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Ever since I bought my used A5 the rear window defroster has not worked. I finally got around to investigating the problem, and I think that it is wired incorrectly. I am out of warranty, so I figured I would try to solve this on my own.

I cannot find out where the rear defroster get positive + power from? Should the window have positive + on one side and negative - on the other?

Does anyone have a wiring schematic for how the rear window should be wired up? Or better yet does anyone have any photos of their car?

See my photos attached of what was hiding under the C-pillar. Both defroster leads on the window are bolted down to ground points, and one on the passenger side shares a stud with some type of electrical coil. The electrical coil/box in the second photo does have positive power on both of the outside studs, but I have no idea what it does.

Any thoughts?
 

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Hi TurboJuice,

You ever solve this? I have the same issue with my 2010 A5, the demister switch comes on and lights up but no power to the actual heating element. If you do find a solution I would appreciate you posting it here!!

Cheers
 

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Discussion Starter #4
Hi TurboJuice,

You ever solve this? I have the same issue with my 2010 A5, the demister switch comes on and lights up but no power to the actual heating element. If you do find a solution I would appreciate you posting it here!!

Cheers
I did figure mine out. For some reason the wire lead from the rear window was bolted down in the wrong location. I am not sure if this is manufacturer error or if the car was ever serviced in the rear?

In the second photo i posted above, the rear window is grounded on the center stud of that noise suppressor device (p/n 8T8035570), which is incorrect. The window lead needs to bolt down to the other side of that device to allow positive power into the circuit when the button is switched.

I would recommend checking a few things:
1. pay attention to the audible sound of the relay turning on when the button is pressed. While in your garage you will be able to hear a 'click' com from the trunk when you press defrost button.
2. remove the d-Pillar covers and check the defroster leads for continuity to make sure you do not have any breaks in the circuit.
3. check for voltage when you switch the button on and off at the device i show in my second image.
 

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Ah that's so strange.

I had my pillars off the other night and had exactly the same layout as yours in the picture.

So you move the window lead one position to left on the suppression device? (as in my hastily edited picture below)

I will have a go this eve and check your suggested points. Do you know which relay it is that controls the heated window?

Cheers

*EDIT

Had some assistance from a work mate and checked the boot for the relay sound 'click' when pressing the demister power button which we could clearly hear coming from relay 643 in the luggage compartment fuse/relay board.
 

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Discussion Starter #6
Ah that's so strange.

I had my pillars off the other night and had exactly the same layout as yours in the picture.

So you move the window lead one position to left on the suppression device? (as in my hastily edited picture below)

I will have a go this eve and check your suggested points. Do you know which relay it is that controls the heated window?

Cheers

*EDIT

Had some assistance from a work mate and checked the boot for the relay sound 'click' when pressing the demister power button which we could clearly hear coming from relay 643 in the luggage compartment fuse/relay board.
Yes, very strange. It would be very odd if yours is connected in the wrong location like mine.

Your illustration is correct. This is exactly what I did with mine. The way it is wired in the photo prevents the rear demister from ever receiving positive power.

I also used this how-to for photos of whats under the D-pillar.
https://snapguide.com/guides/audi-a4a5-bang-olufsen-subwoofer-upgrade/

You can see the wires in the 5th photo.
 

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Good spot on the linked photo!

Yeah having looked again it's as if someone has connected the element wire to the grounding bolt on a re-assembly.

One thing I have been thinking, my car is actually a Category D write off (written off by insurer for cosmetic damage) and I think at some point the rear windscreen may have been replaced or taken out. My bet would be the repair mechanic dropped the ball and connected it up wrong.

For yours though I am stumped, unless the previous owner of your car was suffering from radio antennae interference from the heated window and cocked up a part replacement? (which is an issue for some owners I believe)


***EDIT

Problem solved, was exactly as you described TurboJuice, switched the wires over and it all started working a treat.

Thanks for you help!
 

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Thought mine wasn't working too after a Google 1st post was this one Im looking behind that c panel now &...

Looks like the same,?? & the left hand 10mm nut is loose

Sent from my SM-N910F
 

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One of the panel clips is bent so looks like it been off before, with engine running & demist switch on I get 12v at both sides of the block swapping lead over now... Here goes nothing!

Sent from my SM-N910F

works now ? weird why they're been swapped over in all 3 cases?? Maybe some one having a bad last day at work in Germany lol ?
 

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Discussion Starter #10
I'm glad this thread has helped you!

Very odd that we have all had the same problem.
 

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Slight difference in design but same job, can see how loose nut was almost off
Works but hope it's right, anyone got a wiring diagram of it?
2007 A5 3.0tdi sport Quattro
 

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Slight difference in design but same job, can see how loose nut was almost off
Works but hope it's right, anyone got a wiring diagram of it?
2007 A5 3.0tdi sport Quattro
I think it has to do with when a rear windscreen gets replaced by a non Audi specialist, they're numpties basically and have no clue how the wiring is set up!!
 

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Hi folks

i have the 2009 A5 3.0tdi and got the exact same issue, only noticed during the cold spells recently as never needed to use it since buying the car back in may.
thats great advice and tips, i'm going to give it a try when not so cold out there.

thanks
 

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Hi folks

i finally managed to strip the D pillar cover off today as having no demister is now seriously doing my head in.


And guess what, i have the exact same issue as the rest of yous, there was 1 cable bolted to the black nut in the centre, the nut on the left was very loose.


i just want to confirm that i've done this correct.

1. removed the cable from the centre black nut.
2. bolted the cable onto the left nut
3. tightened up the black nut to the centre

i dont know how to attach pic to this post as i have taken some pic.

cheers
Mike
 

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Just like mine, been fine for last 2 years, sometimes a little interference when radio on but demists now

Sent from my SM-N950F using Tapatalk
 

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hi

just managed to figure how to attach the pic.
the 1st pic is before and the 2nd pic is after i re positioned the cable.

hope this is correct.

Cheers
hi

just managed to figure how to attach the pic.
the 1st pic is before and the 2nd pic is after i re positioned the cable.

hope this is correct.

Cheers
Thanks to TurboJuice, Nedbenson100, and others for your contribution on this subject. I have been trying to figure out for a few weeks on how to solve this issue on my son's 2010 Audi A4 and I finally found the answer here accidentally. I went out today and reconnect the wire that was connected to the center screw, pictured on TurboJuice's previous photos, to the left screw and it worked perfectly. You won't believe the local shop wanted to charge me, $1400.

Hope this is a great feedback for all!
 

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Thanks to TurboJuice, Nedbenson100, and others for your contribution on this subject. I have been trying to figure out for a few weeks on how to solve this issue on my son's 2010 Audi A4 and I finally found the answer here accidentally. I went out today and reconnect the wire that was connected to the center screw, pictured on TurboJuice's previous photos, to the left screw and it worked perfectly. You won't believe the local shop wanted to charge me, $1400.

Hope this is a great feedback for all!
By the way, mikey1756, I had mine attached just like yours and it worked.

Best
 
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