W166 Heated Mirrors

Diesel Benz

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Remove the mirror glass and measure the heating element resistance. Don't need to guess if it is broken.

Find some youtube videos showing how to remove the mirror glass, pretty easy.
 

Major774

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On the back of the mirror (driver’s side) the wiring for the auto dimming and heating features are separate. You can just leave the auto dimming plug unplugged, it will work just fine.
 
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SuffolkSifter

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Thanks Andy,

My mate says the same, maybe a fuse or relay. I believe the auto dim interior mirror is working. I also think that both mirrors failed sounds suss. Trouble is that I don't know much about the workings of the mirrors and with the lack of a workshop manual or a fuse position schematic don't know where to start.

So...

A) Where is the fuse or relay for the heated mirrors?
B) How do I remove the mirror glass without breaking it?

From a www search it looks like the W166 mirrors have two wires for heating and a round connector for the auto dimming.
 
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SuffolkSifter

SuffolkSifter

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Remove the mirror glass and measure the heating element resistance. Don't need to guess if it is broken.

Find some youtube videos showing how to remove the mirror glass, pretty easy.
All I can find is to adjust the mirror fully down, get fingers behind it and pull.
 

Diesel Benz

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Thanks Andy,

My mate says the same, maybe a fuse or relay. I believe the auto dim interior mirror is working. I also think that both mirrors failed sounds suss. Trouble is that I don't know much about the workings of the mirrors and with the lack of a workshop manual or a fuse position schematic don't know where to start.

So...

A) Where is the fuse or relay for the heated mirrors?
B) How do I remove the mirror glass without breaking it?

From a www search it looks like the W166 mirrors have two wires for heating and a round connector for the auto dimming.

As Andy already mentioned earlier in this thread, there is only one common fuse for the door control unit. If it was this fuse, you would lose a lot of the door functions, mirror adjustment, power windows etc.

Google should find you the wiring diagrams. pe72.29-p-2101-97naa for the left door, pe72.29-p-2102naa for the right door (at least for LHD cars, should cover both).
 
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SuffolkSifter

SuffolkSifter

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I have yet to sort this issue out with winter etc. I think that I will pull off the cheaper passenger side mirror (in case i break it), I have a cheap non original heated mirror to test/replace it. If it works, in theory the wiring to the drivers side should work and I will plug in the NS glass to confirm. As for getting a dimming/heated mirror from other than Merc is proving difficult. Need to find a breaker.
 
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SuffolkSifter

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My real cheap passenger side mirror arrived this morning. Mirror came off OK, but the cheapy had the heater tabs in a different place and had nowhere to fit the two metal locating arms, but fitted back OK. I tested the heating circuit and it was open so was faulty. Tested voltage at the wires and had 12v there. But what I found is that I have 12v there all the while. If I lock the doors there is 12v present for around 12 seconds then it drops. Unlock and 12v is there then drops off, is it some sort of test? The temp showing at the time was 13.5c, so what is telling the mirrors to heat?

Removed the drivers side expensive dimming mirror and heating element is open circuit, bugger. But at least I have power at the mirror heating elements.

Just need to know how the power to the heated mirrors is tuned on.
 

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Diesel Benz

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Just need to know how the power to the heated mirrors is tuned on.

Do you mean how to turn power on to test if the (new) heating element works? Bring the car to my place, cold enough. The do come on automatically below a specific temperature as mentioned already. You can try to switch the rear window heating on. Usually mirror heating is also activated. The functional descriptions mention this for some MB models but not for all. I would assume this works for all relatively new models.
 
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SuffolkSifter

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Thanks DB, I have already tested for power at the mirror element and all is OK. It has power without the rear screen being on. What I would like to know is what are the conditions i.e. when a feed is sent to the heated mirror elements? Is it just a temperature level? If so what is the level?
 

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