Remember, when you turn the A/C switch on you are making a request to the powertrain control module. If the PCM sees the request and determines the conditions are correct it will engage the compressor clutch and it will engage the cooling fan. Since neither the fan or compressor are engaging I'm going to initially look for a condition that would cause the PCM to "decide" not to comply with the request. The system on this particular car is not automatic temperature control but a manual system with dual temperature control. The manual system looks at fewer inputs then the automatic so *should* be easier to diagnose.
One very basic check is system pressure. If the system pressure is too low, the PCM will not turn the A/C on. I choose to connect a set of gauges to the system for the pressure test. That way, if the system proves to be low of refrigerant I'm ready to charge it without missing a beat.
In this case the stabilized pressures are approximately 120psi so there is plenty of refrigerant and certainly enough to allow the compressor to engage. Since I have started along this path though it's a good idea to continue the journey before looking elsewhere. *I* know the system has good pressure but I don't know if the *PCM* knows it. The PCM won't be looking at a set of gauges, it will be looking at the pressure signal voltage from the pressure sensor. The pressure sensor is located in the pressure line in the area below the air cleaner housing.
I can check the voltage at the sensor with a meter but if I start with a scan tool and look at the A/C pressure sensor data I'll be looking at what the PCM "sees". If I start at the sensor and do the voltage check with a meter and the voltage checks good, I would still have to check the scan tool data. But, if I check the scan tool data first and it is good then there is no need to check the sensor voltage.
I can't miss an opportunity to check stored trouble codes so once I connect the scan tool I do just that. The PCM has stored DTC P0530. P0530 indicates a problem with the A/C refrigerant pressure sensor circuit. This code certainly tells me that I'm on the right path and I pull up the A/C pressure data items.
The A/C pressure readings are certainly out of the ordinary. The PCM sees no voltage from the pressure sensor and calculates a VACUUM condition (-15 psi)! I think it is time to do some voltage checks at the sensor. The pressure sensor has three wires. It has a 5 volt reference voltage, a ground and a signal return to the PCM. As the pressure changes the voltage on the signal return will vary. The PCM will use that voltage signal to determine system pressure.
With the sensor wiring connected I verify the reference voltage and ground are good at the sensor connector. A check of the signal wire shows no voltage so I'm almost ready to condemn the sensor. I need to verify the signal circuit is not shorted to ground. A short to ground would kill the sensor output voltage even with a good sensor. It won't work to disconnect the sensor and check for a ground on the circuit because the pulldown resistor inside the PCM would ground the open circuit. I wouldn't know if there was a short to ground unless I also disconnected the PCM connector. I didn't disconnect the PCM. I cut the signal wire a few inches from the sensor and checked for voltage on the sensor side of the wire. When I found no voltage output from the sensor then I KNEW I had a bad sensor.
FIX:
Replace the A/C Pressure Sensor.
