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how do auto-dimming mirrors work?

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Old 12-06-2001, 10:29 PM
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how do auto-dimming mirrors work?

i know about the light sensor but how do mirrors get darker? someone mentioned looking for the thick black border around the glass (like on the driverside mirror) and that there is a liquid. can someone please explain the technology to me? thanks

also, my c230k coupe doesn't have the auto-dimming mirror on the passenger side. is this true of other models? how about the sedans?
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Old 12-07-2001, 02:14 PM
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Electrochromic Mirrors:
Sensors in the rearview mirrors compare the intensity of light reflected in the mirror with that of the surrounding light. A large difference indicates glare, which is then reduced automatically by changing the mirror's reflectivity. This is accomplished by changing the electrical current being sent to an electrochromic element in the mirror (which is similar to a liquid-crystal display in a digital watch).
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Old 12-07-2001, 06:32 PM
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The Driver's outside rearview mirror doesn't dim as much as the inside mirror. I think it works, it's just not as dark as the inside. Should they be the same light intensity? If so, does that mean I have a problem? I did bump into my outside mirror (hard) recently while washing. I can't imagine it's that sensitive to breakage, but you can tell I'm worried.
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Old 12-07-2001, 06:41 PM
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If the light intensity near the outside and inside and rear view mirrors are different then the degree of dimming should also be different.
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Old 12-07-2001, 07:46 PM
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thanks for the explanation, renok7.

i assumed that there was only one light sensor - in the rear view mirror, and that that was tied into the side mirror - hence the reflectivity should be similarly reduced. is this not correct?

MB-BOB, are both your side mirrors auto-dimming?
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Old 12-07-2001, 10:04 PM
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You're right Young.
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Old 12-07-2001, 10:56 PM
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Originally posted by young
thanks for the explanation, renok7.

i assumed that there was only one light sensor - in the rear view mirror, and that that was tied into the side mirror - hence the reflectivity should be similarly reduced. is this not correct?

MB-BOB, are both your side mirrors auto-dimming?
Young, No. On the C-Class sedan, only the driver's door and inside mirror are auto-dimming (the pass. door mirror does not). I think it should be the same on the Coupe.

This actually makes less sense to me now. Light directly hits the Left outside mirror from behind, but light has to go through the rear window (5% transmission loss, stock). Plus if the rear window is tinted, even less light falls on the inside rearview. Mine are tinted (35%)

So, I'm thinking the outside mirror, receiving more direct light, would dim MORE than the inside mirror receiving less light. But I'm observing just the opposite. My inside mirror dims MORE. I confused...
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Old 12-07-2001, 11:28 PM
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mb-bob, is your side window similarly tinted? 35% - if not, i think the side mirror would be brighter. also, it could be the angle you have the mirrors set to. more direct light will seem brighter. the side mirror is lower than the rear view so it will seem brighter.

having said all that, i'll have to check to see if i notice a significant difference between the two mirrors next time. i'll let you know then.

yes, the coupe has only the driver side mirror auto-dimming. i thought bmw 3's had both but they just have that bluish tint. i think the audi a4s have both side mirrors auto-dimming.

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Old 12-08-2001, 12:38 PM
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Young, I have 35% tinting on all windows (except windshield, of course). Looking out through the driver's window, the driver's side mirror still is significantly brighter than the inside mirror.

Maybe I misunderstood the earlier posts. I thought that each mirror had its own sensor. I can see the sensor on the inside mirror, of course, but not outside. If we knew how many sensors and their location, maybe that would help.

Yes, I agree that the angle of the mirrors would affect how much light each one measures, but I'm still noodling through that issue, too.

Fun topic, huh?
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Old 12-08-2001, 07:43 PM
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You could try covering the rear view sensor with some thick black tape and drive around to see if the outside mirrors continue to dim. If it does, theres another sensor somewhere!
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Old 12-08-2001, 11:36 PM
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Kenny, you have 'a keen sense for the obvious' !!!

Ok, is this a bug or some misinterpretation by me.

On the C230K, there is a switch for the dome light.

The side on the right shows a door open.

I think that this would mean the light is door operated.

On the left is an icon of an illuminated light.

I think this would mean keep the light on irregardless of door position.

These seem to be reversed, and in fact the middle position seems to be door operated lighting.

Anyone else notice this anomaly?
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Old 12-10-2001, 08:02 PM
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Originally posted by MB-BOB
Young, I have 35% tinting on all windows (except windshield, of course). Looking out through the driver's window, the driver's side mirror still is significantly brighter than the inside mirror.

Maybe I misunderstood the earlier posts. I thought that each mirror had its own sensor. I can see the sensor on the inside mirror, of course, but not outside. If we knew how many sensors and their location, maybe that would help.

Yes, I agree that the angle of the mirrors would affect how much light each one measures, but I'm still noodling through that issue, too.

Fun topic, huh?
mb-bob, i just checked the two mirrors and to me, they look almost identical in reflectivity. i forgot to block the light sensor to see if that would affect the side mirror anyway, i think they are tied together so your mirror may have indeed have a loose connector or something due to the bump. i think you should have it checked out next time you goto the dealer and leave out the bump part
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Old 12-12-2001, 12:27 AM
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young, thanks for the help. I thought that the mirrors were equal when I first got the car. But I don't drive it at night very often and it's been a while now (6-mos) since new.

I'll add this item to the dealer list, and right, I'll leave off the bumping story. Thanks again!
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Old 12-12-2001, 10:34 PM
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no problem, mb-bob. i also just tested the sensor, and indeed, the one in the rearview mirror controls the side mirror. very fast action also.
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Old 11-07-2016, 02:26 PM
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About auto dimming function

May i replace the driver side mirror with auto dimming feature for another without auto dimming in a 2013 C250 Sedan?.
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