Grand Prix V6-191 3.1L VIN M SFI (1998)
A/C Signal: Description and Operation
PCM Controlled Air Conditioning
DESCRIPTION
The air conditioning system uses a compressor with variable displacement. This compressor is referred to as the V-5 type compressor. The V-5
compressor meets A/C requirements without cycling.
The A/C compressor operation is controlled by the Powertrain Control Module (PCM) for the following reasons:
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Improves idle quality during compressor clutch engagement.
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Improves wide open throttle performance (WOT).
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Provides A/C compressor protection from operation with incorrect refrigerant pressures.
The A/C electrical system consists of the following components:
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A/C control head.
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A/C refrigerant pressure switch.
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A/C compressor clutch.
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A/C compressor clutch relay.
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PCM.
OPERATION
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:
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Throttle greater than 90%.
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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).
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Ignition voltage below 9.5 volts.
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Engine speed greater than 6900 RPM for 5 seconds.
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ECT greater than 121°C (250°F).
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IAT less than 0°C (32°F).
DIAGNOSIS
A/C Clutch Circuit Diagnosis located under Diagnostic Tables should be used for diagnosing the electrical portion of the A/C compressor clutch
circuit. See: Testing and Inspection
When diagnosing the refrigerant portion of the HVAC system, refer to Heating, Ventilation, and Air Conditioning.
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.
