Car information: Kia K5 car with 146 thousand miles.
Problem: air conditioner not cold
Diagnose Procedure: use the self-test function of air conditioner to test, and the air conditioner control unit display no fault code, therefore it is impossible that the monitoring part of air conditioner control unit has fault. Then I use GDS VCI diagnostic tool to test the air conditioner related data stream, and Find Air Conditioner Pressure Voltage is 0V, A/C pressure is 0 psi (1 psi=6.895 kPa), Air conditioner compressor is OFF, which means the engine control unit does not receive the pressure signal from air conditioner pressure sensor. According to K5 engine control system related control circuit, I use multimeter to test the voltage of PCM connector terminal CHG-K 54, it is 0V, and test the circuit conduction of APT terminal E19-2 (gray / black cable) to terminal EC 11-57 circuit, and find this circuit is broken (is ground off) , then result in PCM fail to receive APT pressure signal, PCM stop the compressor working through C-CAN control air conditioner control unit, then air conditioner not cold.
Solution: connect the broken circuit and make it firm, then use GDS to test air conditioner system data stream, Pressure Voltage is 1.9V, A/C pressure is 54 psi, air conditioner compressor is ON, this means PCM has received APT pressure signal, and order control air conditioner compressor to work, and then air conditioner cooling system is working again.