MP4.2+6.0

PaulW lives more over on QW, but he posted something on his 2007 S8 I had a mental note of: his big improvement fix to an S8 he had recently bought with so-so A/C cooling turned out to be the lowly (and cheap) exterior temp sensor up in the front grill area.

Long story short, seems like same here and his core fix worked great for me too. Deeper dive with some verbal walk through of the experience and the replacement below.

Starting in early August even in just moderately warm scenarios I noticed A/C just wasn't up to snuff--car felt stuffy and kind of muggy inside (and I am in the SF Bay Area, not the humid parts of the country), it was slow to cool down inside after driving and parking in sun, etc. It did cool though either on longer drives, or if I started from the garage with engine cold, even at mid day when it was hotter outside.

I did the usual. Ran the check for HVAC module codes. None. Ran diagnostics through VAG COM that basically involves cycling all kinds of HVAC solenoids, fans and subsystems. All good. Started to conclude it maybe just needed a refrigerant charge, though I don't like to go there as more a "symptom" of a leak or component failure than a full cure in the longer run.

I also found it was definitely sun and heat load related but I couldn't quite put my finger on it. I tried things like covering the dash solar sensor with a piece of loose paper. Seemed to have a bit of effect. Also noticed main motor radiator fans started cycling in a way I have never run into before--from very quiet to the full-on turbo jet blast rarely heard except on the hottest of days, but then back to quiet, in 60-90 second intervals. Weird. Made me wonder if low pressure A/C switch might be kicking system in and out if not enough charge, but the earlier VAG COM diagnostics did not suggest a pressure issue. Still figured I would be likely headed in for the system check/flush/recharge but I knew that a code scan would show zero. One day with a lot of hot driveway idling (think W12...) I actually also saw the outside temp read way high because of the heat coming off the whole motor and all; normally it doesn't get confused or get heat soaked like that at the sensor, but married with some erratic fan action, maybe it got a lot more affected than usual. Another pointer at sensor as a possible, though with weird motor fan action I was starting to think about the car's (not fun to change) coolant temp. sensor.

From what I ran into, I rolled dice and changed the solar dash sensor and outside temp sensor at same time. Outside temp. sensor is the cheaper of the two; solar sensor is the much easier to install once you know that little cover in the dash center pops off--pried gently from the sides farthest away first, not the front closest to you inside the car. Given PaulW's experience I would say it was probably the outside temp sensor for most or all of issue, but I just wanted to be done and move to other stuff.

Wow, night and day difference was the net right after the change and on first drive!! First time I drove it, I constantly had to RAISE the temp, finally at or above 75 degrees on setting. Checked rear settings too. HVAC fan speed came way up too in auto mode. And, a telltale sign of big time cooling, for about the first time ever (certainly outside winter), I dove back into MMI to actually play with raising the center vent temps. Before I always wondered why one would do that much and just unwind some of the air cooling.

In the intervening several days I have just let it do its thing and didn't want to declare victory or post before some more driving, parking in sun, etc. The latter was the acid test, since I saw the consistent issues after restarts with car parked in sun a while. Well, it cools just like its younger and good days once again, no flaky fan spin ups I've heard. It seems like it has "learned" somehow too--I don't have to dial up temps again like the first drive or two after the sensor change (I usually find 71 is about my settle point, and wander from 69 when hottest up to about 74-75 if a lot of sun load on interior, esp. dash. Last several days 71 or 72 is where it's happy again.

BTW, the front outside temp. sensor was tricky but I pulled it off a step better than PaulW, who started down some of the wheel well pull back stuff as I recall. On older multipart pre fish mouth grills, it looks from Bentley like no huge deal to take upper grill out. But on any fish mouth, Bentley basically says you need to pull bumper skin to get at this. I still have never done that of necessity on any Audi, ever, timing belts included.

In this case, I found after the plastic piece just in back of the grill running across the front sill area was removed (carefully), I could get to the last two hidden larger screws holding the upper bumper skin and grill. Then I pulled forward manually in the area above the sensor to try to get access to it, watching not to force it too hard and not snap something. I had already spotted the sensor through the grill slats in the shadows, so I knew it was just left of the centerline several inches down and in front of the radiator/AC panels. I managed to wedge one and then two MagLite flashlights in the gap I was pulling open, turned backwards as I stuck them in--basically just as a spacer of a type not likely to scratch stuff and rounded to reduce stress on the plastics. With that I could now work freely without painfully trying to hold that gap open with my hands, and I could see the sensor directly and manipulate it pretty well w/ my fingers. I used a flat screwdriver in from the grill slats to press at the side of the sensor where it pressure fits into a small bracket while I pulled up at the connector itself. After 5 minutes of fooling, I got it loose of its positioning bracket. Important to leave wire connected at that stage so sensor didn't fall into the inaccessible abyss down there. Now with sensor out of the bracket, I could raise it toward me and just worked the wire connector lock clip and unplugged it. Only trick to new one was reseating it in the same bracket. Again, I plugged it back in first thing and then positioned it near the round bracket hole it locks into. A couple minutes of pushing at it just right with a long bladed screwdriver and it seated. Buttoned grill back up and from there got the results above. An hour and a half + of fooling and trial and (not much) error, but now with a description of the technique and one experience (I never accessed that area before on the car in any way), it would be 45-60 minutes for me if I did it again.

