WmCartwright

2000 Audi A8 Quattro transmission 126,000 miles, that is slipping in reverse only. I am getting contradictory information about possible resolutions. One transmission specialist suggest changing the fluid and filter would help another says it won't and might make it worse when new detergents break loose even more debris within the transmission. Are their any other possibilities for resolution. To do nothing is not an viable option. I can't drive like this much longer. Auto won't move at all in reverse when it's cold and barely when it warms up. Is worth a chance to change fluid and filter or basically the trans is a write-off?

ocles_inc

In my A8, with a transmission like yours, I've had a very-low fluid level cause slippage, but nothing as dramatic as what you're experiencing, and also not in reverse gear. So, we can probably rule out that a fluid fill (and thus level correction) will fix the problem.

On one of our project cars with this type of transmission, a blue 2000 A6 Quattro 4.2 V8, I've wasted hundreds of dollars changing the fluid and filter multiple times mainly motivated by unrealistic wishful thinking on my part. It didn't fix the problem. However, it also didn't hurt as far as I could tell. After these multiple fluid and filter changes, the car didn't drive any worse than before and it's been driving like this for more than a year now. One day I'll replace the transmission on it but for now, it's a nice -- albeit slow -- way to get around town.

My points are that even though the don't-change-the-fluid story sounds reasonable, I've violated it with alacrity and the blue car is no worse, as far as I can tell. So, I wouldn't suggest you be wary of changing the fluid -- but nor do I think it'll fix as severe a problem as you're experiencing. You'd simply be blowing a couple of hundred dollars that might be better spent.

My little car company owns multiple Audi A6 or A8 Quattro project cars; one of them is a silver 2000 A6 4.2V8 with the same transmission as yours, and the same reverse gear behavior as yours PLUS it misbehaves when driving forward too. So, I don't have any project car history that applies cleanly to your situation, which means we have to figure it out.

I don't know how to be helpful without getting kinda technical so please bear with me. The following information is based on a paper on the ZF 5HP-24 control system, by Phil Whiffin who was at the time a project engineer for Jaguar Cars Ltd. He was focused on the XK8. His paper shows which solenoid valves and pressure regulators are used in each gear. Your Audi has a ZF 5HP-24A. The control systems are very similar fundamentally even though the valve bodies are not simply interchangeable between, say, a Jaguar XK8 and your Audi Quattro A8.

So, as I understand Phil, in reverse gear, the C clutch and D brake are locked up, and so voltage goes to:

• Solenoid 2
• Pressure regulators 1, 2, 3 and 5

In third gear, the A clutch and D brake are locked up, and so voltage goes to:

• Solenoid 2
• Pressure regulators 1, 2, 3 and 4

My point here is that third gear and reverse are quite similar. If third gear works fine on your car, this means we can assume the D brake is fine, hence we should suspect the C clutch.

Indeed, reverse is the only gear that uses the C clutch, so we might be onto something, here.

It might seem tempting to suspect pressure regulator #5 but it's used for gears other than just reverse, and so if reverse is all that's misbehaving, we can exonerate pressure regulator #5.

So, assuming the hydraulics are healthy, you probably have a failing C clutch. I'm not saying "drop the oil pan" but if the C clutch is bad and you were to do so then you might find the magnets looking like hedgehogs from the steel filings that used to be part of the C clutch.

Fixing the C clutch requires removal of the transmission, so I'd suggest this as a last resort (even though I sell these transmissions, used).

Logically we cannot rule out that your valve body is simply not getting the fluid pressure to the C clutch. Sometimes they have a visually discernible crack on part of the valve body housing, and this can cause slippage. You would then simply replace that part of the valve body and then you've fixed the problem (though I'd recommend some additional preventative maintenance tasks at the time, if you have the transmission pan off anyway).

If you can't visually discern a blatant cause, then the next step I'd recommend would be that you replace the entire valve body and if the transmission still misbehaves in the same way, then you know you can rule out the valve body.

The ideal would be to get a good loaner valve body out of a known-to-be-good transmission. I might be willing to temporarily remove the valve body from my daily driver, a black 2000 A6 Quattro 4.2 V8, and send that to you as a loaner unit, though there are a few caveats that we can discuss when the time comes.

To inspect or swap out the valve body, you'd have to drain the fluid and drop the pan, and then use fresh (and Audi-specific, i.e., not Dexron/Mercon) fluid and certainly no additives to fill the transmission. Filling it to the precise level is fairly involved, too. My point is that if you nevertheless feel a burning need to replace the old fluid with fresh fluid, then that would be the time.

Compared to swapping out the transmission, swapping a valve body is almost offensively simple.

For starters, please let me know what you think about the above.

hillpc

This doesn't agree with the information I have. See the chart below, excerpted from an Audi self-study document on the 5HP19 (also called 01V) and 5HP24 (01L) transmissions. This diagram agrees with 2 or 3 others that I have from various sources.

Also search on the F brake. I seem to recall from past reading that breakage of this is a common cause of reverse failure in these transmissions. I don't know how that would affect what you feel in neutral (the F brake is also engaged in neutral).

tozoM8

I've rebuilt hundreds of these transmissions and if there is no codes indicate of electrical failure then it is always the F piston. You will need a rebuilt trans.

ocles_inc

@hillpc, thank you for posting that chart and correcting me. I would love to get my hands on that self-study program. Would you please point me to a source? Applying Audi documentation to Audi situations would be much, much better than applying Jaguar documentation to Audi situations.

@tozom8, thank you. I'd love to involve you in bringing my elephant's graveyard of dead ZF 5HP-24A transmissions back to life. I'll be sending you a friendly private message as such.

hillpc

That "course" (it's an 80-page pdf document) can be accessed at https://issuu.com/asafavidan/docs/pp...omatic_transmi and probably other places.

I haven't taken the time to understand the power flow diagrams. I'll bet that understanding all of that document would make one pretty good at finding out what's wrong. Lot of work, though.

ocles_inc

Wow, this is great! Thank you. I need all the help I can get.

~Tanya

WmCartwright

Thank You! ocles_inc, hillpc, and tozoM8. I am wow!ed by your superior knowledge and generous advice. I did changed the fluid and filter. Reverse is their on level ground, but will slip if reverse on an incline. I'll check the codes to see if their is any electrical issues.