HVAC - No Compressor run

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.