1. At the magnetic clutch relay, check for two power terminals to be present when the blower motor is operating. Check to see if the relay will pass voltage through it.
2. At the high-pressure switch, check the yellow wire for a 5-volt reference with it unplugged. The reference voltage should go to 0 volts when the sensor is plugged in.
3. At the A/C thermistor, check for a 5-volt reference on the black/blue wire with the sensor disconnected. Check for less than 2.0 volts when the sensor is plugged in.
4. Using a scanner, check to see if there is a start input signal showing on. Check for a wide open throttle input from the throttle position sensor and check for an over-heat input. Any of these will prevent the computer from energizing the relay.
5. At the computer, energize the magnetic clutch relay and make sure the compressor comes on.
Tech tip: The computer will shut off the air conditioning system when the thermistor voltage reaches more than 2.0 volts.
Courtesy of Identifix, Inc.