Principles of Operation (How Does it Work?)

Exterior Mirrors
The exterior mirrors are controlled by the exterior rear view mirror control. The exterior rear view mirror control is hardwired to the Driver Door Module (DDM). When the driver exterior rear view mirror is selected, adjustment is carried out by moving the control to the desired position. The DDM receives the adjustment input command from the exterior rear view mirror control and supplies power and ground to the appropriate exterior rear view mirror motor to adjust the exterior rear view mirror. The passenger exterior rear view mirror is controlled by the Front Electronics Module (FEM). When the DDM receives an adjustment command for the passenger exterior rear view mirror, the DDM interprets the command and sends the adjustment command over the SCP communication network to the FEM. The FEM interprets the command and supplies power and ground to the appropriate exterior rear view mirror motor to adjust the exterior rear view mirror.

Memory Operation
The DDM and FEM constantly track mirror position. Each exterior mirror motor is equipped with a potentiometer, which the modules monitor to track the mirror position. This position remains in memory as long as the module retains power. When a position is stored in memory by the operator, the module will retain this position in memory for future recall. When a memory position is recalled by the operator, the DDM and FEM will power the exterior mirror motors simultaneously and monitor the potentiometer circuits. When the stored memory position is reached, the DDM and FEM will remove power from the motors. If the exterior rear view mirror control is operated during a memory recall, the modules will stop the memory recall and respond to the exterior rear view mirror control command.

Heated Exterior Mirrors
The heated exterior mirrors will only function when the rear window defrost is operating. There is no separate control for heated exterior mirrors, they will function anytime the rear window defrost is ON. For additional information on the rear window defrost, refer to Windows.

The heated mirror power is supplied by the rear window defrost relay through a separate battery junction box fuse that isolates the heated mirrors from the rear window defrost in the event of a concern.

Automatic Dimming Mirror
The automatic dimming mirror will tighten or darken depending on the input of the sensors that are internal to the mirror.

The automatic dimming mirror reverse input is supplied by a hardwired circuit to the Rear Electronic Module (REM). When REVERSE is selected, the REM will receive the PRNDL message over the SCP communication network from the PCM stating reverse has been selected. The REM will then supply a power signal to the automatic dimming mirror causing the automatic dimming mirror to cancel any dimming function currently operating and return to a full bright status. When the gear selector is moved out of REVERSE, the automatic dimming mirror returns normal operation and resumes any interrupted dimming functions.

If the REM does not receive the PRNDL information from the PCM, a communication DTC will be logged. As a default, the REM will supply power to the dimming circuit preventing the automatic dimming function from operating until the message and communication is restored.
The reverse message supplied to the REM has a time delay before the message is sent by the PCM. This delay allows for the gear selector to be moved through REVERSE without canceling dimming operations.

This feature is configurable. Make sure the vehicle was built with this function enabled before diagnosing.

Switched System Power (SSP)
The SSP is invoked by both the FEM and REM. This function removes power from relays that provide power to the exterior lamps, interior lamps, and power door locks. This is only accomplished when both the FEM and the REM are in sleep mode. The sleep function of the FEM and REM places the modules in sleep mode when the ignition switch is in the OFF position, and no wake up (inputs) signals occur for 30 minutes. The module will not sleep if the parking lamps or the hazard lamps are active. The following relays are controlled by the SSP function: SSP1, SSP2, SSP3, and SSP4. When either the FEM or REM are not in sleep mode, all SSP relays will be energized. Each relay, when energized, will supply power to multiple features/functions. For additional information, refer to the SSP Relay Index for diagnosis and testing of SSP-related issues. Component Tests and General Diagnostics