Burned up a rear view mirror

Interesting. The passenger side mirror burned up this morning going in to work. That's a new one on me. Didn't blow any fuses (that I know of). I turned on the rear window & side mirror defroster to clear condensation, and it worked fine. Later on, I noticed the side mirror wasn't visible (it was dark). Took these pictures just to prove I'm not delusional, it really happened. The lumpy surface is indeed a lumpy surface, you can feel it with your fingers.

Can't figure out how it managed to draw that much power to generate that much heat without blowing a fuse or such.

What kind of car?

04 Saab 9-3 Aero convertible.

If the other electric defrosters are on the same fuse, they might have gotten a little amps while that thing took a lot but still staying under the fuse's rating. Path of least resistance.

The resistance is in the filaments of the heater though. Even hooked directly to the battery it shouldn't be able to pull the current to generate the heat to do this.

Not arguing that that is apparently what did in fact happen. Just rather baffled by how. Especially so uniformly. As well why it didn't blow a fuse or burn a wire. Though the latter remains to be investigated.

The others did work and clear the glass (rear window and drivers side). Though you may be onto something. I have thought the rear window defrogger was kinda slow, and perhaps getting slower.

There is probably a short in the element. V=IR. If there's a short the resistance will be significantly smaller, and the voltage will remain 12-14volts, so the amperage will increase. The wattage is W=I^(2)R so with the increase of current even with a proportional decrease in resistance, the wattage will still increase.

The short could have been any where in the circuit now that I think about it.

It's also possible that the heating elements are regulated, a pulsewidth modulated circuit to do that is pretty simple.
That lets them sense ice and melt it quickly if needed, without cooking the mirror by being on all the time.
Factory wiring diagram will tell the tale.
The controller could have barfed, and just turned on the elements 100% (ie 'cooked mirror')

erohslc wrote: It's also possible that the heating elements are regulated, a pulsewidth modulated circuit to do that is pretty simple. That lets them sense ice and melt it quickly if needed, without cooking the mirror by being on all the time. Factory wiring diagram will tell the tale. The controller could have barfed, and just turned on the elements 100% (ie 'cooked mirror')

This is my guess too. There is probably a temp sensing circuit in the mirror itself that should shut the power off if the mirrors are getting hot. I bet that part of the circuit failed. A thermistor is a variable resistance device that changes resistance with temperature. It could have shorted or otherwise failed.

That's a side view mirror. If it is a rear view mirror and it only lets you see what you can already see in the actual rear view mirror then adjust it outward. I bet it burned up with anger.

The failed thermistor theory makes sense to me. Good thing it didn't start a fire.

I once set a shorted speaker on fire (like flames and everything) in the door of a Neon...didn't even kill the driver in the radio. New speaker, away we go.

Never seen that before or since.

Well, to my surprise, it's none of the above. Another person suggested I look closer.

Turns out the glass itself fell off the silvering on the back. I've had plenty of mirrors fall of cars before, but I've never had one where the glass itself fell off, leaving the silver. But, that's what happened. When I came out this afternoon, it was all nicely blackened with tarnish.

Which does make me feel better about the heater and the wiring.

foxtrapper wrote: 04 Saab 9-3 Aero convertible.

Ah. There's the problem.