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 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. When the refrigerant high side pressure reaches approximately 1,200 kPa, 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 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
| • | This switch only closes when the refrigerant high side pressure reaches approximately 1,200 kPa. Proper switch operation cannot be verified if the ambient conditions will not allow this system pressure to be achieved. |
| • | An intermittent may be caused by the following: |
| - | Corrosion |
| - | A mis-routed harness |
| - | A rubbed through wire insulation |
| - | A broken wire inside of 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? | -- | ||||||||
2 |
Does the scan tool indicate Heavy? | -- | ||||||||
3 | Disconnect the A/C Load switch. Does the scan tool display change to Normal with the switch disconnected? | -- | ||||||||
4 |
Does the test light illuminate? | -- | ||||||||
5 | Use a fused jumper in order to jumper between the two terminals in the A/C Load switch harness connector. Does the scan tool parameter change to Heavy when the switch is jumpered out? | -- | ||||||||
6 | Check for any poor connections or poor terminal tension at the A/C Load switch harness connector. Refer to Section 8A, Troubleshooting Procedures, Checking Terminal Contact in Engine Control Module Diagnosis for the proper procedure. Was a problem found and corrected? | -- | ||||||||
Does the scan tool parameter change to Heavy when the high side pressure reaches the specified value? | 1,200 kPa (174 psi) | No trouble found. Refer to Diagnostic Aids | ||||||||
8 |
Was a problem found and corrected? | -- | ||||||||
9 | Repair the open/high resistance in the ignition feed circuit to the switch. Is the action complete? | -- | -- | |||||||
10 | Replace the switch. Is the action complete? | -- | -- | |||||||
11 | Check for any 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? | -- | ||||||||
12 |
Important: This vehicle is equipped with a Theft Deterrent System which interfaces with the Engine Control Module (ECM). Program the new ECM must 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? | -- | -- | |||||||
13 |
Does the scan tool indicate Heavy? | -- | ||||||||
14 | Disconnect the A/C Load switch. Does the scan tool display change to Normal with the switch disconnected? | -- | ||||||||
15 | Use a fused jumper in order to jumper between the two terminals in the A/C Load switch harness connector. Does the scan tool parameter change to Heavy when the switch is jumpered out? | -- | The System is OK |
