HVAC - No Compressor run

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.