I have been working on a Carrier heatpump model #HC1CJ030-A. The unit will not go into defrost. I have tested the defrost sensor and it is showing closed. I have bypassed the sensor with no luck. I have shorted the test pins, no luck. I have 24 volts between the defrost terminal and the common terminal. I have replaced the board with a new board, no luck. I work at an apartment complex, so I tested another unit by shorting the test pins, which it went into defrost. I then took that circuit board and installed it on the unit that is giving me the problem. Again, the unit would not go into defrost. The charge is correct on the unit. Any ideas?