A/C Operation: Notes

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Yukon, 2002 Chevrolet Tahoe, 2002 Chevrolet Suburban, and 2002 Chevrolet Avalanche. 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
DEFROST

The A/C system can operate regardless of the temperature setting, however, RECIRCULATION is only available when the HVAC control assembly 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 ground the A/C clutch relay control circuit, the A/C low pressure switch signal circuit needs to be grounded and the A/C request signal circuit needs to have 12 volts applied to it from the HVAC control assembly. A 12-volt reference signal is sent out over the A/C request signal circuits from the HVAC control assembly, through the A/C high pressure switch and then to the PCM when the vehicle operator makes an A/C request. A separate 12-volt reference signal is sent out over the A/C low pressure switch signal circuit, through the A/C low pressure switch and the ground circuit.

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 18 psi (124 kPa) or more than 49 psi (338 kPa).
The A/C high pressure switch pressure is more than 420 psi (2896 kPa).
Engine speed is more than 5500 RPM.
Engine Coolant Temperature (ECT) is more than 250°F (121°C).