P0646

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 P0646 will set if the ECM detects lower than normal feedback voltage when A/C relay is commanded Off with engine cranking/running.

DTC PARAMETERS
DTC P0646 will set if A/C relay feedback voltage is less than 2.6 volts 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 P0646 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 a short to ground in the wiring may exist.

A/C relay coil resistance: 70-115 ohms