An auto-dimming rear view mirror is a great convenience when there is too much light on the mirror. This is not only good for you as the driver, but also for people who are driving behind you. The mirror will dim to a level where the light does not reflect back in your eyes or the eyes of the person following you. This auto dimming feature works much like the small lever that could manually be used to change the angle of the mirror to dim the light.
After the rear view mirror has been in use for several years, things can start going wrong. Much like the other parts of the vehicle, they begin to fail with age. There are also times when the problems are not caused by age, but by faulty parts. If you are having problems with the auto-dimming feature of your rear view mirror, here are some tips to help you troubleshoot this yourself.
Nothing Works on Mirror
Most of the auto dimming rear view mirrors also have other electronic displays within the mirror itself. They can display the time, the temperature, and direction of travel. If nothing is working when you turn on the key, there is a problem with either the wiring or the actual mirror itself. Check the wire connections at the rear view mirror and wiring harness. Also check any fuses associated with the lighting.
Auto Dimming Feature Not Working but Other Displays Are
One of the common problems with the auto dimming rear view mirror is that they just will not dim. If the other displays are working, but the dimming function is not, you should check the sensors. Look at the back of the mirror and locate the sensors aligned there. Clean them off so that there is nothing blocking the sensors from the light. Shine a flashlight onto the sensors and see if the mirror dims. If not, then you may have a faulty sensor.
Dimming Feature Works Sometimes
There may be times when the mirror works like it should, and then others where it does not. The culprit could be a short in the wiring harness or a loose connection. Use a voltmeter on the wiring to see if you can find any fluctuations in the current.
Check Rear Window
If the dimmer does not work like it should when there is light shining on it from the rear of the vehicle, it could be something that is not related to the rear view mirror at all. Many vehicles today are sold with a factory tinting on the windows. If someone adds even more aftermarket tinting to the rear window, this will have a direct impact on the function of the auto dimming feature in the mirror. To check this, shine a light on the mirror from inside the car rather than outside.
Faulty Mirror Trigger
Besides the sensors on the outside of the rear view mirror, there is also a trigger on the inside that tells the mirror to dim. When this fails, the mirror will not dim like it should. Test this with a voltmeter to see if it is working.