Why Auto-dim Rear-view mirrors Fail in our cars

Discussion in 'MirrorJohn' started by MirrorJohn, Aug 3, 2014.

  1. 3 Aug 2014 at 1:52 PM #1

    MirrorJohn Sponsor

    Jul 31, 2014
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    BMW 2005 Estoril E46 M3 Coupe, Audi B7 RS4 Avant, Honda S2000 GT, Honda CBR600RR
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    E46 M3 & E39 M5 - Auto-Dimming Rear View Mirrors – How they work, How they break and How to Fix them


    Having fixed many of these mirrors over the years, I wanted to share some knowledge with fellow Cutters. I will give you some insights in to how and why these mirrors fail and some damage limitation if your mirror does stop working. Plus, assistance in repairing your mirror, saving you lots of money.

    If you own an E46 M3 (Coupe and Convertible) or E39 M5 and haven’t experienced a Mirror failure yet, chances are high that you will do at some point in your ownership. My mirror had already failed when I bought my car back in 2009. I didn’t even realise straight away that it was meant to dim/un-dim! I was lucky that although mine didn’t work, it hadn’t actually leaked out and damaged anything (yet). I caught it in time.

    History Lesson

    To begin, a little bit of History for you. The first version of an Auto Dimming Mirror that came out was actually a Prismatic Mirror. This is the same style of tilt mirror that a lot of cars have fitted as standard (they use the lever at the bottom to tilt):


    However, with these Auto-Dimming versions, there was a light sensor and a small motor inside that tilted the mirror up and down automatically. The mirrors were only available on very expensive cars at the time and became obsolete very quickly when the Electro-Chromatic Auto-Dimming Mirrors were developed. BMW was one of the first Car manufacturers to offer these mirrors on some of their cars. This is the same technology that is used in your mirror if you drive an E46 M3 or an E39 M5.

    There are two Companies that produce Auto Dimming Mirrors for 90% of the Car Manufacturers around the world. Magna Donnelly is one and if you are not familiar, Gentex is the other. The Companies actually originated from the same family and both have Headquarters in Holland. When the Electro-chromatic mirrors first came out, Magna Donnelly manufactured the majority of them. Over time and with a large quantity of these mirrors failing, the bias shifted and now Gentex manufactures the majority of these mirrors. At a guess Gentex now have approx. 80% of the whole Auto-dimming Mirror business Worldwide. BMW was using Magna Donnelly to manufacture their Auto-dimming mirrors, but jumped ship in 2007 and moved all their business to Gentex. They have stayed with them ever since and have experienced a much lower failure rate. Unfortunately our cars have Magna Donnelly Mirrors fitted, so we are left holding the baby so to speak.

    How an Auto-dimming mirror works

    An auto-dimming mirror is comprised of two pieces of glass that are sealed together along the edges and then metal contact strips attached:


    An electronic circuit board is attached to the rear of the mirror with wire leads soldered to the edges of the glass:


    A conductive liquid or gel is then injected and completely fills the area between the two pieces of glass and then sealed at the injection point. The back of the glass is coated with a silver mirror coating and under normal (non dimming) conditions the fluid/gel is completely invisible and transparent. It works just like a normal mirror should, reflecting all light. The mirror housing contains the electronic circuit board and photo-sensors that measure ambient light, one facing toward the front of the car and one facing out towards the rear window. The forward facing sensor tells the mirror whether it's day or night and enables or disables the auto dimming function accordingly. If a sufficient amount of light is hitting the forward facing sensor the mirror will conclude that it's daytime and will not dim the glass at all, regardless of what the rear facing sensor sees. When the front facing sensor see reduced levels of light the electronic circuit board will begin applying very low levels of voltage to each piece of glass and the conductive fluid/gel will begin to darken. The level of tinting is varied almost infinitely depending on the conditions. At night the mirror will be dimmer than during the day, however, when the rear facing sensor detects light (from a cars headlights behind) the mirror will actually dim even darker. This effectively reduces the amount of light that hits the mirror coating and eliminates glare. You may also notice your mirror dims if you drive under tree covered roads or through a dark tunnel. The level of dimming is constantly adjusted depending on light conditions. Another neat trick, is that if you put your car in to reverse at night, the mirror automatically un-dims, so that you can see where you are going! It’s all rather clever really!

