DIY: Rear defroster fix for newer HVAC unit in early B6's

[Matt Devo]

Background: early B6 A4's came with an HVAC unit which directly powered the rear defroster; later B6's and B7's use a relay. If you install a newer HVAC unit into an older B6 (either because the lack off an OFF button sucks, or because all of the soft touch paint on the buttons has peeled off and it's ugly), the rear defroster will not work. When activated, the button will illuminate for a few seconds, then turn off. This DIY will show you how to fix that fairly easily.

Materials needed:
(1) 30-40A single-pole single-throw (SPST) relay
(3) 12-14ga insulated female quick disconnect terminals
(1) 18-22ga insulated female quick disconnect terminal
(1) 12-14ga insulated butt connector
~6" of 12/14ga insulated stranded wire (non-black casing preferable)
~3' of 18-22ga insulated stranded wire (black casing preferable)

Tools needed:
- wire strippers
- pliers
- goo gone


Summary: Power to the rear defroster comes from fuse 26 and into the HVAC on wire F-5. Power goes from the HVAC to the rear defroster via wire F-6. We will re-wire such that F-5 provides constant power to the relay; F-6 will be split to trigger the relay and provide power to the rear defroster.

The DIY:

1) use painters tape to protect ashtray cover and sides of center console near HVAC unit. Remove radio. Remove HVAC.

2) remove 3-4" of the cloth tape from the wires on the 'F' plug, which is located on the far right (of the HVAC unit) and has 2 ~12ga wires going into it. Clean evil residue with goo gone.

F-plug:


cloth tape stripped:


3) cut both wires from the F-plug ~1.5" from the plug. Attach quick disconnect terminals to harness side of bare wires.

4) attach butt connector to wire F-6 (white/lilac) on F-plug; F-5 is unused. attach ~6" of 12-14ga wire to butt connector. Attach quick disconnect terminal to end of extension wire.

5) remove fuse panel access cover and driver side knee bolster. Run 18-22ga ground wire from HVAC area to convenient ground post. attach quick disconnect terminal to HVAC end, trim to length.

6) wire up relay:
F-5 (white/yellow) harness-side to terminal 30
F-6 (white/lilac) harness-side to terminal 87
F-6 from HVAC (extended) to terminal 86
ground wire to terminal 85



7) reconnect HVAC plugs, push relay back/down into chasm below HVAC so it's not in the way. Test HVAC before fully reinstalling.

8) Reinstall HVAC. Reinstall radio. clean up work area.

9) profit :)





edit: found this nice diagram from woorlord on audi-sport.net:




Hope this is useful.

[UFfitz56]

VERY useful! I just figured out my rear defroster isn't working because of this. Thanks!

[swededA4]

I have done this as well. Great DIY Matt.

[Dubsport Inc]

Nice work... Thanks in advance if I ever need it!

[Charles.waite]

Just what I needed. Thanks Matt.

Swapped in a B7 HVAC console (from an RS4!!! It probably adds like 5hp to my blower motor at least!) and just now discovered my defrost wasn't working. It would turn the light on for long enough that I would look away and not notice it going back off. Glad the fix is something simple like this.

Is there any reason that its a bad idea to just shove the relay down into the recess? would some strategic zip-tie application be a good idea? My console area is already pretty crowded with my S2+ adapter so I worry it will be too crammed in there...

[Matt Devo]

my relay is down below the HVAC, almost under the ashtray. No issue with radio adapter harnesses as well.

[Charles.waite]

Cool. I'll just cram it in there then. I'll see if I can scrounge up a SPST relay at an autoparts store. I assume finding those is no big deal.

[SJorge3442]

Cool. I'll just cram it in there then. I'll see if I can scrounge up a SPST relay at an autoparts store. I assume finding those is no big deal.

Do you know what year the unit was that I got from you? I never thought or needed to use the re-defrost yet. I may also have to handle this.

[Charles.waite]

Do you know what year the unit was that I got from you? I never thought or needed to use the re-defrost yet. I may also have to handle this.

Yea. THe one I sent you was the original one from my car. its a 2004. Shouldn't have any issues as Rear Defrost always worked.

