ammonman

Anyone know how hot the side mirrors get when de-fog or de-ice is selected? The insulation on the heating element wire terminals has crumbled away to nothing leaving bare female pins on the ends of the wires. Before I try and find some compound to use to effect a repair I thought I'd see if anyone knows how hot the elements or the wire leads get when on.

Thanks

Mike

CardinalCar

I don't know the specific answer but you can make a reasonable guess.

If you have a multimeter, put it in Resistance mode and measure the resistance of the coil. It's basically just a big long skinny resistor glued to the glass. For the sake of argument let's say it's 100ohms.

Plug that into Ohm's Law:

Voltage = Current * Resistance

12V = Current * 100Ohms

Therefore current = 12/100 = 0.12amps

From there you can figure out the wattage:

Power (watts) = Volts * Current

Therefore 12V * 0.12Amps = 1.44 watts. Imagine a 1.5watt light bulb, you'd hardly be able to feel anything while it was on. Barely lukewarm at most.

So not very much heat. Basically all it has to do is get the glass just above freezing so the ice melts. I doubt you'd be able to feel any radiating heat from it.

Any reasonable insulated wiring will almost certainly be fine if all of the above is vaguely correct.

ammonman

Thanks for the clarity. Now, why didn't I think of that. Evidently been a long week.

Mirror heating element resistance is 5 ohms. Assuming max voltage of 14 VDC, amperage would be 2.8 (14/5=2.8) and power would be 39 watts (2.8 amps x 14 VDC= 39.2). A 40 watt lamp will get quite warm with surface temps over +250F. However, since the 40 watts is spread out over 39 square inches of glass surface I doubt the mirror or wiring will reach those temps. I think liquid tape with a usable working temp of up to +200F will be fine as an insulating compound.

Thanks

RET

I would guess that the resistance would increase as the temperature of the heating element increased, so the power dissapated would be less than predicted from the resistance of the cold heating element.

I would also guess that the actual temperature of the mirror when heated is not very high, certainly not in the forty watt range, but if you really had to know, you could apply power to it and measure the temperature (as well as the actual current draw when warm....). I suspect that most good quality heat shrink tubing would be acceptable as well as self-bonding silicone tape (don't use regular electrical tape).

Ad0911

I find heated mirrors a great safety device. My '78 obviously doesn't have that but with the doorpanels off, it seems like a good idea to install it. On which mirrors was this heating available? Also on the 'square' mirrors like my '78?

ammonman

I think your 1978 does have heated side mirrors. The Service Tech book for MY1978 discusses the wiring and controls. The heating elements are controlled by the same switch and relay that energizes the rear window de-fog/defrost system.

Mike

Vilhuer

AFAIK all 928 came with heated outside mirror(s). Those early cars which didn't originally have passenger side mirror might not have wires connected to it but factory mirror should have heater element build in anyway.

Alan

Whilst what you say is true... the degree C temperature change here is small (of the order of 10's of degrees) so has only minimal impact on resistance..

Conversely a light bulb filament changes temperature by so much from cold measurement to white hot operating conditions it can make for an up to 10x change in resistance.

Alan