Heated mirrors dont work very well

[mike ryan]

Since owning my a4 I have noticed the heated mirrors don’t seem to work very well if at all. They do something but it takes ages for them to clear and only if there is no air passing them i.e. on a motorway. On my model it has the later revision so they are on constantly with the ignition. I have changed the glass for another one and no difference, the heated circuit on the glass has a resistance and there is voltage to them. However when I put my multi meter across it they don’t seem to draw any current. The other strange thing is that both wires seem to have no resistance to the vehicle earth. I would only expect this of one of them. Is there some relay or thermostat that controls them that would cause this? Does anyone have a wiring diagram for the mirrors so I can try and trace it back? I have tried almost everything except getting some more glass from Audi. Can anyone tell me if they have had the same problem or is it just me?

[Alan02]

To measure current draw, you must put the meter in `series` and be able to read Amps, some meters dont read this. This wont be a very high current.

[mike ryan]

To measure current draw, you must put the meter in `series` and be able to read Amps, some meters dont read this. This wont be a very high current.

That is what i did but it only drew about 0.2 amps. I am thinking i might just have to buy some new glass to get them working.

[Zenerdiode]

In the early B5 the mirrors were switched on when the Heated Rear Window was switched on. There was also a 10A fuse hidden in the switch that powered the mirrors. You say the later ones are on permanently? There may still be a hidden fuse in the HRW switch. From memory, the mirror position adjustment knob has to be in the centre position to activate the heaters. Also, the replacement mirror glass regardless of pre/post facelift now come with an integral thermoswitch - no I can't remember what temperature it de-activates the heating element! This can be checked; however, if you invest in a can of freezer spray from an electronic components factor. (RS Components, Farnell, Maplin etc.)

...strange thing is that both wires seem to have no resistance to the vehicle earth. I would only expect this of one of them.

Mmm, there could be a couple of plausible reasons why you're experiencing this; the internal resistance of a lead acid battery is in the order of milliohms, so you could quite easily be measuring through the battery to the vehicle earth. Also, depending on the type of meter (digital or moving coil) and the orientation of the leads, voltages that are present when measuring resistances can play havoc with the readings obtained. This is due to the way the meter functions to measure resistance.

Try swapping your leads around for the terminals that you're measuring, also instead of checking the voltage across the two connections to the mirror heater, connect the meter negative to the vehicle earth and use the positive probe to probe each mirror connector.

Check for voltage at the mirror with it still connected.

If you get the mirror glass out, stick it in the freezer for a half an hour and then check the resistance at the terminals (because of the thermoswitch mentioned above).

I say if, because its notoriously easy to damage a tiny plastic pivot in the positional adjusting motor assembly when you try to lever the glass out. (In the centre of the white bar that you see when the mirror is removed)

And yes, the elements do fail with time.

Blimey, I was on a roll there

[mike ryan]

There are a couple of good points you have mentioned that i will try to have a look at. I still think it will be the mirror glass personally but at £20 plus per side its an expensive gamble. I will try some of your ideas and let you know haw i get on. By the way for your information i am using a digital multimeter but dont know any more as to what type it is.