[SJorge3442]

Yea. THe one I sent you was the original one from my car. its a 2004. Shouldn't have any issues as Rear Defrost always worked.

Thanks!

[Charles.waite]

Thanks again Matt. Did this mod yesterday and the defrost works great. My only holdup was reading the direction wrong and crimping quick connects to the HVAC side, not the harness side (reading comprehension FTL...). Had to run to radio shack and buy more.

I do need to wrap the relay in something as its hitting something back there and is rattling. Also you might want to add to the first post that you need a ring terminal (or something) to properly hook the ground wire up to an existing ground.

[Matt Devo]

there's a ground post behind there somewhere, I just stripped the ground and hooked it behind the existing nut :)

[Charles.waite]

there's a ground post behind there somewhere, I just stripped the ground and hooked it behind the existing nut :)

Oh really? By the time I got to the ground, it was already dark, so visibility into the console recess was limited. I ran mine all the way to the fusebox and ganged it up with my boost gauge ground.

[MantanaB6]

I'm going to necro-bump this thread, as I'm planning on doing this as I recently discovered my S4 is suffering from lack of defrost. Light goes on then almost immediately off. I know I have a new HVAC unit as it also has heated seat switches which unfortunately do nothing. Previous owner neglected to mention these little bits when selling the car (turns out heated seats and rear defrost come in handy during New England winters...). But that is neither here nor there.

What I'm wondering is how the "Radio Aerial" fits into this. Does this mean that the antenna amplifier is not currently being powered, but will be after the mod? So I should theoretically get a boost in my radio reception? Or am I completely misunderstanding that wiring diagram? This isn't really critical to anything, just curious.

[K04-B6]

I'm going to necro-bump this thread, as I'm planning on doing this as I recently discovered my S4 is suffering from lack of defrost. Light goes on then almost immediately off. I know I have a new HVAC unit as it also has heated seat switches which unfortunately do nothing. Previous owner neglected to mention these little bits when selling the car (turns out heated seats and rear defrost come in handy during New England winters...). But that is neither here nor there.

What I'm wondering is how the "Radio Aerial" fits into this. Does this mean that the antenna amplifier is not currently being powered, but will be after the mod? So I should theoretically get a boost in my radio reception? Or am I completely misunderstanding that wiring diagram? This isn't really critical to anything, just curious.

I have just fallen into this problem as well since I installed a HVAC unit from a 2008 into my 2002. The only thing that doesn't work for me is the rear defrost, MY radio reception is still good though so I don't believe this would fix your issue.

Also, I noticed from the description that #6 doesn't quite match what is pictured for the B7 HVAC diagram once wired. I'm not sure if it matters but terminal 85 and 86 needs to be switched on the diagram since it doesn't match the written description.

[MantanaB6]

I have just fallen into this problem as well since I installed a HVAC unit from a 2008 into my 2002. The only thing that doesn't work for me is the rear defrost, MY radio reception is still good though so I don't believe this would fix your issue.

Also, I noticed from the description that #6 doesn't quite match what is pictured for the B7 HVAC diagram once wired. I'm not sure if it matters but terminal 85 and 86 needs to be switched on the diagram since it doesn't match the written description.

Yea I see they have terminal 85 going to ground in the photo, but 86 to ground in the diagram. I'm assuming the photo is the correct way. My radio reception is actually fine, just was wondering if it would be even better performing this fix.

[K04-B6]

Yea I see they have terminal 85 going to ground in the photo, but 86 to ground in the diagram. I'm assuming the photo is the correct way. My radio reception is actually fine, just was wondering if it would be even better performing this fix.

Oh I see what you mean. I wouldn't know about that, I don't think they're linked since I believe the power just connects straight to the defroster at the window. I could be wrong though..

[Matt Devo]

What I'm wondering is how the "Radio Aerial" fits into this. Does this mean that the antenna amplifier is not currently being powered, but will be after the mod? So I should theoretically get a boost in my radio reception? Or am I completely misunderstanding that wiring diagram? This isn't really critical to anything, just curious.

