Posted 09 May 2014 - 11:17 AM
The compressor should be engaged when the selector is on any setting that uses the AC.
There are several relays and switches in the way between the control panel and the compressor, so yes even if the pressure switch shows continuity power could not be making it to the compressor clutch due to one of the other switches or relays.
Obviously make sure all the fuses are good first.
Power for the compressor clutch does not flow through the pressure switch directly. Power for the compressor clutch comes from the ignition switch, through a fuse in the AC relay holder, through the AC relay, through the compressor thermal switch (if equipped), then through an AC cut relay, then to the clutch.
Not all models have a thermal switch on the compressor so you just have to look and see. If it does you'll have a 3 wire plug at the compressor. If there is only one wire at the compressor there is no thermal switch.
The AC cut relay is normally closed so should show continuity at all times. This relay is controlled by the evaporator thermal switch, and will only open if the evaporator core gets too cold.
The pressure switch controls the coil side of the main AC relay. Power flows from the mode control panel, to the pressure switch, then to the AC relay, then through the thermal switch on the evaporator, then to ground through the fan switch in the mode control panel.
You can easily check for 12v at the pressure switch when the control panel is set to an AC position.
If you have the thermal switch on the compressor you can check for 12v there. Also check for continuity through the switch.