HVAC Issue

[tonyt102]

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.

[Adrian E]

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....

[tonyt102]

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.

[The_Laird]

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

[tonyt102]

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.

[Architex_mA8tey]

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

[ainarssems]

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

[tonyt102]

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.

[ainarssems]

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.

[Hodgy by sea]

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,.