A/C Operation

WARNING: This page is about a different car, the 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, 2002 Chevrolet Silverado, and 2002 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The purpose of the air conditioning (A/C) system is to provide cool air and remove humidity from the interior of the vehicle. The vehicle operator can activate the A/C system by placing the mode switch in one of the following positions: Panel, Bi-Level, Defog, and Defrost. The A/C system can operate regardless of the temperature setting, however recirculation is only available when the HVAC control module is either in A/C or Bi-Level mode only.

When the A/C button is pressed a request is made to the powertrain control module (PCM) to turn on the A/C compressor. The PCM turns on the A/C compressor by providing a path to ground through the A/C clutch relay control circuit for the A/C compressor clutch relay. Once the relay closes its internal switch, power from the battery is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. Whenever the compressor is turned off, the A/C compressor clutch diode provides a path for the voltage spike resulting from the collapsing magnetic field of the compressor clutch coil.

In order for the PCM to internally grounds the A/C clutch relay control circuit, two separate request signals need to be grounded. A 12-volt reference signal is sent out over the A/C request signal circuit, through the A/C high pressure switch and grounded through the HVAC control module when the vehicle operator makes an A/C request. The A/C request signal circuit output from the PCM to the HVAC control module can not survive a short to power. A separate 12-volt reference signal is sent out over the A/C refrigerant low pressure cut-out switch signal circuit, through the A/C low pressure switch and the ground circuit. When both these reference signals are grounded, the PCM may activate the A/C compressor clutch.

The PCM will engage the A/C clutch any time the engine speed is below 5000 RPM and the A/C is requested unless any of the following conditions exist:

Throttle angle is at 100 percent.
The A/C low pressure switch is less than 124 kPa (18 psi) or more than 338 kPa (49 psi).
The A/C high pressure switch pressure is more than 2896 kPa (420 psi).
Engine speed is more than 5500 RPM.
Engine coolant temperature (ECT) is more than 121°C (250°F).