Principles of Operation (How Does it Work?)

Exterior Mirrors
The exterior mirrors are controlled by the exterior rear view mirror control switch. The switch supplies power and ground to the selected mirror motor in order to adjust to the desired position.

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

The heated mirror power is supplied by the back window defroster relay through a separate fuse junction panel fuse that isolates the heated mirrors from the heated back window in the event of a concern.

Automatic Dimming Mirror
The automatic dimming feature darkens the interior rear view mirror whenever the front ambient light sensor detects low outside light and glare reaches the light sensor (photo cell) located inside the mirror. The electrochromatic interior rear view mirror starts to return to the normal view once the glare goes away.

The automatic dimming mirror reverse input is supplied by a hard-wired circuit to the REM. When REVERSE is selected the REM will receive the PRNDL message over the 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.