ocles_inc

This thread is about our current project car, a silver 2000 Audi A6 Quattro 4.2 V8 which (with the engine running) shows not even a hint of any enthusiasm to move whether the shift lever is in drive or reverse. We have personally experienced this. The previous owner said this failure had been instantaneous.

While the engine is running and the transmission is installed in the car, there's a wealth of diagnostic capabilities available and I'm trying to exhaust these before I decide to remove the transmission. Besides, it would be ideal if the problem can be fixed without removing the transmission and I'd like to reasonably exhaust these possibilities. Even so, I understand the smart money would probably bet "the transmission is toast." I don't disagree. I just don't wanna get there by guessing.

I figured this might make for an interesting thread.

1. I don't know if a bad-enough transmission control module can cause such a failure but just in case, we removed the transmission control module on the premise that, with it absent, the car should at least be able to work in reverse and forward, albeit in limp-home mode. Our blue 2000 Audi A6 Quattro 4.2 V8 project car has been driving around for more than a year without a transmission control module. Even so, removing it on this silver project car ... didn't help matters at all.
2. We loosened one of the lines to the transmission cooler and whatever fluid was there spurted out at high pressure and high volume. So the oil pump was getting fluid and it was creating pressure.
3. I ran diagnostics using VCDS (though I did so after I'd removed the TCM; that might have some interesting diagnostic information so I should put it back and try again). The engine has some error codes issues but there wasn't any sort of major malfunction indicator, and since the engine started and ran this is not surprising.
4. We considered that perhaps we were only thinking we were engaging D or R; with a dead or loose shifter cable the transmission might have been in P all along. So we looked at the outside of the mechanism right by the side of the transmission and when someone moved the shift lever, there was indeed a visible effect.
5. One possibility is a broken output shaft but we'd have expected to hear or feel some whining or humming, and we didn't feel that though we're not yet ruling that out. We plan to look more closely.

These five points exhausted my insights as to how to diagnose possible problems without pulling the pan. I've been studying the manuals as to how the clutches interact with the planetary gear set, and it seems weird to me that EVERYthing can be slipping. Not even slipping, really, it's not even trying.

6. We dropped the oil pan. Lots of fluid ... so super-low fluid wasn't the issue. Not that this is surprising, either.
7. The fluid smelled burned and looked coffee-colored. This was an Oregon car. Did something (e.g., the radiator) rust through and then water-based coolant maybe contaminated the fluid? Has this every happened with these cars? From hearsay, I gather the clutch friction material attracts water and will draw water out of the fluid and then become soggy and unable to function well or at all. Really, I'd like to get all of the old fluid out but much of it is sitting in the torque converter and dropping the pan doesn't get it all out. I gather the only way is to do multiple fluid changes, to keep diluting the bad fluid. We plan to do that. Then, we plan take a good look at the quality of the fluid and run the engine, and if the fluid gets contaminated noticeably quickly then I should go looking for a culprit.
8. Still not wanting to rule out the dead-shifter possibility, we looked at the shifter shaft actually moving in the valve body housing, when someone moved the shift lever, and yes, there was a visible effect.

The way I'm trying to reason this out is that the valve body is supposed to control the internals of the transmission, and since there's fluid and fluid pressure yet no reaction from the internals of the transmission, maybe there's something horribly wrong with the valve body itself. So we plan to swap that, including the wiring and solenoids, out with a similar unit, refill with healthy fluid and a new filter, and see what happens.

Any technical input would be greatly appreciated though if I don't understand it, I plan to say so. :-)

Thank you.

~Tanya

4Driver4

Your troubleshooting path is definitely different than the one I would have taken. A couple of questions:

Did you check all driveshafts to be sure they are bolted in place and that the CV joints are not broken?

Did you pull codes from the transmission or just the engine?

ocles_inc

> Your troubleshooting path is definitely different than the one I would have taken.

Then your input is likely to add much-needed guidance, and you're being very polite about it, too. Thank you :-)

> Did you check all driveshafts to be sure they are bolted in place and that the CV joints are not broken?

I did not. I'll do that.

> Did you pull codes from the transmission or just the engine?

Just the engine, there is no transmission control module installed. I plan to install one and try again.

Thank you for the help. :-)

ocles_inc

After a busy day, I remain puzzled.

I plugged the original transmission control module back into the car. If VCDS and the instrument cluster display are anything to go by, it's fine but it's also not reporting any bad news.

We checked the fluid for water content; nothing. No debris at the bottom of the pan, either. The magnets have a little bit of sludge on them but nothing terrible. They certainly don't look like hedgehogs.

We had a spare valve body lying around and I don't like the idea of running on a pressure regulator that might allow a pressure spike through and blow out the "A" clutch drum so we installed a replacement valve body with a renewed pressure regulator; with most of the valve body coming from a donor car and the lower front housing coming from the ZF distributor. Besides that way I could eliminate one more variable, albeit an unlikely one.

We kept the car's original valve body wiring for convenience and because I've read that wiring isn't as identical among models as the ZF spare parts catalog might imply.

So after putting in a new filter and new gasket, and adding fresh fluid we started the car, got it to the correct temperature and added the optimal amount of fluid then closed the fill plug.

We put it in reverse. The engine changed RPM (to higher, oddly) and the instrument cluster gearshift position indicator showed "R" ... the car didn't give any tactile indication of trying to reverse. Nothing. We put it in "D" ... same. Not even a labored-yet-futile attempt at moving. Just ... nothing. With the handbrake on, we revved the engine a little. Still nothing.

We asked VCDS about it. No bad news to report.

We peeked underneath with the engine running. The torque converter is spinning.

We put it in reverse with the handbrake on. The rear shaft seems intact but there's no externally visible indication of it spinning or trying to. Each of the two side shafts seems intact but looking at the inboard portions where the transmission flange attaches to the side shafts, there's no externally visible indication of it spinning or trying to.

