What is this on the back of my rear view mirror?

[Techna]

My truck has ACC, all the collision warning/avoidance features, auto dimming functionality and rain sensing wipers. I understood (perhaps incorrectly) that the sensors for these features were behind the housing mounted on the windshield. If I’m right, what is this here for? I’m considering replacing the rear view mirror or possibly modifying the existing one, but would like to know more before I start making final decisions and implementing changes.

 

[scottmoyer]

I could be wrong, but that might be the forward light sensor for either auto dimming the mirror, or if equipped, it could be for auto dimming the high beams. Being in the mirror, I'm thinking it has more to do with light sensing for the auto dimming mirror.

My '87 Camaro has one of the first auto dimming technologies that didn't require you to manually flip the mirror. It has a sensor on the front and rear of the mirror. If it's darker to the front than the rear, the mirror dims. If there's more light to the front than the rear, it returns to normal.

 

[Techna]

I could be wrong, but that might be the forward light sensor for either auto dimming the mirror, or if equipped, it could be for auto dimming the high beams. Being in the mirror, I'm thinking it has more to do with light sensing for the auto dimming mirror.

My '87 Camaro has one of the first auto dimming technologies that didn't require you to manually flip the mirror. It has a sensor on the front and rear of the mirror. If it's darker to the front than the rear, the mirror dims. If there's more light to the front than the rear, it returns to normal.

I appreciate the input. Wouldn’t have imagined that it’d have anything to do with mirror dimming since the sensor is on the front of the mirror, but the light differential explanation makes sense.

 

[stevj]

There is a light sensor in the mirror housing that faces forward, the one in the picture, and one that is part of the mirror that faces to the rear.
The difference of light intensity between forward and rearward facing sensors determines when and how much dimming to apply to the mirror(s).

Steve

 

[Jimmy07]

I appreciate the input. Wouldn’t have imagined that it’d have anything to do with mirror dimming since the sensor is on the front of the mirror, but the light differential explanation makes sense.

That is, indeed, the sensor for auto dim, so it can get the light differential with the one on the glass.

 

[Techna]

This helps a great deal and I won’t miss that feature moving forward. Thanks for clearing it up.

 

[BobK]

Yes, and if your like me and have one of these type of cameras, even though the front facing sensor is not blocked, the auto dimming will not work.

 

[Techna]

Yes, and if your like me and have one of these type of cameras, even though the front facing sensor is not blocked, the auto dimming will not work.
View attachment 82071

I’m installing something similar but my mirror dimming function is probably no longer useful anyway. I put a cap on the truck and between the cap’s privacy glass and then the truck’s privacy glass, very little light makes it through. My rear view mirror isn’t quite useless, but it’s close enough that digital mirror is in the very near future. I’d been considering one that replaces the entire factory mirror, but don’t really want one with an attached front camera due to it being obstructed somewhat by the large housing covering all of the electronics on the front glass. The one I'm looking at closely is the Pormido D50 which has input for three cameras (front is detached) but is strapped to the existing mirror rather than replacing it. That’s not a deal killer, but it’s not really what I want either so fabricating a ball/socket mount to attach to the Pormido may be an option. I did just notice a small hole on the underside of the truck’s mirror and am now left wondering is that’s a microphone access point?