Why when I turn the AC on only the in side unit works? I did check a few things, thermostat the out side unit to see if the disconnect switch was off. I used the meter to test the capacitor and the contact relay they are OK, except I am not getting 24v from the contactor relay? My central AC has heat pump and defrost relay & circuit board. I think the problem is to the circuit board. Please if someone can answer this I will really really preciated

Thank you
DM

What brand & model # is the outdoor unit? With thermostat set to cooling mode & setpoint lowered below room temperature, you should read 24v from "R" to "C" at defrost board. If the unit is equipped with pressure switches, a loss of refrigerant charge would prevent 24v from reaching contactor. Have you checked line-voltage power supply (230-240v at line side of contactor)?

Thanks KC13 I have a Bryant unit MD#FA4ANF030. Yes I did get 24v R' to C' and 240v at line side to the contactor but I am not getting 24v form the contactor (coil to energize the contactor) only when I pres the contactor the fan and the comressor start to run? What I believe to my knowledge the defrost board is not working properly, because the defrost relay has no voltage at all, there should be any voltage to the relay?

Thank you again
DM