A4 Phil

I have a 2002 S6 which has 71K on it. I have driven it with no problems. Today on my 31 mile commute back from work at freeway speeds, the AC dies. I notice the humidity in the cabin and the lack of heat immediately. I try cycling it off and on and nothing responds. 15 minutes later the high temp warning goes off and I notice the needle creeping to the H. I pull into a Safeway and check the coolent. It is fine. No dip in coolent level. I drive home 10 blocks and pulling into my driveway the steam starts billowing out. I cut the engine and coolent bubbles for a bit.

Any ideas? I am sure that the AC and the engine temp must be related. I just did a visual check for belts, the two at the front of the engine and they are still there. What can I check or is this thing not running without a tow to the shop?

TIA.

Redneck Truck

i cant fathom why, at highway speeds, the ac would cause the engine to overheat. if you were moving slowly the whole time, youve likely got a dead fan or two.

A4 Phil

From the little bit that I know, Audi's kill the AC as the engine heat passes a certain threshold. At least that is what I gather from doing a search on the forum.

This means that the overheating started at highway speeds. I should check the fuses but aside from that I have no idea what to do short of towing to the mechanic.

Airbag

Definitely start with the fans ..I would suspect ther is not enough flow through the rad and the condensor if they wer in fact related .. When my fan comes on when it is idling it is loud and moves A LOT of air ... start there.

A4 Phil

Well after a little troubleshooting I have determined that BOTH fans are not working. The primary fan closest to the driver and the aux fan closest to the bumper are both not turning. I have VAG'd it and the computer cannot turn the fans on either. There is also a code being thrown for a bad temp sensor.

The good news is that when you bypass the sensor and apply current, both fans turn.

The not so good news is that I cracked the coolant tank driving the car the ten miles to get it VAG'd.

Hopefully this will turn into simply a sensor replacement. I'd like to avoid major repairs until the timing belt coming up in 20K.

Redneck Truck

Hopefully that's not the only thing that cracked. How did you test the fans in vag com? I'd like to give this a shot on mine, because it seems like the electric rad fan should be turning on and it doesn't.

A4 Phil

My story took a turn for the worse. The temp sensor was replaced. The fans both started working, but then the bad news. The thermostat is stuck closed, broken. After running the engine the radiator is still cold, which means that water is not circulating. That is bad.

Long story short, a thermostat replacement is a serious repair which will take a mechanic seven hours. So at 71K, I am just going to bite the bullet and have the timing belt replaced as well since the same components need to come off. May as well only pay for the labor once.

Ugghh...

monkeytronic

FWIW the C5 4.2 and S6 sport a double-pass radiator which demands a lot more from the water pump than the single-pass radiator all the V6 models are equipped with. In a nutshell, the coolant inlet and outlet are both located on the passenger side of the radiator and coolant travels twice though the radiator: first through the top half of the core towards the driver side then through the bottom half of the core back to the passenger side.

Basically a double-pass radiator requires 16 times more pressure to flow the same volume of coolant through it than a single-pass radiator.

So even at highway speeds where there will be more than enough ambient air being forced through the rad core without need for any fans, the water pump still must keep up with coolant pressure and flow demands or else overheating will ensue.

A4 Phil

Well actually, I appreciate your post a great deal. I like knowing the reason things are happening around me. Well you are absolutely correct about the S6's V8 radiator. I had a bit of a primer by the mechanic today.

I am hoping that with the timing belt replacement and all of the replacements which that entails, the new water pump and thermostat as part of that, the problem will be fixed. At this point I am actually wondering if I should just have the fans replaced as well at the same time. With the entire front end off, it may be the right thing to do.

Anything else I should look at while I am spending the time and money?

Airbag

I have a 2000 a6 and when I did the timing belt i did the idlers, tensioner roller and hydraulic tensioner piston . also replaced both front cam seals.
In hind site maybe your fans weren't coming on becasue your coolant sensor wasn't tellin them too . and your coolant sensor wasn't telling them to becasue your stat was stuck closed ..

i think Redneck meant to say he hopes your coolant tank was the only thing that cracked

pretty cheap part though and makes your engine lok new again