I think whoever made the diagram just wanted to convey that the heating element and antenna are the same physical item; obviously they are powered separately or you'd only have reception when the defrost was on :)

[Matt Devo]

Yea I see they have terminal 85 going to ground in the photo, but 86 to ground in the diagram. I'm assuming the photo is the correct way. My radio reception is actually fine, just was wondering if it would be even better performing this fix.

the short answer is it doesn't matter, since polarity doesn't affect the relay's operation; as long as the trigger and ground are connected to the coil connectors (85 and 86) then you're fine

[K04-B6]

the short answer is it doesn't matter, since polarity doesn't affect the relay's operation; as long as the trigger and ground are connected to the coil connectors (85 and 86) then you're fine

That's what I had figured but just wanted to be 100% sure. Thanks

Mitch

[MantanaB6]

I think whoever made the diagram just wanted to convey that the heating element and antenna are the same physical item; obviously they are powered separately or you'd only have reception when the defrost was on :)

Yea that's what I figured haha. I actually thought about the fact that the antenna would only work is defrost was on which led me to believe it was incorrect, but thought I'd throw it out there anyway!

[DPDISXR4Ti]

I've been searching through the HVAC control unit upgrade threads and there's been a mixed bag of comments, some in conflict with each other. This one seems to be the most succinct yet complete, and I know Matt has a clue, so that's a factor. My car is an early (12/01) with peeling buttons and no "Off" button.

I was initially under the impression that the circuitry changes were required due to the addition of the "off" button starting in 2003. I now believe that is not correct and the change is just related to the rear defroster, correct?

Just to confirm, the addition of the relay is the same, whether I went with a late B6 unit or early B7, yes?

Lastly, I've read about some soft-coding being required(?) to take advantage of some features offered in the B7 unit. Is this optional, and can be done (or not) at a later time?

[DPDISXR4Ti]

Since I've got everything apart for the B7 console swap I thought it would be a good time to do the HVAC swap. As mentioned in my post of 8/25 above, I'm looking for some clarity, and in the mean time I have some more details. I happen to have three HVAC units in front of me now, all with heated seat rheostats, as follows...

B6 early 2002 (no "OFF" button) p/n 8E0 820 043 G
B6 early 2003 ("OFF" button) p/n 8E0 820 043 L
B7 2007 ("OFF" button) p/n 8E0 820 043 BM

I clipped off the wiring harness from the two later units I got at the junkyard, so I can offer the following... It appears to me that the 2002 and 2003 are identically wired and so can plug-n-play without any further wiring modifications. So basically, if all you want is the "OFF" button, this is the easy way to go. Can anyone confirm?

The B7 unit has only 4 wires coming out of the F connector whereas both B6 units have 5 wires. Keep in mind, this is different from what Matt has pictured above (2 wires from F connector), probably because his car doesn't have heated seats.

If you want the external relay for the rear defroster (probably more reliable), and/or a HVAC unit without missing button paint (the B7 units seem to have more durable paint), the B7 HVAC unit is the way to go.

[Matt Devo]

Since I've got everything apart for the B7 console swap I thought it would be a good time to do the HVAC swap. As mentioned in my post of 8/25 above, I'm looking for some clarity, and in the mean time I have some more details. I happen to have three HVAC units in front of me now, all with heated seat rheostats, as follows...

B6 early 2002 (no "OFF" button) p/n 8E0 820 043 G
B6 early 2003 ("OFF" button) p/n 8E0 820 043 L
B7 2007 ("OFF" button) p/n 8E0 820 043 BM

I clipped off the wiring harness from the two later units I got at the junkyard, so I can offer the following... It appears to me that the 2002 and 2003 are identically wired and so can plug-n-play without any further wiring modifications. So basically, if all you want is the "OFF" button, this is the easy way to go. Can anyone confirm?

The B7 unit has only 4 wires coming out of the F connector whereas both B6 units have 5 wires. Keep in mind, this is different from what Matt has pictured above (2 wires from F connector), probably because his car doesn't have heated seats.

If you want the external relay for the rear defroster (probably more reliable), and/or a HVAC unit without missing button paint (the B7 units seem to have more durable paint), the B7 HVAC unit is the way to go.

correct on all counts

[DPDISXR4Ti]

correct on all counts

Great, thanks Matt. I think I'm just going to put the 2003 unit in for now and come back to this at a later date. I need to drive the car tomorrow and the interior of the car looks like a yard sale right now.

