Kpj

Hello all.

I have a 2005 (D3) A8L W12 which is giving me a coolant warning on the dash's mini-mmi. The temp gauge (on the dash cluster) stays all the way to the left on 'C' and does not move while the warning light flashes constantly (red coolant indicator) and the radiator fans run at full power!

I had the vehicle checked out in Traverse City, Michigan while on vacation by Marathon Auto and they found no leak or any overheating (Code #: P0118). They said that the error seems to be with the dash computer as the coolant sensor is sending the correct information to the ECM (engine control module), but the data does not compute at the dash/instrument panel.

I've done numerous searches and cannot find much info on this exact issue with the W12 (I know we owners are few & far between). Any idea what I can do/look for? Is there a way to wipe out or re-boot the main computer?

Any help would be appreciated as I don't want to spend thousands at the dealer for a fix that could be a DIY. Lastly, any Michiganders (in the Detroit area) that can recommend a 'good/reasonable' & knowledgable mechanic that works with this particular car?

Thanks again!

s4master1

tell them to check all 3, ECM, A/c and instrument cluster for temperature readings, if they within 5 degrees difference the sensor is ok, if not, replace the coolant temp sensor.

MP4.2+6.0

I doubt you will find someone specialized in W12's outside of the dealer. The total number in the whole U.S. for all years is probably < 1,000, many of which are still under some kind of warranty. While a different motor, the electric/electronic control system of the S8 may actually be similar (both being high output/high cooling need), and both using two ECU's as I mention below for the W12.

Outside of the dealer (or even if you went that route to get more data), the next thing I would do is scan it with a VAG-COM. You need to access the Audi specific fault codes, and the generic OBD-II stuff isn't going to come close to pinpointing the issue. I haven't had an issue anything like you describe on my W12 (I did on an old '85 C3; more below), but typically if you use VAG-COM you can zoom in much better, often right to the faulty part. Certainly it will sort out for you if it is instrument cluster or ECU generated, or much more likely one of the sensors.

As far as my own history with something similar on an old 85 5000, I learned then that Audi often uses sensors with multiple connections--they may not be fully integrated electronically, even if they are contained in the same sensor. Electrical terminals beyond two are the usual clue. Aside from the ground, one terminal may run the dash gauge, another may send data to the fan, another to the ECU, etc. Newer cars are much better on integrating this onto a single or at least fewer wires, but not always.

Specific to the W12, looking at the motor self service training book I bought some years back, I see several sensors that could be relevant:

G2 Coolant Temperature Sensor
G62 Engine Coolant Temperature Sensor
G382 Drive Circuit Cooling Fan Temperature Sensor

The wiring diagram looks like G2 and G62 may be physically together and share a common signal into the ECU, but that's just a guess from the schematic. G382 looks like it has a wire both from and to the ECU. I don't know where the sensors are physically; look at the Bentley manual for that. Typically in a water line somewhere at least somewhat separated from the block and heads to mitigate heat soak.

I can't diagnose it (and would 100% be using a VAG COM to try), but with a set up like that and what you describe (no gauge reading, warning light, high fans, but not overheating), I would be suspecting a sensor. The sensors all feed ECU 1 by the way (W12's have two ECU's). No, you can't just unplug it to clear something, but again with VAG COM, after you read the error codes, then yes you could clear the codes. Some simpler code readers might allow that too, though I have no idea if they can deal with two ECU's like VAG COM can.

And by the way, having said all this, when you have flaky temp readings, fan operation, etc., the thermostat is also a suspect. The contra to that is you say it isn't actually overheating. On the W12, it's an electronically controlled thing. W12 owners don't seem to have much issue with them (at least that gets onto the boards), but I wouldn't just dismiss it either. Sensors though are the pretty obvious starting point.

Good luck.

Kpj

Thanks for all of the info guys, I greatly appreciate it.

As for the VAG-COMs, are they something a normal person can handle/understand? Or am I better off just surrendering & taking into the dealer for diagnosis? I was also thinking about purchasing the A8 repair/technical guide that the mechanics use to remedy all issues with the car. Are these manuals/documents something of value to the amateur DIYer?

