GM 20852979 Mirror Assembly, Outside Rear View *Service Primer

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Most electric rear view mirrors use two motors to move the glass; one for up-and-down adjustments and one for left-to-right adjustments. Additionally, rear defoggers in some models have heated electrical glass circuits commonly powered by a rear window defogger relay. It has a selectable section that directs voltage towards either left or right mirror. With the ignition ON but the engine OFF, roll down the windows and operate the mirror control switch through all functions (left-right and up-down) for both the left and right side mirrors. Listen carefully for the sound of the electric motors running in the mirrors. In case you can hear motors but your mirror glass does not move, there is probably an issue with drive mechanism inside a mirror. Check if there is no movement from your mirrors and you cannot hear any sounds coming from them. If no sound is heard while the mirrors are dead, check out for the fuse. If fuse is good, take out your mounting mirror control switch without disconnecting connected wires. In models where power mirror switch is on instrument panel, remove the switch from instrument panel to probe behind it and see what's wrong with wires back there. In models where a switch sits in driver's armrest just pull it up with other switches together. Have ignition ON and make sure that voltage in provided at this time on one terminal only. If there is no voltage at this instance look for open or short wire between fuse panel and switch itself. Locate wire which goes from a switch to ground (negative terminal of battery). Leave it connected and connect jumper wire between this wire and ground; then check whether it will work fine now. A bad earth connection can be rectified if after connecting this wire back everything works correctly again; otherwise remove its cover in order to know if any voltage can be found on those wires with test light or not as you have already turned ignition ON and selector mirror switch being put to proper side as indicated. With the ignition ON and the mirror selector switch on the appropriate side, use a test light to check this wire for voltage. Move the mirror switch through all its positions. There should be voltage at one of the switch-to-mirror wires in each switch position (except the neutral "off" position). If there's no voltage in any of these positions check wiring between mirror and control switch for both open and short circuits. If it has voltage then you will need to remove mirror and test it off vehicle using jumper wires. If it fails, replace the mirror.

Start by disconnecting the negative cable from the battery.