When the rear defog input circuit 292 to the body control
module (BCM) is momentarily switched to ground (as when the rear defog switch is pressed),
the BCM will respond by grounding circuit 193, the coil of the rear defog relay.
This action energizes the rear defrost relay providing power to circuit 293
(rear defrost heater elements), circuit 683 (heated mirror relay coil) and circuit 683
(rear defrost LED telltale). Momentarily switching circuit 292 to ground again will
result in the BCM turning the rear defrost relay OFF.
The first time in an ignition cycle that the rear defrost is activated, rear
defrost is activated, rear defrost will remain ON for 15 minutes or until the
ignition switch is turned OFF. If rear defrost is activated again during the same
ignition cycle, rear defrost will remain ON for 7 1/2 minutes or until the
ignition switch is turned OFF.
DTC Parameters
| • | Circuit 193 is shorted to battery voltage. |
| • | The BCM is commanding the rear defog system to the ON state. |
Diagnostic Aids
| • | When a shorted to battery voltage condition exists in circuit 193 and
the BCM has been requested to activate the rear defrost system, the BCM will attempt
to switch the circuit 193 to ground. High current flow will result and the BCM output
will go into a protective state. The BCM output will not allow itself to be activated
for as much as 3 minutes. |
| • | Inspect the wiring harness for damage. Check for broken or chaffed insulation. |
| • | If fault is suspected to be intermittent, wiggling the harness wiring
may help in locating the fault. |
