mishar

A/C compressor can be stacked on purpose because of worn clutch, but in that case A/C would work well in the Summer.

volvopentaman

Yes, of course, sometimes i don't think far enough...

But could it be that the warm/cold air control flap would have got loose from the actuator motor, and it's stuck on the cold?

The actuator motor is moving, and the potentiometer values confirm it.

Because now i can't get any heat at all anymore... heatpipes still warm?

Or could i have some basic setting in the climatronic unit screwed up?

mishar

Have you checked fresh air flaps just behind the cabin air filter? They should be closed when you press recirculation button. If AC is off all heat must stay inside.

volvopentaman

The flaps closed atleast in test mode (had the filter removed and saw it, so no potentiometer hocus-pocus ), so i assume they also close if i choose it from the MMI menu.

I'll check my basic settigns for the heat control flap, and then verify that the actual flap is moving. (as long as i'd know where to look, without taking the whole car's front apart)?

Any idea where to get a good picture or TSB or SSP about the A8 heating unit?

Haven't found anything, and the volkspage.net site only had for the 4D and touareg.

jakematic

http://www.volkspage.net/technik/ssp/ssp/SSP_282_d1.pdf

SSP_208_d1 covers AC but may be useful

Edit: This SPP is referenced re aux heater http://www.volkspage.net/technik/ssp/ssp/SSP_267.PDF

mishar

+1

That's the bast I have.

MP4.2+6.0

They are in the U.S. W12 SSP (SSP 921403). Skimming through the German one for the W12, part 1 (SSP 267) that I found somewhere in the past online, I see them in there though in somewhat different form than the U.S. one. The U.S. one is more a flow schematic, whole the German one is more a 3D pictoral representation, plus some schematics. My high school German is too rusty though to even begin to decipher if it is saying the same kind of stuff as the U.S. SSP is.

The general A8 HVAC system was also discussed at some length in the fairly comprehensive A8 SSP released when the car was intro'ed in the U.S. SSP 992303. A variety of diagrams, descriptions etc of the ducting, air flows, flaps, solenoids and control electronics.

Meanwhile, reading through some of the new posts in last day or two, it does sound like looking at physical flaps and their movement, and not just testing solenoids w/ VAG COM may be in order. You report your outside air flap is working, which I did find physically detached at one point back on my C3. For what its worth, my W12 heats fine and quickly, so I don't think there is some design shortcoming in the car itself you are running into. I'm in the mild SF Bay Area, but up in plenty cold in the winter North Tahoe it's the same. Like my C5 4.2, it blasts plenty of hot when needed, can melt a good coat of ice and snow remnants off the windshield when it warms in defrost mode, and needs to be turned down if I run the temp up initially. I don't notice any real difference in warm up time from 4.2 A6 to W12 A8.

volvopentaman

Did some more diagnosing today.

The car heats up far better, if driven in city traffic, but not near as well as it should.

Ran the diagnostic output test again, an noted that everything worked as should (all actuators and so on), and all values within ranges.

Did some notes on the G150/151 sensors in the dashboard vents. They could display around 0-12c, even if there was 20-40c coming out of the vents. The centre vents measured temp value was very close to real (around 20-30c).

Ran the car hot in a parking lot, and put the heating on "HI", all hoses were warm, even the return hoses. Engine temp was around 100c all the time, incoming heating water to the pump+valve unit was approx 70c, passanger side hoses 50c and driver also. SHOULDN'T THESE BE HOTTER????????????

I mean the engine temp is 100c, but only 70c at max going to the heater...

I should get a temp reading from the hoses when driving on highway...

While driving in the highway, turning off the climatronic and on after a while, gives me heat (heat exchangers get to heat up, if they've gone cold?)

So to sum up, the slower i drive, the better heat i get, best while in parking lot.

The airflow must be cooling down stuff when i'm driving (no problem at 0c, but while below 0c, like -5 and more does trouble). Today the temp was -7 to -12, and i couldn't get the cabin warmer than 11-14c no matter what i did.

So what could cause a cold heater hose?

Maybe I just have an airpocket, but why didn't i get air out of the bleeding screws?

Or is my engine water pump or the electrical pump in front of the engine or the heater pump failing?

Another weird thing is that the passenger side is ALWAYS COLDER than the driver side, the defrost vent is like 10-20c colder, and less airflow, same with all vents on passenger side.

mishar

This is possible only if the water flow is low. Either valves are not working properly, there is air plug or cores are clogged (the least likely).

MP4.2+6.0

Yes (no), this really does not add up. The operating temp including back at the heater inlet should be above at least the thermostat closing point, which will be at least in the mid 90 C range.

My various ideas as of now would be:

thermostat: probably put my greatest weight here and would probably take a chance on changing it out even if not sure if it were my car with a frustrating issue like this. And UNLIKE the huge pain in the butt that the 4.2 thermostat is to get at (or Audi V6's I've worked on for that matter), as I understand W12's its relatively easy since it is in the intake manifold area. It is also an electronically controlled one rather than the old mechanical type, so it fluctuates very rapidly (as in within seconds or even less, not a minute plus). On cold days if your thermostat really should be closed at times but it is not, that will cool system in general at speed since the radiator(s) are running wide open. For that matter, the W12 is set up with that thermostat tightly regulating and enhancing performance--water temp is high generally by not just running the thermostat at mostly full open to enhance themo efficiency of motor, but will immediately then dump to full open on hard accelaration anticipating heat build up and load to follow quicky, etc. And the water temp sensor may also be somewhat heat soaked so not so quick to see the differences (or for that matter dumbed down by the programming given the thermostat rapid change characteristics the W12 has)

air in system: along lines you are thinking

bad electric secondary circulation pump: dont know that much about the particulars of this thing in the W12, but I know they have them for post shut down circulation, and tied into that two part valve under the plenum area. You would need to see those TSB's I referenced to review applicability for running rather than just post shutdown scenarios. I think I recall it was set up to be a booster pump in part, at least at low engine revs.

By the way, I guess never say never, but I tend to doubt its the engine water pump. At its core, it remains a mechanical part with the typical impeller. Yes, the impeller could be disintegrated or somehow come loose on the shaft, but again seems like rare corner case compared to a basic thermostat failure.