Cutlass V6-191 3.1L VIN M SFI (1998)
When A/C is selected through the A/C control head a 12 volt signal is supplied to the A/C request input of the PCM. The A/C compressor clutch relay is
controlled through the PCM. This allows the PCM to increase the engine idle speed just prior to A/C clutch engagement for better idle quality. In
addition the PCM will command the cooling fans on during A/C operation. The PCM monitors the A/C refrigerant pressure. If the A/C refrigerant
pressure, and engine operating conditions are within a specific calibrated acceptable ranges the PCM will enable the A/C compressor relay. This is
accomplished by providing a ground path for the A/C relay coil within the PCM. When the A/C compressor relay is enabled battery voltage is supplied
to the compressor clutch coil. The PCM will enable the A/C compressor clutch whenever the engine is running and the A/C has been requested, unless
any of the following conditions are met:
^
Throttle greater than 90%.
^
A/C head pressure greater than 427 psi (4.27 volts) or less than 41 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).
^
Ignition voltage below 9.5 volts.
^
Engine speed greater than 6900 RPM for 5 seconds.
^
ECT greater than 121°C (250°F).
^
IAT less than 0°C (32°F)
A/C Clutch Circuit Diagnosis should be used for diagnosing the electrical portion of the A/C compressor clutch circuit. When diagnosing the refrigerant
portion of the HVAC system.
A scan tool will be used in diagnosing the system. The scan tool has the ability to read the A/C request and refrigerant pressure inputs to the PCM. The
scan tool can display when the PCM has commanded the A/C clutch on. The scan tool should have the ability to override the A/C request signal and
energize the A/C control relay.
