Zone A/C Difference in Temp?

[CornersWell]

All,

Will try to do more homework on this, but thought I'd throw it out there and see if anyone has any thoughts.

Just returned from extended travel and started up the RS6. Of course, since it’s now warm here, I tested the A/C. Was set for 70 in both driver’s and front passenger's zones. Blew seemingly cold air to passenger's zone, but clearly warm(er) air to driver’s. Eventually, the air for the driver’s zone did cool down (some), but was still not blowing at the same temp for the passenger’s. So, it’s warmer on the driver’s side. After driving it for a bit and parking, when I restarted, one was cold and the other was hot. Again cooled down, but not to the same temp.

It’s been a while since I've had an a/c service on it. I assume it’s just one a/c system, so I don’t know why one side would blow colder than the other. Maybe the driver’s zone temperature sensor has gone bad?

Thanks,

CW

[nubcake]

Probably one (or more) air flap motors gone bad.
Just one heater & AC core.

[CornersWell]

Thanks, Nub.

That could well be the problem. I watched a couple of videos describing the issue, and that's consistent with my experience. I'll have a closer look now that I know what I'm looking for, but it seems like an easy fix, if it is. Just have to get to the actuator, which is a bit of a job, apparently, for the driver's side.

CW

[hahnmgh63]

Those servos are pretty reliable but they're all getting older now. Depending on which one, they're not fun to get at.

[CornersWell]

Scanned with VAG, but HVAC throwing no codes.

Indy shop thought it could be the blower box. If so, that would require dash removal. $$$.

Not sure what's next, but I have an appointment with them tomorrow. Hopefully, we'll know more after that.

Looks like central vent is fed from a single duct, which splits in the last few inches. Not sure why one side would be perceptibly different in temperature, then.

HVAC ran at full tilt for an entire 1-hour drive in 85 degree heat. Driver side air blowing was still warmer than passenger side.

CW

[CornersWell]

Was back at the shop yesterday. Still throwing no HVAC codes. Ran a test program that specifically run the servos/actuators that was able to get three HVAC codes thrown. Ordering those parts. Of the three, two show NLA. Purchased one part through their supplier and two on eBay. Awaiting shipping. Once in hand, appointment will go on the calendar. We'll see. The good news is that they were able to observe the problem first hand and confirmed that I'm not insane.

CW

[hahnmgh63]

When you get the servos see if you can post the manufacturer and/or the non-Audi part numbers and I bet we can find replacements that way. I would imagine they are used in a few different German models/manufacturers.

[CornersWell]

Here you go...

Parts I needed (3 of 4 servos) were...

4B1 820 511
4B1 820 511 A
4B1 820 511 C

Here's photos of the NOS in box (C) and the used (A). The indy got the other one for me.

CW

[lswing]

Of all things......this is now happening to me, doh! :mech: Of course it has to be the drivers side...

Was thinking that my AC needed recharging, and it was odd that it still got cold at times. Well sure enough last week I felt the air vents on the passenger side and they were nice and cold! Driver side blowing air temp of 80 degrees or whatever, ugh. So the cold ac is not getting routed to my side of the blower/fan/switch system or however. Please keep us posted, and I'll do the same as we try and figure this out. Luckily not my main car and my commute is short.

[CornersWell]

Traveling for a couple of weeks, so RS6 doesn't go in, again, until 8/4. Will post after servo replacement.

CW

[lswing]

Three of the four look relatively accessible...

<iframe width="560" height="315" src="https://www.youtube.com/embed/eKFB0fRdUnU" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

[CornersWell]

Helpful. Thank you. In order to save time at the shop, I went ahead and changed the "A" and "C" myself. Easy. Shop is doing the difficult-to-reach one as I write this. Hopefully, problem will be solved.

CW

[CornersWell]

Well, yesterday was a LONG day for the tech. We believe the problem is now fixed. Fingers crossed, as some warm weather is due in the next week. So, I'll confirm that. BUT, here's the report...

The problem is, most likely, the driver's side servo (which is this part number: 4B1 820 511...note the lack of an alpha number code at the end). The problem is that this servo is buried behind a structural support beam and plate. VERY difficult to access. I was charged 5 hours of labor. Arrived at 9a. Didn't leave until 530p. I added an oil change, since I was there, and it was almost due, anyway. That was a few more $, but the total invoice was in the $1,150 range.

As I said, the "A" and "C" servos are confirmed to be easy enough, per the video, and my own experience. But, the " " part is a b*tch.

CW

[lswing]

Well, yesterday was a LONG day for the tech. We believe the problem is now fixed. Fingers crossed, as some warm weather is due in the next week. So, I'll confirm that. BUT, here's the report...

The problem is, most likely, the driver's side servo (which is this part number: 4B1 820 511...note the lack of an alpha number code at the end). The problem is that this servo is buried behind a structural support beam and plate. VERY difficult to access. I was charged 5 hours of labor. Arrived at 9a. Didn't leave until 530p. I added an oil change, since I was there, and it was almost due, anyway. That was a few more $, but the total invoice was in the $1,150 range.

