Diagnostic Aids

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The air conditioning (A/C) compressor clutch will not engage under the following conditions:

The A/C high side line pressure is over 2 957 kPa (429 psi).
The A/C low side line pressure is under 138-172 kPa (20-25 psi).
The throttle angle is at 100 percent.
The engine speed is more than 5,500 RPM.
The engine coolant temperature (ECT) is more than 121°C (250°F).
The ambient air temperature (AAT) is less than 5°C (40°F).
The engine is idling at a low unstable RPM.

Compressor clutch engagement cannot be used to determine the status of the low pressure switch. The low pressure switch is one of several inputs to the HVAC control module for A/C request authorization. A/C request is one of several inputs to the powertrain control module (PCM) that control A/C compressor clutch engagement.

To accurately determine what pressure the A/C low pressure switch opens and closes at use Kent Moore GE-47742 which will allow the technician to measure the switch point pressure at the switch.

Using a scan tool, monitor the "low pressure switch" status while monitoring the "low side" pressure at the switch to determine the switch points of the low pressure switch.

The low pressure switch "connector seal" must be removed before plugging it into the switch for testing. The "plunger effect" of plugging the connector with seal into the switch induces a pressure on the back side of the switch, this pressure will skew the opening/closing characteristics of the switch 5-10 psi until the pressure bleeds off. The time required for the connection induced pressure to bleed off can take 20 minutes or longer.