Skylark V6-3100 3.1L VIN M SFI (1996)
Powertrain Control Module: Description and Operation
PCM Outputs
PCM CONTROLLED AIR CONDITIONING
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
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 )
When engine temperature exceeds 1O5°C (221°F).
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When A/C is requested (ambient temperature must be above 90°C (48°F).
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When A/C refrigerant pressure is greater than 190 psi (2 volts).
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When the ignition is off and engine coolant temperature is greater than 140°C (140°F).
The PCM will complete the ground paths for the high speed cooling fans (Fan 2) relays 1, 2 and 3 under the following conditions:
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When engine temperature exceeds 113°C (235F).
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When A/C refrigerant pressure is greater than 275 psi (2.9 volts).
When certain Diagnostic Trouble Codes (DTCs) are present, the PCM may command the cooling fans to run all the time. It is important to
perform the Diagnostic Circuit check prior to diagnosing the engine cooling fans.
