Heat Pumps and Electric Heating - York heat Pump Compressor NOT Running

My heat pump compressor does not run when thermostat calls for heat. I opened the access panel on condensing unit and manually closed the contactor. Compressor and fan runs as long as I hold contactor in. When I release unit will stop running. Electric backup heat is only thing that runs with air handler blower. Could this be the defrost timer electronic board? Is the ambient temperature sensor normally open or closed? Is the coil sensor contact normally open or closed?

I assume this unit has worked before now. Defrost timer should not cause this. Compressor always runs while in defrost and outside fan is off. You need to check voltage on the relay coil which should be about 24 VAC. If you do then the relay coil is open or a bad connection. If you have a defrost board with a lite see if it is blinking indicating power low voltage ac is in the unit. Check where all connections from TStat is made usually small wirenuts. I have seen the signal wires break where the nut goes on. Most of the time the signal cable from the TStat runs through the air handler and connected on through to outside. You need to check these also. Make sure breaker to outside unit is not tripped. Measure for the 220 VAC coming from outside disconnect. The fact it runs when you hold contactor in pretty much eliminates the main power.