211 heated door mirror question

[24 VRT]

Have now replaced broken mirror motor and fitted the replacement mirror following 'jousting' incident, and all is well - mirror moves in both planes, moves back to it memory position and the auto dimming on mirror works. However, for some reason the mirror heater seems to be on whenever the ignition in on, and doesn't go off (and no I haven't forgotten to turn off the HRW/rear screen demister).
I've heard that on some Mercs it comes on for a very short time on start up, but then goes off - I don't know if that is the case for mine - I haven't had her long enough to have misted/frosted windows and mirrors and hence be able to know. The replacement mirror gets gently warm from start up and stays that way, even after a 30 minute journey.
There were just the two spade connections to the back of the mirror, and I can't believe they needed connecting in a certain order - the power should flow either way through the heating element, and if there was a fault in the cables then surely it would go open circuit and thus there would be no heating?
The only thing I haven't tried is turning on the HRW to see if it turns off the mirror heating but if that was happening, it would indicate someone has played with the HRW wiring circuit, which I think is unlikely as there is nothing in the service history/paperwork to suggest this.
Has anyone any ideas or can shed some light on this for me?

 

[Trickythemerc]

There may be a thermostat in the mirror glass, certainly is one on my CLK that automatically turns the mirror demist on. Not sure at what temp it operates at though.

 

[24 VRT]

There may be a thermostat in the mirror glass, certainly is one on my CLK that automatically turns the mirror demist on. Not sure at what temp it operates at though.

Thanks for that - its about 14 degrees outside, so therefore I wouldn't have thought it should trigger. I'll move it to sit in the sun for a while and see if that stops it. If it is a faulty mirror glass thermostat, then I'll have to speak to MB in Truro who ordered it for me when I was down there. If mine has this thermostat, it may be another reason why the thing was so expensive!

 

[mersum1es]

Its dimming feature which pumps up price 5x... see offside mirror price vs. driver side... AFAIK the is no thermostat, heating is controlled by ecu according outside temp (nothing to do with rear demister). Though cant remember if there was max time set. I can try dig some WIS docs about mirror control...

edit: found. So probably your outside temp sensor is playing up...

GF88.79-P-2000T

The outside temperature is measured by the outside temperature
indicator temperature sensor (14) and forwarded to the driver's
side SAM control module with fuse and relay module (N10/1).

The latter relays this information to the CAN, class B bus. The
front left door control module (N69/1) and right (N69/2) evaluates
the information and switch the mirror heaters (M21/1r1 and
M21/2r1) ON or OFF accordingly.

In addition to the outside temperature, the humidity (dew point)
is also used for control. If the outside temperature value received is implausible, the mirror
heater is always switched on. If the dew point value is not
available, the switch-on value is set to 13 °C.

1 Outside temperature G2 Generator N69/1 Left front door control module
2 Circuit 61 (engine running) M21/1r1 Mirror heater N69/2 Right front door control module
M21/2r1 Mirror heater B14 Outside temperature indicator N10/1 Driver's side SAM control

 

[24 VRT]

Its dimming feature which pumps up price 5x... see offside mirror price vs. driver side... AFAIK the is no thermostat, heating is controlled by ecu according outside temp (nothing to do with rear demister). Though cant remember if there was max time set. I can try dig some WIS docs about mirror control...

edit: found. So probably your outside temp sensor is playing up...

GF88.79-P-2000T

The outside temperature is measured by the outside temperature
indicator temperature sensor (14) and forwarded to the driver's
side SAM control module with fuse and relay module (N10/1).

The latter relays this information to the CAN, class B bus. The
front left door control module (N69/1) and right (N69/2) evaluates
the information and switch the mirror heaters (M21/1r1 and
M21/2r1) ON or OFF accordingly.

In addition to the outside temperature, the humidity (dew point)
is also used for control. If the outside temperature value received is implausible, the mirror
heater is always switched on. If the dew point value is not
available, the switch-on value is set to 13 °C.

1 Outside temperature G2 Generator N69/1 Left front door control module
2 Circuit 61 (engine running) M21/1r1 Mirror heater N69/2 Right front door control module
M21/2r1 Mirror heater B14 Outside temperature indicator N10/1 Driver's side SAM control

Thanks for this - I don't think the outside temp sensor is playing up though. The temperatures outside have fallen recently and this morning were around 13°C when I changed the mirror - dashboard showed outside temp as being 13°C - so in theory it was meant to make the mirror heater come on, which was happening. I didn't think to see if the nearside mirror heater was also on.
Can't get to the car for a while, but I'll recheck when I do get to it taking into account the outside temperature - hopefully our outside temps will rise a bit before autumn/winter really kicks in! In any event, the information you have provided gives me something to work on. Thanks again.

 

[Major774]

The heated mirrors are automatic and is activated by temperature, there is no manual off button.

So just let do it's thing.