HVAC Issue
 

Hi all.

Been away from the forum for a while. Sold my last S8 of 4 years about a year ago in favor of a newer B6 A6 S-Line Avant 3.0TDI... big mistake. Not sure what it was but I just haven't gelled. Anyway, after a year of looking, I've just bagged myself a rally good Final Edition however, I have a HVAC issue.

Here are the symptoms:

On my way home, there was a constant hot air to the foot well which couldn't be turned off although this hasn't happened since.

Auto function is intermittent. Even when it appears to work, warm(ish) air comes through the foot well but the center vents and screen vents blow cold air. Sometimes when I press auto, the blower stays on one bar and doesn't do anything.

I've read several threads some suggesting that maybe the control unit itself maybe at fault. When I first bought the car I did a full scan and there wasn't any codes present. I've since done a calibration and scan and it's shown up the following codes:

00728 - Potentiometer in Positioning Motor for Left-Hand Central Vent (G136)
09-00 - Adaptation Limit Surpassed
00734 - Potentiometer in Central Vent (G142)
30-00 - Open or Short to Plus



Not sure if it's got anything to do with it but the engine temp doesn't seem to run as high as my previous S8, it never really gets to 90, more like 75/80ish. I've got a feeling it's the thermostat but any other suggestions would be appreciated.

Cheers.

Think you've hit the nail on the head on both fronts - thermostat is probably the first order of business, assuming the cluster is reporting the temperature accurately (measuring values should tell you)

When was the cambelt last done as it's usually changed at the same time - not sure on the V8s if it's accessible without removing the belt? Think by some good fortune it might be....

Cheers Adrian.

Cambelt was replaced 50k ago when the engine was replaced by Audi. I bought the cam locking tool for my last S8 so might have to go down that route... Great.

Might try and get hold of a control unit to try too as the cold air to the top and warm in the foot well may not point towards the thermostat.

Oi, where are the photos? Did I miss them somewhere (wouldn't be the first time)?

Haven't got any good photos as yet. I've got the 20's away getting refurb'd at the mo. They'd actually just been done but I wasn't keen on the colour. I've got some temporary 18" A6 alloys on currently with winter tyres fitted so might just run these until next next year.

Does anyone know if the last letter in the part number is important? I've found a couple of cheap control panels to try. The number on my unit is 4D0 820 043Q but most of the ones I've found have an N at the end, not Q.

In a lot of cases a revised letter at the end is simply a later updated version of the same part number but I would give audi a call and ask them the question before you commit to buy +++

M is for cars with single DIN radio and no heated seats
N single DIN radio, heated seats
P double DIN radio, no heated seats
Q double DIN and heated seats

Cheers for that.

I always thought that all FL A8/S8's were double din???

Anyway, for £30 it might be worth a risk even if it doesn't fit properly, just to see if it solves the issue.

I have not seen any with single Din or without heated seats for that matter, but that's what ETKA says about part numbers last letter.

You might be able to swap the front from your old unit and make it work.

Hi Antony,It was two years ago,you should have said you were after another S8 ,I know where there is a beauty,well looked after and everything done,:ROFL:.

I tell you what, I'd have had it back in a shot!!! That is a really good example.

Managed to get hold of a new control unit for £17 so I'll give that a try. Spoke to my guy at the dealer today and a new one is £1700 apparently!!! Hopefully it'll sort the issue, I can't handle the lack of heat.

We'll the new module didn't make any difference but it was worth a try. Not sure what to try now really. Just noticed today that there is no heat coming from the passenger footwell either. I do have VCDS which doesn't pick up any faults. Any more ideas?

Well today I flushed the coolant system to no avail after a few people suggested there could be a blockage.

What I have noticed is that if I switch the engine off and back on, warm air will come through to the passenger side but turns to cold air 10secs later.

Also got a new fault code today - 01274. Cleared it, fiddled with the climate control for a few mins and it never came back.

I'm at a loss. Any ideas?

01274 - Air Flow Flap Positioning Motor (V71): Faulty

Possible Causes
Wiring/Connectors from/to Air Flow Flap Positioning Motor (V71) faulty
Positioning Air Flow Flap Positioning Motor (V71) blocked
Positioning Air Flow Flap Positioning Motor (V71) faulty

Possible Solutions
Check Air Flow Flap Positioning Motor (V71)
- See Measuring Value Blocks (MVB)
- Perform Output Test
- Perform Basic Setting
Check/Repair Wiring and Connectors

Yeah, saw that on Ross-tech site. Had a check and the 2 flaps appear to be fine. Plus that code hasn't reappeared after a fair bit of fiddling.

:Confused:

So, I'm totally at a loss.

Stripped all of the panels from the tunnel to gain access to the flap servos and all appear to moving and directing air as they should. I just can't work out how the temperature is controlled though???

Also plugged in VCDS and looked at some of the measuring blocks. Found that the left vent temp sensor was reading silly temps of around 35 degrees whereas the driver side was the standard 12 degrees. Decided to switch the sensors over which made no difference, the passenger side was still reading very high. Not sure if this is anything to do with the symptoms of no temperature control (always warm air to the drivers footwell and cold air to the passengers).

Also just found that hot air will come through to the passenger side if I use the rest function????? So confused!

I really don't want to take it to a garage, I hate being beaten.

Sounds like the sensor for the passenger side is shot.. If it thinks it's too hot, it'll always send cold air to that side and warm to the other to keep the balance..

What happens each side if you set the temperatures to Max or Min which would override the sensors?

If switching sensors did not have any effect, then sensors themselves are OK, fault must be in the wiring or climate control unit. If it reads 35 C already it makes sense that it thinks it is already too hot so it's blowing cool air instead of hot but if you switch to max temperature it should be providing warm air.

It would also be interesting to see what temperatures the system reports (and how it behaves) with the sensors disconnected, each side in turn.

Thanks for the ideas.

When the sensor is unplugged, the system reads 0 degrees. Just unplugged it and it made no difference. Also tried a different climate control unit and it was the same.

I was about to try setting one side to Lo and the other to Hi when the two sides synchronised with each other??? Now I can't find out how to turn it off. My A6 has this function but as soon as you adjust the passenger side, it resets. Any ideas?

I have heat!

Managed to get a few mins with the Master Tech at Audi Teesside who although couldn't tell me exactly what was wrong, he did explain exactly how the system works and how the temperature is controlled.

Came home and had a good scratch of the head. I decided to blast the heater cores through at the bulk head. I've already done a system flush but I don't think it can get everything out. Firstly used an air line and out came a fair bit of gunk then attached a hose and thoroughly flushed it. All sorted, I have heat!

Turned out most of the symptoms were just because the system was confused by the sensor readings as only the bottom 1/3 of the heater core was getting warm. Now everything works as it should.

Always good to see a successful (and cheap!) solution. Outright failure of an item is (usually) relatively easy to diagnose but, as with your case a partial failure can be very difficult to get to the bottom of. +++

Did you figure out how the two sides synced, as it's one thing I'd like to do on mine, since I'm the only one in the car for 99.9% of the time, and during the other 0.1%, my mum doesn't fiddle with the settings. I knew how to do it on the A4, but the same trick doesn't work on the A8.

I didn't unfortunately. I'm wondering if it was like a fail safe when the climate control detected a problem. When I went back to it in the morning, the sync had turned off.