My question is in reference to a Trane condenser M#2TWR3018A1000AA. I need to know how the ambient air sensor works in tandem with the coil sensor to initiate defrost.Some questions would be: 1.Why is there not 24 volts on terminals for either sensor? 2.If you take the air sensor out, the system will come on,but will it go into defrost?

Thank You

1. It may.

TRANE info is hard to find online, I don't have a source.

The ambient air sensor more than likely PREVENTS the HP from coming on at a given temperature usually around freezing.

Coil temp, more than likely terminates the defrost cycle.

Defrost probably initiates after a certain amount of run time in heat mode.

These may be switches or they could be a sensor. If they are switches and they are closed, they would indicate 0 V (AC? Or DC?). If they are sensors, they are probably resistive and would change the DC value across them.