Continued thanks is being sent from here

MP4.2+6.0

Go to the Ross Tech website to see about the VAG COM. http://www.ross-tech.com/vag-com/index.html About as friendly (and comprehensive) as these things get. Makes the stuff the BMW/Mini crowd has to contend with (as I am learning with a Mini) look like you need to work in assembly code by comparison--in German! First time or two you pinpoint an issue or two, you will likely have paid the tool cost.

You can't really buy the Audi full shop manual as a practical matter, but the Bentley disk based manual that runs on PC's is the closest approximation. Also includes the Audi TSB's that are a helpful resource of common/recurring problems and their fixes. A lot of the manual is probably beyond what you would ever dive into as far as many things it covers, but as shop manuals go, its pictures and descriptions are generally solid, and often source from Audi to begin with.

Kpj

Well, the 8 is in the shop being worked on. The dealer informed me that it is the 'bottom' coolant sensor that is failing. Of course, it's the one where the engine needs to be removed :/

Not looking forward to the bill....

mishar

For every sensor that need engine removal in order to be changed somebody in Audi should be fired. Starting from chief engineer all the way to the President.

MP4.2+6.0

I got your message too.

First, I'm not clear what "bottom" sensor really means. If it meant it were below the motor or radiator, then I would bet you could get to it somehow, perhaps with some intermediate disassembly if it were toward the front.

I also looked at Bentley. I use a Mac and Bentley is marooned on an old PC that still runs XP; had had trouble printing w/ it, but I did take a few screen pictures if needed. If so, send me a message w/ your e mail.

If they basically mean the coolant temp sensor, looks like pretty bad news on the W12. It appears to be behind the right cylinder head (drivers side on a US/left drive car), up above the tranny. On a 4.2 (at least my C5), this sensor is back there too, but it's more centered between the heads and there is more space to get at it.

D3's did shift the motors back relative to the front axle line by relocating the torque converter. Probably has tightened up this space in general. Then looking at my W12 under the hood, that space is really tight, very close to the firewall, plus there is an aluminum insulation piece fitted around the whole rear and side of the motor back there. Also, there are wiring harnesses and what look like vacuum/smog hoses. Thus really a challenge no matter what. Clearances are less than 1" most likely. From the intake manifold down to the tranny area could be the better part of a foot, so that's a very inaccessible area overall. I didn't pull anything off mine, but if I were to try to get at it, I would say taking off the intake (both upper and lower portions) might begin to open up some space. The heads are really wide on a W motor though (thus there is a lot of space to try to reach somehow (left to right) behind the head ); thus unclear if that might even work. Valve cover would be next thing I might try to pull to yield a little more space.

The manual does talk about removing the motor to get access to this area in general, but it's not specific to the sensor. The sensor only seems to be documented in the context of the rear cam drive, as a step on the way to getting at that cam stuff. I looked in the engine cooling section, but didn't see anything specific there. The W12 SSP (self service program--what Audi uses to orient/train it's techs) documents the sensor as far as basic electrical schematic, but not location.

Early suggestions:

1. Try to get dealer mechanic to point you physically to where the sensor is. If you are friendly with the service writer, they may pull the guy in, especially since facing a motor removal you would logically want to exhaust all the other options. See if it it matches what I am saying.

2. From the pictures, it looks like it is fastened into some hoses (as in, it doesn't screw into a hard pipe or part of the motor/head. You could consider buying the sensor and the hoses. It could be trying to change it out would be easier somehow if you undo the other end of the hoses (rather than the sensor end). If you intend to have it serviced one way or another anyway, you won't lose money buying the parts first. With the parts in hand, you can now see visually what exactly you are going for. Also, as another variant and how to get more info, just go to the parts counter and have the guy show you the parts image for this area. That will help orient you too, and those images are usually not the same as the Bentley diagrams either. You could probably get them to print it for you.

Mister Bally

Any chance you could remove carpet next to or behind the brake pedal, remove the pedal assembly and use a hole saw to make an access hole? I realize some structural integrity may be sacrificed, however, with a car out of warranty, I'd consider it. Especially since R & R of the W12 has to be over $3k and possibly over 4.

s4master1

Any chance you could remove carpet next to or behind the brake pedal, remove the pedal assembly and use a hole saw to make an access hole? I realize some structural integrity may be sacrificed, however, with a car out of warranty, I'd consider it. Especially since R & R of the W12 has to be over $3k and possibly over 4.


WTF!!!