heated mirrors

[nadz325i]

question,

i remember reading somewhere (cant reember where) BMW original heated mirrors on the e30, contain a resistor (small box on the back of glass, my glass mirror has this) that cause the heating function to stop working?

can anybody confirm true or false about the resistor?

because i gots 12v on the wires but no continuity (sp?) between the 2 terminals on the glass.

i got a copy heated mirror and thats got no box/resistor on the back of it, but assuming its built in into the glass.

also mirror glass tint available standard non tint, Brown and Blue? i know blue is one but is there one non-tinted or brown?

Thanks.

[nickso]

someone tell him cos ive got the same problem

where did you get your copy mirror from?....bmw want Ԛ£90 plus vat

[jcjdavis1]

Hi Try ECP for mirror glass non heated are about Ԛ£10 heated are Ԛ£38.50+vat.
try www.euro car parts.co.uk

[Brianmoooore]

Don't think you'll have much success repairing heated mirror glasses.
Keep your eyes open in scrapyards. You can often make out a faint circle, about 40mm across or so, in the middle of heated mirror glass, which gives them away.

[DanThe]

Ive just wired a pair of heated glass' into my mirrors but neither seem to be warming up

Got continuity between, and pos and neg on each teminal

[Globulator]

I always thought a bright idea would be to put a small 5w-10w bulb in there if the resistor is gone, might look interesting at night though! Otherwise a wirewould resistor from Maplins might do - DanThe - what resistance (ohms) do you measure on yours?

[DanThe]

I didnt mate, just pulled the earth wire off and tested the pin for current

[DanThe]

Just been out and measured, not getting anything on my meter so must be fooked
Good job they didnt cost me anything

[nadz325i]

oh well

what colour glass tint did BMW do them in?

standard? Brown?? Blue?

ive seen normal standard non tint and blue? but brown tint?

[DanThe]

Ive only ever seen blue

[Brianmoooore]

I always thought a bright idea would be to put a small 5w-10w bulb in there if the resistor is gone, might look interesting at night though! Otherwise a wirewould resistor from Maplins might do - DanThe - what resistance (ohms) do you measure on yours?

If there is a resistor in there somewhere, then it will be a special type known as a positive temperature coefficient type (PTC). This is to reduce the current as the glass heats up and provide regulation.

[DanThe]

So this resistor could be regulating the current to the minimum so as no effect is being made on the temp of the glass.

Are these resistors available from places like R+S components Brian? Might be worth a crack if they arnt too dear

[Globulator]

Agreed - never use carbon based resistors, because they (and bipolar transistors) conduct more as they heat up, so you get 'thermal runaway'. That's why bipolar transistor power amplifiers have series wirewound resistors to create an overall flat or positive temperature coefficient - i.e. they conduct less as they get hotter.

A standard wirewound may not turn off quick-enough for the glass so you need to think about a lowish power so the glass doesn't get too hot and crack..... i.e. don't roast it!!

As long as you stick to moderate powers with wirewound (sometimes marked as WW) resistors or light bulbs you'll be fine. If you're going to buy WW resistors from Maplins or RS (usually in square ceramic packages) make sure they can't short out (even if the glue melts...) and make sure they are biggish - i.e. go for a 25w resistor for 5-10w usage as it'll spread the heat out better. To compute values, R = 14 x 14 / W, so for 5W you need a 39ohm value.

[DanThe]

I took the glass out of the E34 im breaking today and they are heated, done a couple of tests,

At room temp I got 10 Ohm and 1166 Ohm

Put them in the freezer for a couple of hours, 6 Ohm and 955 Ohm

Bit of a giveaway but the high resistance glass doesnt work, the 10 Ohm works well and once thawed out reads 13 Ohm.

Just about to pull one apart and have a mooch inside

[DanThe]

Well thats put the repairable resistor theory to bed