DESCRIPTION & OPERATION
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Note. Powertrain Control Module (PCM) may also be known as Vehicle Control Module (VCM).
To provide improved idle quality, improved Wide Open Throttle (WOT) performance and A/C system protection, the A/C compressor clutch is controlled by the PCM.
For proper control of cooling fans, A/C compressor clutch and Idle Air Control (IAC) valve, a refrigerant pressure sensor is used. PCM uses signal provided by sensor to monitor high and low side refrigerant pressures. If PCM detects a fault in refrigerant pressure circuit, A/C compressor clutch will be disabled.
The A/C compressor clutch relay is controlled by PCM. This allows PCM to raise idle speed before engaging compressor clutch, or disable compressor clutch during WOT, high engine RPM, high power steering loads and hot engine restarts. PCM also disables compressor clutch if coolant temperature becomes excessive.
Description
When A/C is selected, a 12-volt signal is supplied to A/C request input of PCM. A/C compressor clutch relay is controlled through PCM. PCM monitors A/C refrigerant pressure. If A/C refrigerant pressure and engine operating conditions are within a specific calibrated acceptable range, PCM will enable A/C compressor relay. This is done by providing a ground path for A/C relay coil within PCM. When A/C compressor relay is enabled, battery voltage is supplied to A/C compressor clutch coil.
PCM will enable A/C compressor clutch whenever engine is running and A/C has been requested, unless
- Throttle angle is greater than 90 percent.
- A/C head pressure is greater than 414 psi (29.1 kg/cm 2 ).
- A/C head pressure is less than 35 psi (2.5 kg/cm 2 ).
- Ignition voltage is less than 10.5 volts.
- Engine speed is greater than 4500 RPM for 5 seconds.
- Engine coolant temperature is greater than 257°F (125°C).
- Intake air temperature is less than 41°F (5°C).
A/C clutch relay is PCM controlled to delay A/C clutch engagement after A/C is turned on. This allows PCM to adjust engine RPM before A/C clutch engages. PCM will engage A/C clutch any time A/C has been requested unless coolant temperature is high, A/C system pressure is low, A/C system pressure is high, wide open throttle, or high engine RPM.
When A/C-heater control panel is placed in A/C mode, a 12-volt signal is sent to PCM. When PCM receives this signal, PCM will ground A/C clutch relay control circuit to energize A/C relay. This is shown on scan tool as A/C REQUEST YES.
When an A/C request has been detected by PCM, PCM will ground A/C clutch relay control circuit. As a result, relay contacts close, allowing current to flow through relay to A/C compressor clutch. When A/C REQUEST has been detected by PCM, cooling fans will be turned on when A/C pressure is greater than a predetermined value.