Rear window demister not working

[Jamestnewson]

I've got an issue with my rear demister which doesn't seem to be working.

I've taken the rear trim off and tested that there is continuity in the circuit across the window and also that there is continuity in the wiring loom from the left to the right side connections, but still no joy.Has The 30A fuse is good and I was wondering if there is a relay which controls this hidden somewhere. The light on the switch works when pressed so I guess I might need to also test the switch for the voltage when on.

Anyone else had this issue? I assume the wing mirrors need to be in the central switch position for these the defrost too.

Cheers for any responses, it's appreciated.

 

[DoubleB6]

So as I understand it on the B6 the AC unit output a signal which then activated a relay elsewhere in order to supply current to the defroster

In the B7 this relay was moved inside the AC unit. The wiring diagram in the link below supposedly shows the connections for the layout

The supply comes in a white/violet wire from fuse 26A (not sure if the location) to pin F5 on the AC unit. I believe this is then routed through the relay and comes out on pin F6

If you’re getting voltage on F5 but then nothing on F6 when the rear defroster button is pressed then it looks like there is a problem with the AC unit

I’d recommend you verify that the above layout is the case with yours and for example do a continuity test between the F6 white/violet wire and the wire supplying power to the defroster

These are not factory wiring diagrams and it could well have changed in the production run, but I page 30 on the link could be off some use to you:


View: https://drive.google.com/file/d/1SicEUxnRJxERgRuQNdtfsXG4WuNS7XNn/view


Mirrors
According to the information from the B6 the heated mirrors will come on automatically if outside air below 25degC or if the rear defroster is manually turned on. There is no mention of this in the B7 release features docs so I assume it is the same B6/B7. Page 61 in B6 release features below:

http://www.volkspage.net/technik/ssp/ssp/SSP_254.pdf

 

[Jamestnewson]

So as I understand it on the B6 the AC unit output a signal which then activated a relay elsewhere in order to supply current to the defroster

In the B7 this relay was moved inside the AC unit. The wiring diagram in the link below supposedly shows the connections for the layout

The supply comes in a white/violet wire from fuse 26A (not sure if the location) to pin F5 on the AC unit. I believe this is then routed through the relay and comes out on pin F6

If you’re getting voltage on F5 but then nothing on F6 when the rear defroster button is pressed then it looks like there is a problem with the AC unit

I’d recommend you verify that the above layout is the case with yours and for example do a continuity test between the F6 white/violet wire and the wire supplying power to the defroster

These are not factory wiring diagrams and it could well have changed in the production run, but I page 30 on the link could be off some use to you:


View: https://drive.google.com/file/d/1SicEUxnRJxERgRuQNdtfsXG4WuNS7XNn/view


Mirrors
According to the information from the B6 the heated mirrors will come on automatically if outside air below 25degC or if the rear defroster is manually turned on. There is no mention of this in the B7 release features docs so I assume it is the same B6/B7. Page 61 in B6 release features below:

http://www.volkspage.net/technik/ssp/ssp/SSP_254.pdf

That’s super helpful info, thanks. I’ll have a look at the diagram and test the pins to see if the ac unit matches up.