24 volt coil. If you are not getting any voltage to the contactor coil then this could be a blown fuse on your furnace or air handler control board.

    Contractor · January 24, 2017

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