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]

GREAT. 🙂

[Ron_Nielson]

The new board arrived a few minutes ago, I installed it, and the HVAC now works like a new one. It took both the thermo and the board to get the job done successfully. Thanks to all for the help and ideas.

[Ron_Nielson]

A new thermostat arrived today and I installed it. The new one is similar but not the same as the older unit. I now have B+Voltage on the cooling request line but now it's the board that is not functioning correctly. Just ordered a new board and it should arrive in the next week or so. Might even get an RV trip in after the HVAC is running again. Wouldn't that be nice?

[wnjj]

Ron Nielson wrote:
wnjj
Again, thanks for the info.

Would you happen to know the part number for the relay? (jt box 8330-385) I have never soldered on a circuit board but I have a brother-in-law who builds electrical devices. His job is to repair stun guns and police body cameras so he is well prepared for the task.

Sorry I don’t but if you post a picture of the relay I could check out the specs on it. You need only match the footprint, coil voltage and amp contact rating. Relays are usually fairly universally and easy to swap. I did one on the control board of our Norcold fridge a few years ago.

[Ron_Nielson]

wnjj
Again, thanks for the info.

Would you happen to know the part number for the relay? (jt box 8330-385) I have never soldered on a circuit board but I have a brother-in-law who builds electrical devices. His job is to repair stun guns and police body cameras so he is well prepared for the task.

[wnjj]

Ron Nielson wrote:

wnjj wrote:
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.

Thanks for that idea. I have checked and there is no voltage (well, .05V this morning) when the line from the thermostat is disconnected from the board terminal. And I know that if I wire around the relay, the compressor runs.

It’s possible a shorted relay took out the thermostat.

Just to be clear, you tried a jumper on the relay coil (Y terminal) with the thermostat out of the circuit? You should see voltage on the source you jumped from (fan) before you connect the jumper. Then if you still see voltage when you connect it to the Y pin, it means the relay coil is open. If the voltage drops to near 0, the coil is shorted and pulling even the fan signal down.

[Ron_Nielson]

Ron Nielson wrote:

dougrainer wrote:


Regardless of the tstat, IF YOU HAVE 12 VOLTS AT THE UPPER CONTROL BOARD ON "Y", THE COMPRESSOR SHOULD COME ON. If NOT, your control board or the Capacitors are defective, NOT YOUR Tstat. BUT if you do NOT have 12 volts from the tstat on "Y", then find where the open is. Do you have 12 volts at the tstat on the wire for "Y"? If NOT then odds are your tstat is bad also. Doug

I would like to temporarily wire around the relay long enough to verify that the compressor will physically run. I think I just need to add a short connecting wire between the 110V input and the relay output circuit. Don't know what I might be doing to the balance of the board, if anything, just don't want sparks and fire. OK to do that?

Yes, I will test the actual output of the thermostat by disconnecting that wire from the board.

The compressor runs fine. Nice to see that I don't have to buy a new HVAC system, although I would prefer to have a 15K instead of the current 13.5K.

I'm now thinking that both the thermostat and the board (maybe just the relay?) are bad. The new thermostat should arrive in a couple of days.

[Ron_Nielson]

wnjj wrote:
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.

Thanks for that idea. I have checked and there is no voltage (well, .05V this morning) when the line from the thermostat is disconnected from the board terminal. And I know that if I wire around the relay, the compressor runs.

[dougrainer]

Ron Nielson wrote:

dougrainer wrote:


Regardless of the tstat, IF YOU HAVE 12 VOLTS AT THE UPPER CONTROL BOARD ON "Y", THE COMPRESSOR SHOULD COME ON. If NOT, your control board or the Capacitors are defective, NOT YOUR Tstat. BUT if you do NOT have 12 volts from the tstat on "Y", then find where the open is. Do you have 12 volts at the tstat on the wire for "Y"? If NOT then odds are your tstat is bad also. Doug

I would like to temporarily wire around the relay long enough to verify that the compressor will physically run. I think I just need to add a short connecting wire between the 110V input and the relay output circuit. Don't know what I might be doing to the balance of the board, if anything, just don't want sparks and fire. OK to do that?

Yes, I will test the actual output of the thermostat by disconnecting that wire from the board.

There are 2 heavy duty large wire gauge wires with Spade terminals on the relay. You can just pull them off the relay and interconnect them. Then apply 120 power to the RV. The compressor should start and run. Make sure you have NO 120 to the RV when doing this. Only connect 120 after you have jumped the 2 wires. Doug

[Ron_Nielson]

dougrainer wrote:


Regardless of the tstat, IF YOU HAVE 12 VOLTS AT THE UPPER CONTROL BOARD ON "Y", THE COMPRESSOR SHOULD COME ON. If NOT, your control board or the Capacitors are defective, NOT YOUR Tstat. BUT if you do NOT have 12 volts from the tstat on "Y", then find where the open is. Do you have 12 volts at the tstat on the wire for "Y"? If NOT then odds are your tstat is bad also. Doug

I would like to temporarily wire around the relay long enough to verify that the compressor will physically run. I think I just need to add a short connecting wire between the 110V input and the relay output circuit. Don't know what I might be doing to the balance of the board, if anything, just don't want sparks and fire. OK to do that?

Yes, I will test the actual output of the thermostat by disconnecting that wire from the board.

[30sweeds]

To add to what Doug said,if the compressor relay is bad,replace board.right?You probably know that the boards aren't that cheap. I had multiple relay failures(freeze sensor was bad and cycled the compressor every few seconds). and got tired of buying boards.So...if you have soldering ability,you can get the exact same relay on eBay or other places for a couple bucks and fix your board.Desolder,solder...really easy.

[dougrainer]

Ron Nielson wrote:
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.

Regardless of the tstat, IF YOU HAVE 12 VOLTS AT THE UPPER CONTROL BOARD ON "Y", THE COMPRESSOR SHOULD COME ON. If NOT, your control board or the Capacitors are defective, NOT YOUR Tstat. BUT if you do NOT have 12 volts from the tstat on "Y", then find where the open is. Do you have 12 volts at the tstat on the wire for "Y"? If NOT then odds are your tstat is bad also. Doug

[wnjj]

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.

[Ron_Nielson]

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.