PCM Controlled A/C Circuit Diagnosis

Circuit Description
The Air Conditioning (A/C) system consists of a HVAC control assembly, separate A/C refrigerant high and low pressure switches, separate A/C high and low temperature sensors, a control relay, the compressor clutch and the PCM.
When the HVAC control assembly is placed in the A/C mode, a request signal is sent to the PCM. The PCM will then energize the A/C clutch relay, unless abnormally high or low A/C pressure is detected by the A/C refrigerant pressure switches. The PCM also monitors the A/C low temperature sensor to cycle the A/C clutch ON and OFF. If the PCM detects an A/C evaporator temperature less than 2°C (36°F), the A/C clutch will not be enabled. The PCM will also energize the cooling fans ON when A/C is requested. Refer to Electric Cooling Fans. The A/C compressor relay is controlled by the PCM so that the PCM can increase idle speed before turning ON the clutch or disable the clutch when high engine coolant temperature is detected or high engine RPM is detected. Component Tests and General Diagnostics

Diagnostic Aids
^ If DTC P1537 is stored as history, it is most likely that one of the A/C pressure switches opened. Check for improper refrigerant pressures.
^ If any PCM DTCs are set, perform those diagnostics before using this table. The A/C clutch is disabled for many DTCs.
^ The PCM will energize the A/C clutch whenever the engine is running less than 4700 RPM and A/C has been requested, unless any of the following conditions are met:
^ NC head pressure greater than 414 psi (2854 kPa) or less than 38 psi (262 kPa) as determined by the A/C refrigerant pressure switches.
^ NC evaporator temperature less than 2°C (36°F).
^ Engine at wide open throttle.
^ High power steering load.
^ Low ignition voltage.
^ Engine speed greater than 4700 RPM for 6 seconds.
^ ECT greater than 125°C (257°F).

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
3. The outside air temperature must be great enough for the A/C compressor clutch to engage.
5. This step uses the Scan Tool to determine if there is an open or short.
6. This step checks the power feed circuit.
7. This step bypasses the relay.