When do heated mirrors come on?

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paul_c

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My car is a W203 C-class Estate (270 CDI Elegance SE). As part of its equipment, it has heated door mirrors. Now, my passenger side mirror seems to work in that it will demist in misty conditions, but the driver's side didn't work. So, I duly ordered a new mirror glass including the heating element.

I took off the old glass/element, and found it wasn't connected - well it was, only one connection (brown/grey) instead of two. Also I found a 4-terminal connector but one of these, the brown/grey, had another wire coming out the back of the block and terminated with a small female spade (which was connected to the mirror glass/element).

I looked up online and found a wiring diagram, which revealed the 2 connectors for the heating element are the brown/grey and red/black. So I added a little wire from the 4-way block to the existing glass/element. However, during testing I put a multimeter on the terminals, expecting 12V at some stage, but I got readings of nothing (0V). I tried with the ignition on, engine running, front & rear screen demister on, etc.

So I took off the passenger door mirror too (which I know does demist) and it too gave 0V readings. But it works!! So the question is.....

When does the glass element work?

I guess I can wait for the next misty/cold evening to see if my added wire has fixed the issue, but I was a bit disappointed that my multimeter couldn't check the wiring was okay etc.

I've also checked the relevant fuses, none were blown etc.
 
They come on automatically below 15c or you can put them on manually by switching on the heated rear screen
 
IIRC the mirror heaters come on with IGN on and engine running at temps below approx 15degC
 
The mirrors have a temp sensitive switch on the back so come on only when the temp drops to a certain point.
 
The mirrors have a temp sensitive switch on the back ...

Old school implementation, it used to be like that. The W203 does not have that any more but the door control unit drives the mirror heating element based on temperature info via CAN (from outside temp sensor).
 
Great - I'll try it when the temperature drops a bit. I did also try it with the heated rear screen on and got no voltage reading - hopefully it does actually work though.
 
IIRC the mirror heaters come on with IGN on and engine running at temps below approx 15degC

They come on automatically below 15c or you can put them on manually by switching on the heated rear screen

15C seems a bit high.

Is it 5C?
 
Its 9 or 15. They don't just come on to de-ice but to demist
 
"When do heated mirrors come on?"

Every fourth week.
 
Its 9 or 15. They don't just come on to de-ice but to demist

9 seems familiar, I work evenings and often drive home around 1am, on a few particularly cold nights I've noticed that the passenger one demisted but the driver's didn't.
 
WIS document (GF88.79-P-200T) states that switch on value is set via outside temp sensor BUT also the humidity (dew point) is used. It's not said which sensor detects humidity... IF dew point is not available, switch value is 13C. If temperature is implausible, mirror heat is on all the time.
 
WIS document (GF88.79-P-200T) states that switch on value is set via outside temp sensor BUT also the humidity (dew point) is used. It's not said which sensor detects humidity... IF dew point is not available, switch value is 13C. If temperature is implausible, mirror heat is on all the time.

Thanks for all the info so far. I've just checked, the car is saying its 14 deg C so I can't check them now, and in any case they're not misted up. I'll have to wait for poorer weather and see if my 'fix' has worked.

Don't know if to send back the cheap aftermarket replacement mirror for a refund, or just keep it for now - I bought it before I diagnosed the broken/missing connection because I feared breaking the old one in removing it (but I didn't).
 
Early MBs the mirror heaters were permanently fed and were switched on/off by a bi-metal switch mounted on the back of the mirror glass, the figure for these 4 degrees C.
The later cars mirror heaters are fed off of the CAN system and the figure for turning on/off I believe is 14 degrees C.
Your fault is almost certainly a wiring issue in the mirror, especially if they are the folding type, I have always found the mirror wiring of atrocious quality.
Looking at your post again you mention a 4-way plug, it sounds like your car had a dimming glass option and a previous owner modified the wiring so they could fit a standard mirror glass, from memory what colour are the other 2 cables on the 4-way plug? 2 pink wires, each with different trace colour by any chance.
 
Thanks for all the info so far. I've just checked, the car is saying its 14 deg C so I can't check them now, and in any case they're not misted up. I'll have to wait for poorer weather and see if my 'fix' has worked.


I forgot to mention that that WIS applies for W211...

If W203 system is similar, what if you unplug outside temp sensor, you are getting -117 reading or something --> mirror heat on... ? Well, maybe better wait colder nights. :cool:
 
Early MBs the mirror heaters were permanently fed and were switched on/off by a bi-metal switch mounted on the back of the mirror glass, the figure for these 4 degrees C.
The later cars mirror heaters are fed off of the CAN system and the figure for turning on/off I believe is 14 degrees C.
Your fault is almost certainly a wiring issue in the mirror, especially if they are the folding type, I have always found the mirror wiring of atrocious quality.
Looking at your post again you mention a 4-way plug, it sounds like your car had a dimming glass option and a previous owner modified the wiring so they could fit a standard mirror glass, from memory what colour are the other 2 cables on the 4-way plug? 2 pink wires, each with different trace colour by any chance.

The 4 wires were:

red/black
green/black
blue/black
brown/grey
 
paul_c said:
My car is a W203 C-class Estate (270 CDI Elegance SE). As part of its equipment, it has heated door mirrors. Now, my passenger side mirror seems to work in that it will demist in misty conditions, but the driver's side didn't work. So, I duly ordered a new mirror glass including the heating element. I took off the old glass/element, and found it wasn't connected - well it was, only one connection (brown/grey) instead of two. Also I found a 4-terminal connector but one of these, the brown/grey, had another wire coming out the back of the block and terminated with a small female spade (which was connected to the mirror glass/element). I looked up online and found a wiring diagram, which revealed the 2 connectors for the heating element are the brown/grey and red/black. So I added a little wire from the 4-way block to the existing glass/element. However, during testing I put a multimeter on the terminals, expecting 12V at some stage, but I got readings of nothing (0V). I tried with the ignition on, engine running, front & rear screen demister on, etc. So I took off the passenger door mirror too (which I know does demist) and it too gave 0V readings. But it works!! So the question is..... When does the glass element work? I guess I can wait for the next misty/cold evening to see if my added wire has fixed the issue, but I was a bit disappointed that my multimeter couldn't check the wiring was okay etc. I've also checked the relevant fuses, none were blown etc.
Where did u find wiring diagram for mirror I have bought heated mirror and don't know which wires to use to connect with one that had anti dimmer
 
The 4 wires were:

red/black
green/black
blue/black
brown/grey

Brown /Grey and Red / Black = Mirror Heater. The other two are the indicator

Although the diagrams show the indicators as being striped with yellow
 

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