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Under voltage at Tstat

I have been trying to find an electrical problem with a Trane AC system. Measuring between R and Common is 18 volts, not the required 24volts. Though measuring between R + Y (Cool) = 24 volts and R + Y2 is 24 volts. I would expect that should be zero, the transformer is supplying 24+ volts, otherwise I wouldn't get those readings. When I jump R to Y, the compressor comes on fine. Voltage between R + C (Common) is 18...not 24. Transformer is 27V with no load and 24V connected.

Any ideas on how to find the problem? I am completely befuddled.

Steve from Denver, CO


  • Tim McElwain
    Tim McElwain Member Posts: 4,608
    You are correct R & C should be the same as measured directly across the transformer it self usually 24 to 30 volts depending on the transformer. There is a resistive load somewhere between R and C at the equipment causing the drop in voltage. Disconnect each wire and ohm them out and see if one is greater than the other in resistance. That one will be your problem. Also make sure no wires are touching metal surfaces causing a loss (but not a direct short).
  • Ironman
    Ironman Member Posts: 7,350
    Obviously, the common wire is where the resistance is. Look for a bad connection or use another wire in the cable if it's available.

    The voltage between "R" and "Y" will be 24v when the thermostat contacts are open and 0v when they're closed.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Steve Garson_2
    Steve Garson_2 Member Posts: 708
    Found the problem: one of the blue wires that was connected to the air handler terminal board (common) was no longer connected. Took lots of time but that was it. Thanks all.
    Steve from Denver, CO