Circuit Description
A/C Request Circuit -- When the HVAC controls are in any position that enables the A/C compressor, and the A/C compressor refrigerant switch is closed (low and high), the A/C Request circuit is connected to B+. The ECM energizes the A/C compressor control circuit when a high signal voltage is detected on the A/C Request circuit. The ECM will de-energize the A/C compressor control circuit during the WOT maneuvers, when the ECT exceeds a predetermined threshold or when the A/C Request circuit signal voltage goes low (either by a pressure switch opening, or the HVAC system is in a non-A/C mode).
A/C Load Signal -- The ECM also receives input on the A/C load signal circuit. This is accomplished through a pressure switch connected to B+ which is normally open. During high ambient temperature conditions or any other conditions which would cause very high refrigerant pressures, the switch closes, sending a B+ signal to the ECM. The ECM will then raise the idle speed by approximately 50 RPM in order to help compensate for these conditions.
A/C Compressor Control Circuit -- The ignition voltage is supplied directly to the A/C Compressor Control relay coil. The ECM controls the relay by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the component being controlled. When the ECM is commanding a component on, the voltage of the control circuit should be low (near 0 volts). When the ECM is commanding the control circuit to a component off, the voltage potential of the circuit should be high (near the battery voltage).
The relay controls the high current flow to the A/C Compressor Clutch. This allows the ECM driver to only have to handle the relatively low current used by the relay.
Diagnostic Aids
An intermittent may be caused by the following:
| • | Corrosion |
| • | A mis-routed harness |
| • | A rubbed through wire insulation |
| • | A broken wire inside the insulation |
Refer to Symptoms for Intermittents.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||
Does the scan tool parameter toggle between Yes and No? | -- | No trouble Found. Refer to Diagnostic Aids. | ||||||||
3 | Does the scan tool always indicate Yes? | -- | ||||||||
4 | Disconnect the A/C Compressor Refrigerant switch. Does the scan tool display change to No with the switch disconnected? | -- | Go to Section 1, Heater, Ventilation and Air Conditioning (HVAC) | |||||||
5 |
Does the scan tool parameter change to Yes when the circuit is jumpered to B+? | -- | Go to Section 1, Heater, Ventilation and Air Conditioning (HVAC)
| |||||||
6 |
Was a problem found and corrected? | -- | ||||||||
7 | Check for a poor connections or poor terminal tension at the ECM harness connector. Refer to Electrical Diagnosis in Engine Electrical, and Trouble Shooting Procedures for the proper procedure. Also, refer to Checking Terminal Contact in Engine Control Module Diagnosis . Was a problem found and corrected? | -- | ||||||||
8 |
Important: This vehicle is equipped with a Theft Deterrent System which interfaces with the Engine Control Module (ECM). Reprogram the new ECM with the frequency code of the theft deterrent module currently on the vehicle. Replace the ECM. Refer to Engine Control Module Replacement/Programming . Is the action complete? | -- | -- | |||||||
9 |
Does the scan tool parameter toggle between Yes and No? | -- | The System is OK |
