BMW M3 (e46) & M5 (e39) AutoDim Rear View Mirrors
BMW was one of the first Car manufacturers to offer Electro-Chromatic Auto-Dimming Rear View 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 due to many failures jumped ship in 2007 and moved all their business to Gentex. They have stayed with them ever since and have experienced a very low 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 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 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) Liquid Leaks & Ruins interior
The time between these steps might be weeks, months, or years, but I would replace it with the first symptoms.
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 may 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. 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.
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. 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, it eats through that too. It really is nasty stuff!