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Ron_Nielson's avatar
Ron_Nielson
Explorer
Sep 15, 2021

HVAC - No Compressor run

The Coleman Mach 3 13.5 BTU HVAC has worked perfectly from 2008 until now.

Troubleshooting my RV HVAC unit. The thermostat says COOLING ON. I have fan control from the thermostat, but no compressor run.

The compressor resistance values properly check out on the 3 terminals with none being shorted. Capacitors tested to their face values.

I have 110V power coming into the relay on the board but no power out. So the relay isn't turning on. Testing at the board on the GL (Fan) terminal to the B- terminal shows that the fan runs at B+ voltage. Less than 1-volt shows at the Y (compressor) terminal. That would tell me that the thermostat is not sending the signal to turn on the relay to run the compressor. So, then I jumped the GL terminal to the C terminal at the board thinking that that should restore the necessary message from the thermostat. Nothing. Just nothing, no change, no compressor.

So it looks like it might be a thermostat problem, but if I jumper B+ power to the thermostat terminal (C) at the board, it looks like a compressor problem. I have not yet tried to wire around the relay at the board and I'm not sure if I should do that. Not sure what the ramification of that might be.

Bad Thermostat? Bad Board? I'm lost or missing something. Suggestions?
  • Ranger_Smith's avatar
    Ranger_Smith
    Explorer
    Sep 15, 2021
    Did you check the low pressure cutoff make sure it is closed. Also do you have 12 volts to the board. I think that is required on the board also
  • dougrainer's avatar
    dougrainer
    Nomad
    Sep 16, 2021
    There is no low pressure cut off on his AC. You should have 12 volts on the YELLOW wire or "Y" terminal at the upper control board. If you do NOT, just jumper the 12 volts from either the GL(low) or GH(high) fan wire. Fan on LO and the GL will have 12 volts. GH and on HIGH it will have 12 volts. IF the Compressor relay fails to close and engage the compressor the upper board is probably bad. Doug
  • 3_tons's avatar
    3_tons
    Explorer III
    Sep 16, 2021
    dougrainer wrote:
    There is no low pressure cut off on his AC. You should have 12 volts on the YELLOW wire or "Y" terminal at the upper control board. If you do NOT, just jumper the 12 volts from either the GL(low) or GH(high) fan wire. Fan on LO and the GL will have 12 volts. GH and on HIGH it will have 12 volts. IF the Compressor relay fails to close and engage the compressor the upper board is probably bad. Doug


    Doug, I love reading your prescriptions -you are truly amazing :) !!

    Many Thanks,
    3 tons
  • hypoxia's avatar
    hypoxia
    Explorer
    Sep 16, 2021
    Ron, that is one of the most concise descriptions of a problem I have seen as well as your troubleshooting status. We are fortunate to have Doug on this forum!

    Well done!
  • Ron_Nielson's avatar
    Ron_Nielson
    Explorer
    Sep 16, 2021
    Thanks for the replies. I have been reading Doug for years and appreciate all the help he has offered.

    One thing that quite surprises me is that the signal from the thermostat to the C term on the unit has no(.8V) voltage on it. I expected to find a B+ voltage there. Why no voltage?
  • Ranger_Smith's avatar
    Ranger_Smith
    Explorer
    Sep 16, 2021
    Ron Nielson wrote:
    Thanks for the replies. I have been reading Doug for years and appreciate all the help he has offered.

    One thing that quite surprises me is that the signal from the thermostat to the C term on the unit has no(.8V) voltage on it. I expected to find a B+ voltage there. Why no voltage?


    I believe the "C" terminal is the common thus no voltage on it . . . I could be wrong

    The c wire is an extra wire that can be used to provide a return path for continuous power for any application. It's normally used to provide a return path to power the thermostat. The “c” stands for common. It is often labeled as “c” on thermostat backplates.
  • Ron_Nielson's avatar
    Ron_Nielson
    Explorer
    Sep 16, 2021
    Yes, I misspoke. I was referring to the Y terminal. No signal there when in cooling mode. Thanks for noticing and keeping anyone else from being confused.
  • Ranger_Smith's avatar
    Ranger_Smith
    Explorer
    Sep 16, 2021
    Ron Nielson wrote:
    Yes, I misspoke. I was referring to the Y terminal. No signal there when in cooling mode. Thanks for noticing and keeping anyone else from being confused.

    If you read Dougs post there should be 12 volts there. If not put fan on high or low and jump the terminal to y and see if compressor relay kicks in. I believe it is either GH or GL
  • Ron_Nielson's avatar
    Ron_Nielson
    Explorer
    Sep 17, 2021
    Ranger

    That's exactly what I did. Jumped 12V from GH (and GL separately) to the Y terminal and there was no response.

    I have the situation where one test would indicate that the board has failed (the above test) and another test would indicate that the thermostat has failed (only .8V on the Y term supplied by the thermostat). Seems like an impossible situation unless both the board and the thermostat are defective.

    I have ordered a thermostat to see if that fixes the problem. Like I previously said, they're only $20 or so and I need to get this thing fixed. If it turns out that that a new thermostat produces the correct result, I'm done, otherwise, I'll see if I can find a new board. They're pretty scarce right now.
  • wnjj's avatar
    wnjj
    Explorer II
    Sep 17, 2021
    When you jumped to the Y terminal, was the thermostat still connected to Y? If so it may have been pulling that down too much. While you are jumpered, trying checking the voltage on the Y terminal at the compressor.

    The opposite could also be true. Perhaps the relay coil at the compressor is shorted and pulls any source down. Try measuring the Y output from the thermostat with nothing connected to it.

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