For 1990-2009 cars

### Circuit Description

The A/C relay is located in the underhood fuse block (UHFB). The engine control module (ECM) controls the A/C compressor clutch by supplying a ground to the A/C relay coil whenever the ECM receives an A/C request over the controller area network (CAN) link from the body control module (BCM) with the A/C system pressure within operating range. When the A/C relay is commanded OFF, circuit not grounded, the voltage level at the ECM should be high, ignition voltage. When the A/C relay is commanded ON, circuit grounded, the voltage level at the ECM should be low. The ECM A/C relay feedback circuit uses a pull-up voltage, 2.6-4.6 volts, which allows the ECM to individually differentiate between an open, short to ground, or short to voltage. DTC P0646 will set if the ECM detects lower than normal feedback voltage when the A/C relay is commanded OFF with the engine cranking/running.

### Conditions for Setting the DTC

DTC P0646 will set if A/C relay feedback voltage is less than 2.6 volts when A/C relay is commanded OFF when:

• | The condition exists for longer than 0.5 seconds.

• | The engine speed is greater than 40 RPM.

• | The battery voltage is between 7.5-15 volts.

DTC P0646 diagnostic runs continuously once the above conditions have been met.

DTC P0646 is a type D DTC.

### Diagnostic Aids

Important: DTC P0646 LAST TEST on the scan tool will not update until a fault initially occurred in the ignition cycle.

To locate an intermittent problem, use the scan tool to monitor DTC P0646 LAST TEST with the engine running and the A/C button on the console OFF. Wiggling the wires while watching for a change from NOT RAN or PASSED to FAILED may locate the area where an open in the wiring may exist.

The A/C relay coil resistance is 70-115 ohms.