How do heated mirrors work?

[TheBlizz]

I know theres an heater element in them that warms them up. I'm more concerned with how they operate.

In my 80 quattro, the mirrors would warm up when I turned the rear defroster on. Not the case with my 96 a4. Do they work by temperature? I don't see any other switch that would activate them.

Now I've read about the "browning" which is caused by the heating element expiring. Would that cause the element to stop working? Both my mirrors have a bit of browning around the edges, which doesn't bother me much, but if that's causing them not to warm up, then I'll be bothered.

[blacka4]

they should work with the rear deforster. or at least that is how my 99.5 worked. if they are not working then I think you might have a problem. I know I had some major browning issues with mine and they continued to work even though the heater was coming off the glass.

[TheBlizz]

So what does audizine recommend? Fuse? Common problem?

[walky_talky20]

I was curious about this as well. I just looked at the diagram. It would appear that the mirror heater elements are power *at all times*. I fully expected them to be powered via the rear defroster switch, but that is not the case. They have constant ground and get power through the load reduction relay (so powered whenever the key is in the ON position). Very surprising.

Knowing this, I may now decide to put a switch in line. I thought the unnecessary heat cycling may cause premature failure of the very expensive Auto-Dimming glass.

EDIT: Upon closer inspection, it appears the very early B5 A4's did indeed use the rear defogger switch to control them. The switch itself appears to have an internal fuse or breaker just for the mirrors. It looks like it went to the "always on" mirror heating for the 1997 model year. Although it shows the defogger switch for that year was still capable of controlling them (via pin 3), it just wasn't used.

[J Ozzie]

they work when the Snowflake comes on when it gets cold in the mornings and such! So helpful for warming up your hands too hahah.

[99blueb5]

pretty sure there was also an option for heated mirrors also so not every a4 had them

[Dallas09]

I read somewhere (I think it was the owners manual), and the writeup for the heated mirrors with rear defogger switch mod, that the mirrors are automatically heated depending on the temperature (maybe they are individually controlled at the mirror, and thats why there is always power running to them based on what walky said?).

[TheBlizz]

Ok so either the mirrors work via temperature, or via rear defrost switch or they're always on.... Anyone got a definative answer?

I am going to pull the rear defrost switch tomorrow and see if there indeed is a fuse in it.

[frameshift18]

If you really want you could check the voltage of the two wires that plug into the mirror. Check it with the defroster on/off, and with fuse in/out.

[walky_talky20]

Please note that I said the switch "appears" to have an internal fuse. That is just what my diagram shows. It could be a breaker or a surface mount fuse (ie: not replaceable and not visible). I have no idea. This heated mirror business is new to me - it is the first time I've looked into it.

I also read the manual, and just assumed there was indeed some temperature control. It never occurred to me *what* was actually doing the temp control. Which is odd, I usually want to know how *everything* works on my B5, lol.

If anybody wants the diagrams I'd be happy to share.

[rookie18]

It should be wired in series with the heater element. At least this is what I remember seeing when I replaced one of mine.