The input shaft might have snapped (hard for me to magine) but probably then the transmission control module would have had something to say about things.

The torque converter might be failing to transmit the motion into the transmission but then again the transmission control module would probably have sensed and reported on it.

It's hard for me to imagine the clutches all failing instantly. Besides, if the input shaft is spinning and the output shaft isn't, that would be the classic clutch "A" drum failure and I know that triggers an error reported by the transmission control module. And yet, I saw nothing of the sort.

The transmission control module announcing that all is well tells me that up to and including the point where a sensor is measuring the output shaft speed, everything is fine.

So I'm veering toward suspecting whatever is downstream of that such as the Torsen unit and differentials. On a RWD ZF 4HP-22 the tail shaft flange is where the story ends. If that turns, then it's up to the drive shaft from there on. By contrast, on the ZF 5HP-24A, my impression is that where the transmission ends, by normal RWD standards, the Quattro mechanisms takes things further downstream, so that's where I plan to look next.

On the last ZF 5HP-24A I dismantled, I also don't recall seeing any electrical sensors or wiring on the the Quattro mechanism, as in the rearmost section of the unit, so if there's were a problem, like gear teeth having sheared off then I'd guess there would not be any electrical feedback as such.

Please shoot my hypothesis full of holes as much as it deserves to. I need all the help I can get. :-)

Thank you.

4Driver4

When you pulled codes, did you specifically query the TCU, or just the ECU?
Does the speedometer needle move?
What about using measuring blocks and/or output tests...done anything there?

ocles_inc

First of all, thank you for being so tenaciously helpful :-)

> When you pulled codes, did you specifically query the TCU, or just the ECU?

Sort of neither ... I ran the VCDS general diagnostic analysis in which it runs through the various components and reports on what it finds, e.g., your steering wheel controls are dead, you're missing intermittently on cylinder six, your driver door central locking isn't looking too good ... that sort of thing. In the Automatic Transmission section, all was allegedly well.

> Does the speedometer needle move?

Oh wow, good point! No. I see what you're getting at. I think your question just killed off a slew of bad assumptions on my part.

> What about using measuring blocks and/or output tests...done anything there?

Very minimally ... while we were putting in new fluid, I checked the ATF temperature in the advanced measuring area, to make sure it was between 30C and 50C but that was it.

Which values would you suggest I focus on, please?

Though I'm enthused to fix this one car, the greater goal for me is that I'm trying to really understand what I'm doing and why, and you're helping me think things through, which I greatly appreciate.

Your advice is very humbling and very welcome. Thank you again.

~Tanya

4Driver4

I'd start by trying to move the car a bit...drop in Drive and rev a bit...to see if you can get it to throw some codes. It's very odd to see a car that won't move at all, not even limp mode, with no codes unless there is a broken CV joint.

For VAG, I'd start by running all the output tests. It will walk you through them in sequence. After that, sit in the car with it running and look through the measuring blocks. Make sure the TCU "sees" what gear you put it in, check any shaft speeds noted.

I'm not a transmission expert, but I can usually find some clues just by poking around in VAG-COM. In a situation like yours, I typically check fluid level and drive shafts, then I go right to VAG-COM.

ocles_inc

> I'd start by trying to move the car a bit...drop in Drive and rev a bit...to see if you can get it to throw some codes. It's very odd to see a car that won't move at all, not even limp mode, with no codes unless there is a broken CV joint.

That's good to know. I plan to rev the engine higher while someone else is below the car, staring intently at the inner CV joint flanges. It'd be great news if that's really the problem, wow.

> For VAG, I'd start by running all the output tests. It will walk you through them in sequence.

Okay, that I did -- assuming VAG.COM and VCSD are the same thing ... the Ross-tech software for Audi, yes?

> After that, sit in the car with it running and look through the measuring blocks. Make sure the TCU "sees" what gear you put it in, check any shaft speeds noted.

I plan to do that. :-)

> I'm not a transmission expert,

Wow, you could have fooled me. :-)

> but I can usually find some clues just by poking around in VAG-COM. In a situation like yours, I typically check fluid level and drive shafts, then I go right to VAG-COM

Very logical. Thank you again!

ocles_inc

Alrighty, I'm reporting back.

The transmission control module indicator matches whatever position I move the shift lever to (except for 2nd, which it reads as Z4, but that's probably not central to the puzzle we're trying to solve).

I revved the car to 2000 rpm in both D and R while my tech looked at the inboard shaft flanges; neither moved.

Then your idea of checking and recording the speed sensor readings ... that pointed to the "smoking gun" ... the engine speed read as 2000 rpm or so, yet the input speed as per transmission speed sensor was ... zero. Output also zero. This was based on the G182 sensor reading.

That matches the "nothing feels like it's happening inside" sensation, as if there's no hint of movement when moving from P to R to N to D. If nothing inside the transmission is turning that would explain it.

So, since we know that the torque converter is moving, this means I have a broken input shaft, yes?

I gather the input shaft is part of the clutch A drum, which is a part that often fails on these units, and there's a variety of options for sourcing these parts, including "Made in China" which would presumably make an input shaft failure more likely than on an original ZF unit.

Today we learned that this is an FBC variation transmission on a car made 11/1999 which means it originally came with an ECF variation -- so someone has had this transmission out of the car and that makes it more likely that someone has messed with it, perhaps including with the clutch A drum, which makes the involvement of an aftermarket input shaft more likely, which makes the failure of an input shaft more likely, compared to if these had all been untouched ZF internals.

This seems like the likely hypothesis to me, anyway. :-)

Thank you for helping me get there.

4Driver4

Unfortunately I think you're right about the A drum. Ouch.