Original post by: Matt ,
low pressure switch works to engage clutch to start compressing r134, forcing it thru condenser, then lines, drier, and evaporator, then back to compressor (drier might be after evaporator). the higher the pressure (the more r134 in system), the more the engine fan runs, no doubt. just notice how often the fan increasingly kicks on as the system is being charged. but i've had a low pressure switch in my personal vehicle be the fault for the exact same problem because the valve core-which is the same as a tire valve stem core-started to unscrew. the pressure would not push that "button" in far enough to disengage clutch, but the high pressure switch was. disengaging clutch.