    If you are unsure if your mirror is working properly or not, the easiest way to test it is to completely cover the front (forward facing) sensor with your finger/hand. As long as the ignition is on/engine is running, you should see the mirror go dark within a couple of seconds. Make sure you also check the fuse that powers the mirror and make sure it isn't blown. ‘Fuse 21 iirc - in the glovebox. 5A’

    How and why these mirrors fail

    The vast majority of the mirrors that have experienced failure were produced between the late 90's and up to the mid 2000's by the Magna Donelly Corporation. They were supplier at the time for BMW's rear view mirrors and made all the mirrors found in our E46 M3’s and in the E39 M5’s. At the time, they used a liquid between the two pieces of glass. What happens is the seal along the edge of the glass is breached due to a failure in the material and air will begin to make it's way inside and produce a ‘bubble’ or discolouration in the mirror.


    The mirror will also stop dimming. It is worth noting that sometimes you don’t see any bubble at all. When mine failed, the glass looked perfect but it was effectively dimmed all the time. It made seeing things in the mirror quite hard as it was always so dark! The bubble on mine came later and then gradually got bigger and bigger!

    If you still have the old mirror which has not been repaired/refurbished and notice that the dimming feature no longer works, remove it immediately. In my experience, the order of occurrence is:

    1) Dimming feature stops
    2) Liquid appears behind glass
    3) Ruined interior

    The time between these steps might be hours, days, weeks, months or months, but I would replace it with the first symptoms. Sometimes it can miss all the tell-tale signs and leak suddenly and violently. One poor member on here was doing a track day with a perfectly fine working mirror. The next moment it leaked out all over his gear surround. Talk about putting you off your racing!

    Whether the liquid inside the mirror will begin to leak or how long it takes depends largely on where the breach occurred. If the breach is on the upper side of the mirror it may take a while for any liquid to make it to the bottom of the case and begin to leak out. If the breach is on the lower portion of the mirror the liquid will escape much more rapidly and since it's directly hitting the bottom of the case you often see it leak the same day the seal is breached. There's a popular misconception that the mirrors first fail and then begin to leak over time and this isn't always the case. Once the seal fails the liquid is escaping and simply following the laws of gravity. The environment that the car is in can have an impact on how long the mirrors last. Heat is known to accelerate the process, so if you own a Vert (and actually drive with the roof down) or leave your car parked in the sun on hot days, the mirror can fail sooner. It’s more prone to happening in the summer, but can happen at any time of the year. I have customers all over the world, including some very cold Norther European contries; so please dont think that heat is the cause of this. It is due to a manufacturing defect and all mirrors are affected. From the pictures below you can clearly see where the seal failed on this mirror - with a small amount of pressure the two pieces of glass completely separate.


    I don’t know why there is such a manufacturing issue with all these mirrors. You would think that Magna Donnelly would have resolved it over the 5 years that the E46 M3 was manufactured, but they didn’t. You can now see why BMW went over to Gentex to make all their Auto dimming mirrors in 2007. Although the numbers are considerably less, I have still repaired some mirrors from post 2007 cars including Audis and other BMW's (1 series and 6 series).

    Unfortunately with these Magna Donnelly Auto Dimming mirrors fitted to our E46 M3’s and E39 M5’s, it is not IF they will FAIL but WHEN! If you notice your mirror has stopped dimming, is permanently dimmed or (even worse) you have bubble(s) in it; the best thing you can do is remove it from your windscreen as soon as possible and get it out the car! Instructions for removing your mirror are further down the page. If there is any visible signs of the liquid leaking from the mirror clean it off right away and wrap the mirror with an absorbent towel to avoid or limit damage to the case. I would foolw my guide to split the mirror and remove the defective dimming glass unit as soon as possible. The longer you leave it, the less chance you have of saving your mirror. The liquid that is inside these mirrors is the equivalent of Alien blood!



    I am not being over-dramatic either! It will eat through the mirror casing, leaving a horrible mess:


    If you are unlucky enough that it leaks out the casing on to your centre console/gaitor/gear surround, it eats through that too. It really is nasty stuff!


    Many people are now switching on to this issue and fixing the problem before it occurs. Preventative maintenance is always best and it could actually save you money in the long run. If you do wait till your mirror does fail, it could be too late. It could leak suddenly without warning and end up costing you a fortune as you may need to replace the mirror and various trim parts in your car. It has happened to quite a few members now
    Last edited: Sep 15, 2017

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