DTC P0645, P0646, or P0647: Air Conditioning (A/C) Clutch Relay Control Circuit: Notes

WARNING: This page is about a different car, the 2004 Pontiac Aztek and 2004 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.

Ignition voltage is supplied directly to the air conditioning (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.