Dometic AC compressor not starting

[mkurtz]

I have a Dometic 57915.531C roof AC unit that the compressor is not coming on. The fan runs at both speeds without issue. It has an Analog control unit 3107541.009. I found and downloaded the service manual and ran through the troubleshooting section. The compressor and capacitors test as they should. The control box has a 6 pin plug that when tested without AC unit plugged into it, pin 5 (common) and pin 1 (blue-compressor) show 115 volts regardless of the thermostat switch (on or off). However, when the roof unit is plugged back in, the voltage drops to zero regardless of the switch. Up on the roof unit with the thermostat set to cool, I have fan as expected, but I have no voltage coming to the compressor. Is it safe to say that the control board is shot?
Thoughts?
Thanks in advance!

[mkurtz]

dougrainer wrote:
1. You cannot test for voltage with a Multimeter at those pins. You have to use a 60 watt light bulb in a socket with 2 wires to test for voltage.
2. On the control board there is a black relay. The is a BLACK wire and a BLUE wire(12 gauge). With NO 120 connected to the AC, jump those 2 wires together and then supply power to the AC. IF the compressor starts and runs, either you COLD(comp) wire from the wall tstat is not sending the signal or your control board is indeed bad.
3. Also, with NO POWER to the AC, take the WIRE CAP off the compressor and verify that all 3 wires are intact and not burned. Do this FIRST before item 2 is done. Doug

Thanks Doug!!
I tested with the bulb as instructed. I had a light across the fan pins but nothing to the compressor pin.
I had the cap off the compressor earlier to check the compressor. All three wires were intact and continuity back to where they originated.
I completed your #2 jumping the relay wires and Viola! I have compressor!! I will check the wire, but have a feeling it's the board.
Thanks again!!
Matt

[dougrainer]

1. You cannot test for voltage with a Multimeter at those pins. You have to use a 60 watt light bulb in a socket with 2 wires to test for voltage.
2. On the control board there is a black relay. The is a BLACK wire and a BLUE wire(12 gauge). With NO 120 connected to the AC, jump those 2 wires together and then supply power to the AC. IF the compressor starts and runs, either you COLD(comp) wire from the wall tstat is not sending the signal or your control board is indeed bad.
3. Also, with NO POWER to the AC, take the WIRE CAP off the compressor and verify that all 3 wires are intact and not burned. Do this FIRST before item 2 is done. Doug