Chart 9 Rear Defogger Inoperative

CIRCUIT DESCRIPTION:
The rear defogger is activated by the "REAR DEFOG" button on the ECC panel. when the button is pressed, a serial data signal is sent to the BCM on CKT 800. The BCM calculates the "ON" time for the defogger and sends a serial signal to the programmer. The circuits, in the programmers then supply the ground for the rear defogger relay. The rear defogger relay, when activated, allows current flow to the defogger elements on the back glass

TEST DESCRIPTION:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Checks to see if the BCM is able to send and receive serial data between the HVAC programmer ECC panel.
2. Checks to see if the rear defog button is functional by using it to exit diagnostics.
3. Checks for power at both relay terminals.
4. Checks for an open in CKT 291 or faulty programmer. The test light should normally light when the rear defog button is pressed, showing that the serial signal was processed and the ground was supplied by the programmer to the relay on CKT 291.
5. Determines whether the fault is in the relay or in the output to the defogger or ground. Normally, by jumping the terminals, the defogger will operate if the circuits are complete. See "ELECTRICAL DIAGNOSIS" for power flow at connector locations.