HVAC System - Automatic

DTC P0645

Ignition voltage is supplied directly to the A/C compressor clutch relay. The powertrain control module (PCM) or engine control module (ECM) controls the relay by grounding the control circuit of the A/C clutch relay through an internal solid state device. This device is called a driver. The driver supplies the ground for the component that is being controlled. Each driver has a fault line which is monitored by the PCM/ECM. When the PCM/ECM commands a component ON, the voltage of the control circuit should be near 0 volts. When the PCM/ECM commands the control circuit to a component OFF, the voltage potential of the circuit should be near battery voltage. If the fault detection circuit senses an unexpected voltage, this DTC will set.

The PCM/ECM will monitor the control circuit to the A/C clutch relay for the following conditions:

* A short to ground
* A short to voltage
* An open circuit
* An open relay coil
* A relay coil that is shorted internally or which has a very low resistance

If the PCM/ECM detects any of these malfunctions, this DTC is set, and the affected driver is disabled.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0645 Air Conditioning (A/C) Clutch Relay Control Circuit.

Conditions for Running the DTC

The ignition voltage is between 9-18 volts.

Conditions for Setting the DTC

* A short to ground, an open circuit, or a short to battery voltage is detected on the control circuit to the A/C clutch relay.
* The condition is present for at least 30 seconds.

Action Taken When the DTC Sets

* The PCM/ECM will not illuminate the malfunction indicator lamp (MIL).
* The PCM/ECM will store those conditions which were present when the DTC set. These conditions are recorded as Failure Records data only. This information will not be stored as Freeze Frame data.

Conditions for Clearing the DTC

* A history DTC will clear after 50 consecutive fault-free ignitions have occurred without a malfunction.
* The DTC can be cleared by using a scan tool.

Diagnostic Aids

Important: Inspect PCM/ECM engine grounds for being secure and clean.

If DTC P0645 cannot be duplicated, review the Failure Records. Observe the vehicle mileage since the diagnostic test last failed. This may help to determine how often the condition which caused the DTC to set occurs. This may assist in diagnosing the condition.

If the condition is not present, refer to Testing for Intermittent Conditions and Poor Connections in Diagnostic Aids. Component Tests and General Diagnostics

Test Description

Step 1 - Step 13:

The numbers below refer to the step numbers on the diagnostic table.
2. Listen for an audible click when the relay operates. Command the ON and the OFF states. Repeat the commands as necessary.
12. The PCM/ECM utilizes electrically erasable programmable read-only memory (EEPROM). Program the new PCM/ECM before you replace the PCM/ECM.