Chart 4

Diagnostic Chart 4:

Schematic 4:

The ECM contains an output driver module (ODM) which controls the main relay coil circuit ground. The ODM monitors the feedback voltage level on circuit 5069 at the ECM when the device is being commanded On or Off. When the device is commanded Off (circuit not grounded), the voltage level at the ECM should be high (ignition voltage). When the device is commanded On (circuit grounded), the voltage level at the ECM should be low. Chart 4 diagnoses a high circuit voltage fault if the ECM detects a higher than normal feedback voltage when the main relay is commanded On.

DTC PARAMETERS
DTC P1640 will set if main feedback voltage is greater than 3.5 volts when main relay is commanded On when:
^ Condition exists for longer than 6 seconds
^ Engine is running

DTC P1640 diagnostic runs continuously with engine running.

P1640 is a type B DTC.

DIAGNOSTIC AIDS
To locate an intermittent problem, use Scan to command A/C RELAY On while monitoring MAIN RELAY CKT SHORT TO VOLTS. When a fault (short to voltage or shorted solenoid) exists, the Scan tool will display YES.

Main relay coil resistance: 70-115 ohms