[fR3ZNO]

I finally got around to doing this for a B7 HVAC unit. Thanks Matt for the DIY! I was initially thrown off by the pics in the OP expecting to see 2 wires on my F plug while it actually had 5, but as stated earlier that's because I have heated seats whereas OP's car doesn't.

I got everything wired up and the rear defrost light stays on unlike before where it used to shut off shortly after hitting the button. But I've tried it a few times in the morning on my way to work, and it doesn't seem like the defrost is actually working, as in the element isn't heating up. I'm pretty sure everything is wired correctly. Could the fuse be blown? Which fuse in the fusebox is F26 as per the diagram? I don't recall seeing anything on the fuse listing on the cover about rear defrost.

Edit: ended up being a bad fuse. also F26 correlates to #26 in the fusebox (duh). replaced it and all is well now.

[DPDISXR4Ti]

Finally got around to putting the B7 HVAC unit in today. I found that the metal frame was grounded so I just put the ground circuit under one of the T-25 headed screws that fastens the frame.

Everything mostly seems to be working. The defrost light stays on continuously when I switch it, but I am hearing the relay click intermittently without me doing anything. Does it just do that when temperature is above some level or the engine is not running perhaps? I checked fuse #26 and it's fine.

[DPDISXR4Ti]

A bit of an update.... Took the car for a ride yesterday and had the radio on (FM station). I then turned the rear window defroster, only to have the radio go into full static mode. Turned the defroster off, and the radio returned to normal reception.

Bad ground? Bad relay? I'm pretty sure I have things wired correctly. Does it truly not matter whether it's pin 85 or 86 which is grounded?

[DreamingofZ]

I have a late 2003 with the Off button. I picked up a 2006 HVAC unit from the local salvage yard. Rather than wire in a relay I simply swapped the front control panel (buttons and surround - no circuit boards). Took me about an hour (the 7 locking tabs are finicky). System works as intended since it is the original internals.

[DPDISXR4Ti]

I have a late 2003 with the Off button. I picked up a 2006 HVAC unit from the local salvage yard. Rather than wire in a relay I simply swapped the front control panel (buttons and surround - no circuit boards). Took me about an hour (the 7 locking tabs are finicky). System works as intended since it is the original internals.

Good to know that's possible. That said, the factory clearly had a reason for redesigning the circuit (adding the relay) on the B7, so the approach that Matt laid out here is the better way to do it.

...unless you're me and it's not working right! I actually haven't gotten back to this project and I really should as colder weather approaches. My first step will simply be trying a different relay. My concern is whether there might be something different on the very early cars (mine is a 12/01 build), that prevents this modification from working properly.

[DPDISXR4Ti]

Here I am one+ year later and I'm trying to get this working. So tonight I took everything out and went for the parts-canon approach since I had spare parts. Tried another B7 control unit and then tried swapping in a different relay. No change at all. When I turn the rear defroster on the LED stays lit but I can hear the relay (I presume) clicking at random intervals from 2 to 8 seconds. I actually see the cabin lights dim ever so slightly when it comes on (I'm assuming).

Any suggestions on diagnostic steps?

[DPDISXR4Ti]

So I *think* I've solved this. First off, I'm inclined to believe the cycling on and off of the relay may have been a function of temperature. It was ~55 degrees in my garage when I tested this and perhaps the control unit knows this and knows there's no reason to be defrosting the window at that temperature. Or, perhaps it's just that the engine wasn't running and delivering good amperage and so here to, the logic of the control unit decided not to deliver power continuously. An additional factor could have been that my battery was weak (I've since replaced it). Lastly, the new Bosch relay may have made a difference vs. the one I had in there originally. No more static through the radio when the defroster is on.

Long story short, it seems to work as it should now. The lesson is don't test with the engine not running in a warm garage. And, use a Bosch relay. The one I'm using actually came from a late 80's BMW. I've collected a few of these basic 30A SPST relays over the years and have successfully used them in a couple different projects.