HVAC - No Compressor run

[Ron_Nielson]

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]

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]

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]

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]

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?

[hypoxia]

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!

[3_tons]

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

[dougrainer]

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

[Ranger_Smith]

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