Here is a way to test the IPDM: Turn the ignition key to the on position, press your door switch 10 times, turn the ignition off then turn it back to the on position. If you do this right your horn will beep and the oil light will start flashing. The defroster ,headlights,cooling fans,wipers,and compressor clutch will cycle, so make sure that your hood is open so you can here the clicks of the compressor clutch. This is called the auto active test and this tests the main relays in the IPDM. If the compressor clutch clicks then its NOT the IPDM. Then i would suggest do what Jim said and test your pressure switch.