As I said, the "A" and "C" servos are confirmed to be easy enough, per the video, and my own experience. But, the " " part is a b*tch.

CW

It's a pain finding out just which one controls what, not sure if it's in a bentley or etka manual maybe.

Here are some links i found of the part you're referring to...4B1 820 511...note the lack of an alpha number code at the end

Appears to just control the driver footwell though, and I think from reading about the other A-passenger footwell, C-central/dash flaps, and D-fresh air part options that you replaced, SOME of those also control the drivers side vents, probably dash and defrost, just not driver footwell? Driver and passenger footwell appear to be separate.

https://www.audiusaparts.com/oem-parts/audi-adjust-motor-4b1820511

https://parts.audiusa.com/p/Audi__A6/Adjust-motor-HVAC-Air-Inlet-Door-Actuator-HVAC-Blend-Door-Actuator/48791381/4B1820511.html

https://www.ecstuning.com/b-genuine-volkswagen-audi-parts/servo-motor-left/4b1820511/

So my guess is you will get cold air through the dash vents, and defrost vents, but not your driver footwell. Wouldn't be the worst, just toggle off the footwell fans so it's not blowing warm air at least.

Luckily i've got another car to drive as it this problems sucks when it's 90+ out...my footwell and dash drivers side blowing full on warm air, all passenger vents nice and cool, doh!

[rah]

Glad you had it sorted out!

i am running into the exact same issue, and my California trip a month ago was a complete nightmare due to it. I posted on here in the classified for those parts but nobody chimed in so i figured i would ask you. I cannot find these parts anywhere right now and my tech has not been able to get it out of Germany either. Where did you find yours if you don't mind me asking? i am looking for the 4B1-820-511 and the 4B1- 820-511L to solve my situation. I have no A/C on the driver side and also does not come in to the cabin except from time to time (almost never)

Thank you

[CornersWell]

Indy obtained one (new) from their parts supplier. ~$200. Indy stated that one of the others I needed was NLA through their part suppliers. The other, new, was almost $500, IIRC.

So, I obtained the other two on eBay. One was new ($85), the other used ($20). ~$105 total.

Indy's diagnostic scan showed 3 of 4 servos throwing codes. Those 3 have now been replaced. 90+ degree temps outside currently. 100+ heat index. Blowing ice cold on both sides. Even after shut down, sitting and re-start. Problem seems to be fixed.

CW

[CornersWell]

FYI...

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=4B1+820+511&_sacat=0

CW

[rah]

Indy obtained one (new) from their parts supplier. ~$200. Indy stated that one of the others I needed was NLA through their part suppliers. The other, new, was almost $500, IIRC.

So, I obtained the other two on eBay. One was new ($85), the other used ($20). ~$105 total.

Indy's diagnostic scan showed 3 of 4 servos throwing codes. Those 3 have now been replaced. 90+ degree temps outside currently. 100+ heat index. Blowing ice cold on both sides. Even after shut down, sitting and re-start. Problem seems to be fixed.

CW

Good to know, this gives me a little hope and a solution for replacement of the 2 needed that are throwing codes in mine. I will go through E bay and follow the link you provided. I found the two needed that i can see, Bosh made and around $25 a piece. Hopefully they will fit (showing C5 A6/Allroad/S6 and RS6 compability) and the problem will be solved. Thanks again!

[CornersWell]

Good luck. At $25 each, not impossible. The video helps to make this an easy DIY project, too, as opposed to a trip to the shop. At least for the "A" and "C" servos.

CW

[lswing]

I'm going to be tackling this soon, wanted to bump the thread. Any more input?

[CornersWell]

Everything is working without issue at this point. So, I think you have a good plan!

CW

[mbagge01]

I would appreciate the help on anyone who can direct me on how to fix my AC problem. My AC fan runs constantly, as soon as the car turns on and I have two faults in vagcom which will not clear. One is

"Coolant fan control 2, open or short to ground"
"Databus power train, missing message from AC controller, implausible signal".

Is the AC control unit bad, and if so how much work is involved to get it out. I see it mounted up against the fan but does the bumper and other parts need to come off to get to it. Could my issues be something else.

[nubcake]

"AC control unit" is the thing with buttons in your center console, not the fan control module you're speaking of.

[CornersWell]

Nub,

When I'm back from travels, I'll be in touch. I still want to do the tune. I have all the cables you told me to procure.

Sorry it's taken so long!

CW

[JakeLight]

I would appreciate the help on anyone who can direct me on how to fix my AC problem. My AC fan runs constantly, as soon as the car turns on and I have two faults in vagcom which will not clear. One is

"Coolant fan control 2, open or short to ground"
"Databus power train, missing message from AC controller, implausible signal".

Is the AC control unit bad, and if so how much work is involved to get it out. I see it mounted up against the fan but does the bumper and other parts need to come off to get to it. Could my issues be something else.

https://www.rs6.com/showthread.php/34078-WTB-RIGHT-SIDE-RS6-cooling-fan-pls-pm-me?p=294445&viewfull=1#post294445

^ This is probably what you're looking for regarding the fan code

-_/