Rear Screen and wing mirrors - heating elements

[ciaranthemurph]

As the weather is a little rubbish I've decided that it would probably be for the best to finally get my rear screen demister working (it hasn't since I bought the car). I took out the old fuse and found that it had blown so popped in a new 30A one and went for a shot. It blew straight away . I'm rubbish with a multimeter so I was wondering if anyone would know what would be causing this. Is the defrost button supposed to hold in when pressed as mine just pops back out.

Thanks in advance.

[Fat Audi 80]

Sounds like you have an earth problem somewhere, which could be a break in the rear screen element or one of the mirrors.

The button does come back out again but should stay illuminated. It won't stay illuminated if you are blowing the fuse straight away.

I must admit, I have not repaired a rear screen before, so can't give you any more detailed info.

Cheers,

Steve.

[S2central.net]

This should help tracing the wires...

http://www.s2central.net/S2_Schematics/B/B95.PDF

Maybe changes a bit over the years but link from http://www.s2central.net/S2_Schematics to check yours...

Must be a short circuit somewhere to blow fuse immediately... Top suspect is the wiring where it enters the front doors - may have been nipped/damaged at some point and giving you a short path to earth.

The switch itself could be faulty - but thats very unlikely - more likely some exposed wires somewhere. With the fuse out and access to all the relevant connectors it should be easy to buzz this one out.

[S2central.net]

Idea... Note connectors T5d and T5e that are in the A-pillars enroute to the mirrors...

A resistance check across pins 4 and 5 of each of those connectors will tell you if the heater element is short circuit to earth.

Similar check from pin 6 of the switch (E15) will tell you resistance of the rear screen.

And a check of resistance seen by pin 3 of the switch (E15) should be BOTH mirror elements in parallel.

Note there is an internal resistor in the switch that adjusts the load placed by mirror elements - it doesn't appear that can be tested separately as the pins aren't exposed.... Actually yes you can...

At E15 switch open (OFF)
Check pin 5 on switch does NOT connect to pins 3 or 6.
Check also that pin 2 does not have a circuit to another pins on the switch.

WIth E15 switch closed (ON)
Check pin 5 to pin 6 is short circuit (0 ohm)
Check pin 5 to pin 3 is 10k ohm
Check pin 5 to pin 2 has resistance appropriate for instrumentation bulb - approx 100-200ohm i expect

[Skull]

There are 2 types of switches. The one that stays in is oldschool, the latest one, stay out, and have a 10 minute timer inside, so they automatically turn off. They are longer too.

[ciaranthemurph]

Thanks guys, I'll have a look tomorrow, if it's not raining!

[macspring]

Skull,

There are 2 types of switches. The one that stays in is oldschool, the latest one, stay out, and have a 10 minute timer inside, so they automatically turn off. They are longer too.

Thanks for the info, wondered why the different length.

Kind regards.