VW PASSAT B5 AC compressor suddenly stops working

A VW PASSAT B5 GSi was brought to my garage last night. The driver told me that the air conditioner in the car suddenly stopped working while he was driving, and the AC compressor just did not work.

Since AC system in the car had self-diagnostic function, and would stored trouble codes if there was something wrong with the system, I first applied a VAG diagnostics tool, VAS 5054A to read DTCs from the system. After connecting the tool to the car diagnostic socket, I pulled a trouble code: 00792, which indicated AC pressure switch fault. This meant the pressure switch or other control circuit was damaged.

I removed the headlamp on the right side, turned off the AC pressure switch, and read the voltage from No.2 terminal of the pressure switch. According to the circuit diagram, the voltage at the terminal should be 12V when the switch was on. But as the multi-meter showed, the real voltage there was actually 0V. It was likely that the problem was caused by the lack of voltage at the terminal.

After finding the fault location, I went on a further inspection and found that there was a 4-pin change-over plug connecting the pressure switch. I removed the decorating shade of the oil storage tank of power steering pump, and noticed that the 4-pin change-over switch was loose.

I replaced the 4-pin change-over switch with a new one and plugged it tightly. Then I started the car, turn on the AC switch, and checked how well the AC system worked. The system could work normally. The VAS 5054A is a good tool.

Tips: The problem was mainly caused by the loose 4-pin change-over switch. The loose switch meant bad connection between the No.1 terminal pressure switch and the No.2, No.3 and No.4 terminals. And therefore the AC would cut off the power supply for its relay and electromagnetic clutch of the compressor, forcing the compressor to stop working.