Rear Screen Heater 80 B3

William Blyth · 30-09-18, 06:08 PM

I've recently replaced the heater matrix on a 1991 Audi 80 b3 1.8 S, and now find that my rear screen heater only works when the dipped lights are on.

The only thing I can think of is that I've found a single cable with no obvious fitting location at the back of the heating controls. Any electricians out there with any ideas?

Mellis46 · 30-09-18, 07:01 PM

Does that cable have a white, funny shaped plug on the end of it? If so its the connection for the 3 auxilliary gauges - it doesnt connect to anything if you haven't got those.

Everything got plugged back in properly behind the dash when you built it back up? Checked all the fuses etc?

William Blyth · 30-09-18, 07:22 PM

The wire was single cable. with a single female connector. I thought I had labelled all the wires as I took them apart, so I was surprised to find an unmarked cable when reconnecting. I was hoping that it may have been an earth cable. Thankyou for your help, at least I can now disregard that cable. That at least leaves me to find another solution to the "rear heater only working with main lights on".

William Blyth · 08-10-18, 10:38 AM

Demister switch now checked and all OK.

Fuse checked - OK.

Checked not connected to Fog Lamps by mistake -OK

Illumination light came on when ignition was turned on

Pressed switch NOTHING

Turned on dipped beam- It works

Davids · 08-10-18, 10:58 AM

No matter how random, tried all the earths, bad earth under a tag and its using the dipper beam as a return path.
Seen it happen before

Just a thought

mikes2 · 08-10-18, 12:35 PM

Disconnect the two fog lamp switches and try it again

William Blyth · 08-10-18, 08:54 PM

Fog lamp switch removed-no change. Demister switch changed to a good spare- no change. Rear foglamp swapped with demister- no change. No obvious earth problems in that area.

All I can think of, is an earth behind dash area, does anybody know if there is an earth for demister in that area or any other ideas please.

Mellis46 · 08-10-18, 09:26 PM

The main earth binding posts are on the left hand side of the passenger footwell forward of the door.

To my mind this has to be something disturbed when removing / replacing the dash. It's possible the switch wiring looms have been caught behind something and squished possibly? It might be the case the dash has to come back out so you can check more easily for damage...

William Blyth · 08-10-18, 09:52 PM

I think your right it seems like an earthing issue. I suppose I have two options. 1 Take it all to pieces again and see, or 2, as its a car really just used for shows, leave it alone and just go down the pub

Thanks everyone

30-09-18, 07:01 PM	#2
Mellis46 Aero Advocate Classic Audi Club Member Join Date: Aug 2014 Location: Lancashire Posts: 1,448	Does that cable have a white, funny shaped plug on the end of it? If so its the connection for the 3 auxilliary gauges - it doesnt connect to anything if you haven't got those. Everything got plugged back in properly behind the dash when you built it back up? Checked all the fuses etc? __________________ -1987 90 2.2E / Sapphire Blue (Rest in Pieces) -1989 90 quattro with Sport Equipment / Satin Black -1997 A4 Avant quattro TDI 110 / Tropical Green (Sold)

30-09-18, 07:22 PM	#3
William Blyth Grown up member Classic Audi Club Member Join Date: Jun 2018 Location: Lincolnshire Posts: 28	The wire was single cable. with a single female connector. I thought I had labelled all the wires as I took them apart, so I was surprised to find an unmarked cable when reconnecting. I was hoping that it may have been an earth cable. Thankyou for your help, at least I can now disregard that cable. That at least leaves me to find another solution to the "rear heater only working with main lights on". Last edited by William Blyth; 01-10-18 at 11:34 AM.

08-10-18, 10:38 AM	#4
William Blyth Grown up member Classic Audi Club Member Join Date: Jun 2018 Location: Lincolnshire Posts: 28	Demister switch now checked and all OK. Fuse checked - OK. Checked not connected to Fog Lamps by mistake -OK Illumination light came on when ignition was turned on Pressed switch NOTHING Turned on dipped beam- It works Last edited by William Blyth; 08-10-18 at 10:42 AM.

08-10-18, 12:35 PM	#6
mikes2 4 ring whore! Classic Audi Club Member Join Date: Nov 2007 Location: Polmont Posts: 2,617	Disconnect the two fog lamp switches and try it again __________________ 1991 90 quattro 20v sport 1991 90 quattro

08-10-18, 09:26 PM	#8
Mellis46 Aero Advocate Classic Audi Club Member Join Date: Aug 2014 Location: Lancashire Posts: 1,448	The main earth binding posts are on the left hand side of the passenger footwell forward of the door. To my mind this has to be something disturbed when removing / replacing the dash. It's possible the switch wiring looms have been caught behind something and squished possibly? It might be the case the dash has to come back out so you can check more easily for damage... __________________ -1987 90 2.2E / Sapphire Blue (Rest in Pieces) -1989 90 quattro with Sport Equipment / Satin Black -1997 A4 Avant quattro TDI 110 / Tropical Green (Sold)

08-10-18, 10:58 AM	#5
Davids 4 ring whore! Join Date: Jan 2017 Posts: 3,124	No matter how random, tried all the earths, bad earth under a tag and its using the dipper beam as a return path. Seen it happen before Just a thought

08-10-18, 08:54 PM	#7
William Blyth Grown up member Classic Audi Club Member Join Date: Jun 2018 Location: Lincolnshire Posts: 28	Fog lamp switch removed-no change. Demister switch changed to a good spare- no change. Rear foglamp swapped with demister- no change. No obvious earth problems in that area. All I can think of, is an earth behind dash area, does anybody know if there is an earth for demister in that area or any other ideas please.

08-10-18, 09:52 PM	#9
William Blyth Grown up member Classic Audi Club Member Join Date: Jun 2018 Location: Lincolnshire Posts: 28	I think your right it seems like an earthing issue. I suppose I have two options. 1 Take it all to pieces again and see, or 2, as its a car really just used for shows, leave it alone and just go down the pub Thanks everyone