Rear defrost not working

[tdip84]

Does anyone have a fuse diagram for a b8 S4? My rear defrost isn't working and I wanted to start with checking the fuse. The button doesn't even light up when I hit it.

[hilmar2k]

https://www.autogenius.info/audi-s4-2013-fuse-box-diagram/

[tdip84]

That's for 2013, mine is a 2011, but when I even go to 2011 diagram, there's no fuse for rear defroster

[hilmar2k]

Well, you could start just pulling fuses and look for the blown out one.....

[jygesq]

Well, you could start just pulling fuses and look for the blown out one.....

I would NOT try that! codes might be stored as a result.Blown fuses could be seen on close inspection as the fuse will show the fuse is broken. High amperage fuse , should be show a break in the fuse. The actual fuse may be in some other place , like near the e-box in the engine cowl, or even a fusible link under the front seat floor,I believe under a plastic cover there may be some high amperage links You could check audi parts diagrams.

[tdip84]

So I found the fuse and the relay. Checked both and both are good

[hilmar2k]

I would NOT try that! codes might be stored as a result.Blown fuses could be seen on close inspection as the fuse will show the fuse is broken. High amperage fuse , should be show a break in the fuse. The actual fuse may be in some other place , like near the e-box in the engine cowl, or even a fusible link under the front seat floor,I believe under a plastic cover there may be some high amperage links You could check audi parts diagrams.

I was being facetious. Guess I didn't make that clear.....

[LYKUNO]

So I found the fuse and the relay. Checked both and both are good

Does the defroster switch's indicator light illuminate when the switch is operated? Assuming it does, get a voltmeter and check for DC voltage at the rear window (although you might need to remove the C-pillar covers to access the heating element connectors though). Also, if I recall correctly, the defroster is on a timer, and will shut off after some predetermined amount of time. Perhaps that functionality may be involved in your issue?

[tdip84]

Does the defroster switch's indicator light illuminate when the switch is operated? Assuming it does, get a voltmeter and check for DC voltage at the rear window (although you might need to remove the C-pillar covers to access the heating element connectors though). Also, if I recall correctly, the defroster is on a timer, and will shut off after some predetermined amount of time. Perhaps that functionality may be involved in your issue?

The defrost button doesn't illuminate. Think it's possibly the HVAC unit itself?

[tdip84]

Every other button on unit works fine

[LYKUNO]

If the defrost button doesn't light, then it's either not getting battery (+) or ground (-). Could be a bad switch or a wiring connection or a fuse. The 2011 owners manual doesn't specifically list the rear window defogger's fuse location, but there are some fuses that control modules (some with relays) that might be involved. And in addition to the fuse panel on the end of the drivers side dash, there is another fuse panel on the passenger side of the dash, as well as one behind the cubby in the right side of the trunk.

It would be really helpful if someone here had access to an electrical schematic that could help pinpoint the problem, or offer advice into how to gain access to that switch that isn't illuminating.

[rand0mher0]

There might be a code in the module to point you in the right direction. Worth checking.

[andivic]

So I found the fuse and the relay. Checked both and both are good

Could you please share the rear defroster relay and fuse location ? I have the same issue
Thanks

[DenverAudi]

I have a unique one for anyone who can assist - the 6(?) pin connector from the three wires soldered to the window doesn't plug in anywhere that I can find. The defrost button works but the window doesn't heat up. I'm 100% positive its because this connector isn't plugged in - any help where to find its mate?

[Smac770]

There's not a 6-pin plug involved with the rear window defroster, that I see in the wiring diagrams. What is the specific vehicle involved (S4 forum, but profile says '09 A4; is this a wagon or sedan; B8.0 or B8.5)? Where are you finding this 6-pin plug around the rear window?

[DenverAudi]

Whooops my bad. A4 (B8) sedan. I found a picture in another forum that has it plugged in. It’s passenger side high on the window and the plug is under the C pillar trim. Just hanging out. Dunno how to attach photos.

[DenverAudi]

There's not a 6-pin plug involved with the rear window defroster, that I see in the wiring diagrams. What is the specific vehicle involved (S4 forum, but profile says '09 A4; is this a wagon or sedan; B8.0 or B8.5)? Where are you finding this 6-pin plug around the rear window?

https://ibb.co/vdXFDDf

This is not my car. I found this in another forum but it does include what I am talking about.

[Smac770]

https://www.ebay.com/itm/384860611992

The defroster grid connections are going to be the lower ones, the large wires with the inline FM filters, such as on the right end of your linked pic.

The left side upper 2-pin and mid 1-pin are for the R112 FM1/AM and FM2 feeds. The right side upper 3-pin is for DAB and TV3 purposes, not used in the US.

The defroster supplies 12v to the right side connection, grounds on the left side connection. I'd check for 12v presence where the harness cable connects to the block there which the window defroster lead is attached. If nothing there, could be either the defroster fuse or the defroster relay, both there in the right rear relay panel (the big 40A fuse and one of the two relays, the other relay is the 12v sockets relay; if you suspect the relay, swap them and see if it starts working).

[DenverAudi]

https://www.ebay.com/itm/384860611992

The defroster grid connections are going to be the lower ones, the large wires with the inline FM filters, such as on the right end of your linked pic.

The left side upper 2-pin and mid 1-pin are for the R112 FM1/AM and FM2 feeds. The right side upper 3-pin is for DAB and TV3 purposes, not used in the US.

The defroster supplies 12v to the right side connection, grounds on the left side connection. I'd check for 12v presence where the harness cable connects to the block there which the window defroster lead is attached. If nothing there, could be either the defroster fuse or the defroster relay, both there in the right rear relay panel (the big 40A fuse and one of the two relays, the other relay is the 12v sockets relay; if you suspect the relay, swap them and see if it starts working).

Thank you sir - very much appreciated.