P0645

Diagnostic Chart:

Schematic:

The A/C relay is located in the underhood fuse block (UHFB). The 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 CAN link from the 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 P0645 will set if the ECM detects an open A/C relay coil control circuit when A/C relay is commanded Off with engine cranking/running.

DTC PARAMETERS
DTC P0645 will set if A/C relay feedback voltage is between 2.6 and 4.6 volts (ECM pull-up voltage) indicating an open circuit when A/C relay is commanded Off when:
^ Condition exists for longer than 0.5 seconds
^ Engine speed is greater than 40 rpm
^ Main relay voltage is between 8.0 and 18.0 volts

DTC P0645 diagnostic runs continuously once the above conditions have been met.

P0645 is a type D DTC.

DIAGNOSTIC AIDS
To locate an intermittent problem, use Scan tool to monitor DTC P0645 LAST TEST with engine running and A/C button (on console) Off. Wiggling 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.

A/C relay coil resistance: 70-115 ohms