Compressor HVAC: Testing and Inspection

Steps 1 - 4:

Steps 5 - 15:

Steps 16 - 25:

Steps 26 - 34:

Steps 35 - 43:

A/C Compressor Control Circuit:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) receives an A/C request signal from the Body Control Module (BCM) over the Class 2 serial data line. When A/C is requested, the PCM provides a ground path to the control circuit of the A/C clutch relay. When the relay circuit is grounded, the A/C compressor clutch relay is energized. The PCM delays grounding the relay circuit for 0.3 second, so the PCM can adjust the engine idle RPM for the additional load.

The PCM will temporarily de-energize the A/C compressor clutch relay for one or more of the following conditions:
^ A hot engine restart.
^ The Throttle Position (TP) is more than 90 percent.
^ The engine RPM is more than 6000 RPM.
^ During an Idle Air Control (IAC) valve reset.
The A/C compressor clutch relay will remain de-energized when one or both of the following conditions exist:
^ A DTC P0530 is set.
^ There is no A/C request signal to the PCM.

DIAGNOSIS
A scan tool can be used to display the following parameters:
^ A/C Request-This displays YES or NO if the PCM has received an A/C request from BCM.
^ A/C Clutch-This displays if the PCM has commanded the A/C clutch on or off.
^ A/C High side-This displays the A/C refrigerant high side pressure in VOLTS and PSI.

DIAGNOSTIC AIDS
^ If DTCs P0530, U1016 or U1064 are set, do not use this table.
^ Refer to Function Test in HVAC for diagnosis of the refrigerant portion of the system.
Function Test - HVAC System - Manual
^ If the complaint is insufficient cooling, refer to Cooling Insufficient, A/C System in HVAC.
Cooling Insufficient - A/C System
^ An A/C refrigerant pressure outside the range of 43 to 428 psi, will cause the A/C compressor clutch to be disabled by the PCM. Observe the A/C refrigerant pressure parameter on the scan tool for 2 minutes with the engine idling and the A/C on. If pressure goes out of range, refer to A Diagnostic System Check - HVAC Systems - Manual in HVAC System.
Initial Inspection and Diagnostic Overview

TEST DESCRIPTION
The numbers below refer to the numbers on the diagnostic table.
3. The PCM will only energize the A/C relay when the engine is running or momentarily during cranking the engine in order to minimize the slugging noise. This step will determine if the relay or the control circuit is malfunctioning.
4. By disconnecting the relay, the system can be divided into 2 sections. If the clutch does not disengage, the malfunction lies on the compressor side of the system.
5. This step determines if the signal is reaching the PCM from the A/C control panel from the Body Control Module (BCM). The signal should only be present when an A/C mode or a defrost mode has been selected and the blower motor is turned on.
6. A test lamp that does not illuminate indicates that the A/C compressor clutch relay is malfunctioning.
8. This step determines if the system is normal or if the A/C refrigerant pressure sensor reading is out of range.
12. The A/C request signal is communicated through the BCM to the PCM over the Class 2 serial data line. The BCM also receives other information in order to decide if the A/C request should be enabled. This step tests for the A/C request switch for the correct function. Additional steps must be completed in order to perform this step correctly. The blower motor must be turned on and the heater, the vent, and the A/C control selector switch must not be in the off position. With the A/C selector switch in the off position, the scan tool should read A/C request off. If the A/C selector switch is in the on position, the scan tool may read Clutch enabled or the following conditions: Off, Low RPM - off, Low Ambient - off, High Press - off, Low Battery Voltage - off, High Coolant Temp. If any of the above conditions are present on the scan tool when the A/C selector switch is enabled, the BCM senses a change.
18. Connecting the ignition feed circuit and the A/C compressor clutch feed circuit together bypasses the PCMs control of the A/C compressor clutch. An A/C compressor clutch that fails to engage at this point indicates that the malfunction is in the A/C compressor clutch feed circuit, the compressor coil, or the compressor coil ground circuit.
19. Locate and repair any shorts that may have caused the fuse to open before replacement.
20. With the engine running and the A/C relay commanded on, the PCM should ground the A/C relay control circuit. This will also illuminate the test lamp.
34. The replacement PCM must be programmed and the Crankshaft Position System Variation Learn procedure must be performed.
36. This step determines if the A/C refrigerant pressure sensor is out of range causing the A/C compressor clutch to be disabled or if the A/C charge is to low to allow A/C operation.