Component Diagnostic Charts/Tests

Chart C-10, A/C Compressor Clutch Control Electrical Diagnosis (1 Of 2):

Chart C-10, A/C Compressor Clutch Control Electrical Diagnosis (2 Of 2):

A/C Pressure Sensor Circuit:

CIRCUIT DESCRIPTION:
The A/C clutch control relay is ECM controlled to delay A/C clutch engagement for a fraction of a second after A/C is turned "ON." This allows the ECM time to adjust IAC counts before the A/C clutch engages, to compensate for the additional load on the engine and maintain the correct idle speed. The ECM also causes the relay to disengage the A/C clutch whenever the following conditions exist:

^ WOT operation.
^ High coolant temperature.
^ High engine oil temperature.
^ High engine speed.
^ High A/C pressure.

To determine when the A/C has been requested, the ECM applies a reference voltage through the pressure cycling switch, to the A/C control head. When the A/C switch is turned "ON", the reference circuit is grounded and the voltage in the circuit goes low (near zero), indicating A/C request to the ECM. This is displayed on a scan tool as "YES" when A/C request is selected. When the pressure cycling switch opens (low evaporator pressure), the circuit voltage goes high and A/C request is canceled.
When a request for A/C has been detected by the ECM, the ECM will ground the compressor clutch relay drive circuit, engaging the clutch.

The ECM reads the A/C refrigerant pressure sensor signal and will set trouble codes 66 &/or 67 if a failure in the sensor or sensor circuit is detected. An open, short to ground, or short to power in the sensor or signal wire will cause a Code 66. If the compressor clutch is engaged and there is no change in the pressure sensor signal, a Code 67 will set.

When A/C is requested, the ECM will also turn the cooling fan(s) "ON" unless vehicle is traveling fast enough to cool the radiator and condenser without fan assistance.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. Checks the ECM's ability to control the A/C clutch
2. Checks for grounded circuit 459 to the ECM.

DIAGNOSTIC AIDS
Before using Chart C-10, make sure there are no ECM trouble codes stored. The ECM will not allow A/c clutch engagement if there are stored Diagnostic Trouble Codes.