For 1990-2009 cars
HVAC Components
A/C Compressor Controls

### Circuit Description

When the A/C button is pressed ON, battery voltage is supplied to terminal 2 of the A/C compressor refrigerant pressure switch. The A/C pressure switch is closed when the pressure in the refrigerant line is approximately 180 KPa (36 psi). With the pressure switch closed, battery voltage is supplied to terminal 58 connector C2 of the Engine Control Module (ECM). After receiving the A/C request input, the ECM grounds the A/C compressor relay coil which energizes the relay closing the relay contacts. With the relay contacts closed, the circuit is completed allowing battery voltage to be applied to the A/C compressor clutch coil. The A/C compressor clutch coil energizes and the compressor clutch will now engage.

The heater and A/C control monitors the ECM when it energizes or de-energizes the A/C compressor relay coil. The heater and A/C control monitors the circuit at terminal 9 connector C1. This circuit is in parallel with the control side of the relay coil.

### Conditions for Setting the DTC

• | The heater and A/C control monitors the request circuit for a short to ground every 40 ms on a continuous basis.

• | The request circuit stays at or near zero volts.

### Action Taken When the DTC Sets

• | The heater and A/C control stores a DTC 36 in memory.

• | The heater and A/C control internally disables the output.

### Conditions for Clearing the DTC

• | The conditions for the fault are no longer present.

• | Using a scan tool.

• | A history DTC will clear after 20 consecutive ignition cycles if the condition for the fault is no longer present.

### Diagnostic Aids

• | If the DTC is a history or an intermittent. Try to perform the test shown while moving wiring harnesses and connectors, this often can cause the malfunction to appear.

• | When moving related harnesses and harness connectors, visually/physically inspect wiring and connectors for the following:

- | Wire insulation that is rubbed through causing an intermittent short.

- | Wiring broken inside the insulation.

- | Poor mating of the connector halves or a terminal not fully seated in the connector body (backed out).

- | Improperly formed or damaged terminals. All connector terminals in the related circuits should be carefully reformed or replaced to insure proper contact tension.

- | Poor terminal to wire connection. Inspect for poor crimps, crimping over wire insulation rather than the wire.

• | Check the power and ground circuits before replacing the heater and A/C control.

### Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

1. Tests for voltage to the ECM and determines if the circuit is complete.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the HVAC Systems Diagnostic System Check? | -- | Go to Step 2 | Go to Diagnostic System Check
2 |

1. Disconnect ECM connector C2.
2. Turn ON the ignition, with the engine OFF.
3. Measure the voltage at terminal 58 while pressing the A/C switch ON.

Does the voltage measure near the specified value?

12 V

Go to Diagnostic Aids

Go to Step 3

3 | Test the A/C request circuit for a short to ground. Refer to Testing for Short to Ground in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 4 | Go to Step 5
4 |

1. Install a scan tool in order to clear DTCs.
2. Operate the A/C system within the Conditions for Setting the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 5

System OK

5 | Inspect for poor connections at the harness connector for the heater and A/C control. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 4 | Go to Step 6
6 | Replace and program the heater and A/C control. Did you complete the replacement? | -- | Go to Step 5 | --