Last, I didn't try any before- and after- resistance tests on either sensor, or try to install and test serially. Nothing was visually wrong with either, though outside sensor seemed to have a thin layer of accumulated greyish brown road grime--like you could find on any part in that area behind the grill but not subject to engine oils and such. If I were do do it on the total cheap, given what I now know, I might simply spray the sensor with a strong garden hose nozzle through the grill as a simple attempt to clean. Also while both sensors are simple two wire affairs, given stuff I saw HVAC performance wise and the weird radiator fan spin up and all, the readings must feed in a very important way into some electronically programmed system maps, even though both before and after the outside temp display driven off that outside sensor seemed right.

Net, if A/C is disappointing but no obvious codes, leaks, etc. think about at least the $15 outside temp sensor, even if it seems to read fine. As PaulW found earlier this year and reposted a few times that I saw, it was a big improvement here too in the remaining warmer days. It's back to nothing to apologize for in any part of the HVAC and it just plain works, as it should be in a nice car.

Brozee

Good post here MP4.2; this could also have been an issue I expereicned with the A/C on my W12 as well. Although, I almost always had the solar sunroof disabled as I found it didn't help that much and I thought the wear on the fans vs. the benefit received from the modest air output didn't weigh out enough for me. In the end I ended up having the A/C recharged and it seemed to fix the issue. Although it was never as cold and direct as my fathers 2004 old accord (never recharged) or my buddies new car.

ltooz_a6_a8_q7

MP4.2,
Now since you mention it, sometimes my AC seems to blow so cold and sometimes it doesn't - Do you have the picture of where and what this temperature sensor look like? I may just get one, but won't change it until the AC is totally haywire in the future.

Thanks,

Louis

Brozee

My air would work fine if it were parked in a garage or in my parking garage at work. When parked in the sun then using the air, that is when it wouldn't work well.

MP4.2+6.0

in sun was mine, especially if run once. And I don't mean the sun warming the interior air--I could leave windows down and still have a similar issue.

Probably came down to the sensor(s) somehow not generating the expected signals/resistance at various specified temps, even though at baseline temps and no external heat confusion (engine heat soak, solar load just sitting there, etc.).

Since PaulW also found same thing (and from QW maybe another one or two tracking from his post), seems like a strong linkage between common A/C performance maladies that can't be found by standard diagnostics and the sensor.

ltooz_a6_a8_q7

Yes, now I think of it, a few days ago, it was hot, I parked under a tree, the air was so cold out of the norm - I always park in the open under the sun and it wasn't cold noticeable as when I parked under the shade.
I need to study about this temperature sensor thing. It seems the control system of the AC has a bug somewhere.

Thanks guys,

Louis

Galco

MP, would you be able to provide a picture of the location of the outside ambient temp sensor? I believe your fix is exactly what is keeping my 2006 from optimal cooling. I've read your description a few time (its awesome) but still question which sill cover you are referring to. Also, perhaps after putting an eye ball on the sensor, with help of a photo, the rest of your process description will make even more sense.

As usual, your advise and detail are most appreciated!!!

Thanks in advance!

1Wasserwerks

Nicely written.
I'll have to log that in the back of my head with all the other tidbits from these cars...

MP4.2+6.0

To tie posts together, after this original post from last year the A/C erratic issues returned on some later year warm days. Per a later post, I had the system refrigerant checked during the (mild CA) winter and it was down by approx. half. The system only holds something like 630g, so down not a lot by older standards now means it is literally half empty in a D3 system. The service was last winter, and it has performed flawlessly w/ the return of summer and warmer weather.

Notice my earlier original post itself referred to a post by PaulW, found more commonly on QW. Well from his posts there, he later found the same need to get the refrigerant recharged, and it was done the first time w/ shop equipment and confirmed as low. Apparently he has a leak, since he had to do it again, this time DIY, but at least w/ a can with a gauge on it. Here is his later reply in that thread where he says what he did, plus you can get back to his starting post and other replies: quattroworld.com Forums: This is a great product In my case, performance has been consistent ever since the recharge, so hopefully mine is just the more expected with time and miles.

Per Galco's reply today, here is a picture. On mine--a W12; fish mouth grille in all W12 years if you don't have fish mouth--the outside temp sensor is exactly straight behind the far left ring of the four ring grille element. The wire comes down to it from above, which I can also see (I think) through the grille slats just above the four rings. Look in the shadows of the very upper part of the ring area though the slats and you can make it out in this picture:

ohsigmachi

Is this part also known as the "Ambient Air temperature sensor"?