If you have an Audi A-8 or another vehicle that uses the ZF 5HP-24A transmission, you might experience a sudden loss of reverse. Although this is a pretty well built beefy unit, the "F" clutch piston can deteriorate to the point of failure. I'm not quite sure of the root cause, whether it's a lack of lubrication to it or a manufacturing issue, but it can happen and has happened on quite a few units. There is a service bulletin on that issue.

My father's 2002 A-8 had this happen and since I rebuild GM transmissions all day long, I figured I'd tackle this monstrosity and repair it myself. Adding to my incentive was the fact that my dad wanted to trade it in and Audi doesn't rebuild their transmissions, they just replace them at about $8000.00 total!

This job is fairly complicated and I wouldn't recommend it for any but the most experienced technicians. This is a view of my experience, I would guess it took me about 25 hours or so! The job starts by disconnecting the battery and installing an engine support fixture that will hold the engine up when the front subframe is removed. I used a GM engine fixture with a couple of bolted hooks to the engines intake manifold bolts. A bar stretches across the two strut mounts and secures the engine to them. The lower subframe is then removed which is about an hour or two to expose the transmission. It is then just a matter of unbolting the tranny.

Tranny removed

Exposed engine flywheel

After the unit is removed, we put it on the bench and remove the rear transfer case which is fairly straight forward. You should drain the transfer case oil, which is separate from the regular transmission fluid. This transmission is a Quattro all wheel drive unit so it has a torson style transfer case bolted to the back of it to drive the rear wheels. The front wheels are driven from a gear in the rear which drives a worm gear to a shaft that is under the bump out on the side of the unit. This shaft runs to the front where it goes into a conventional style differential mounted on the side of the unit. This differential then has an output shaft that runs through a tube to the other side of the unit to power the other front wheel. In a non-all wheel drive model, the rear transfer case is not used but the side differential still is used as it becomes a front wheel drive tranny with the transmission mounted longitudally in the car.

To rebuild this tranny, you really need to get the factory repair manual from ZF. It is cheap and priceless, the ZF part number is #5HP24AREP. I got all my parts from the local ZF distrubutor for hundreds less than I would have paid at Audi. In Ct the distributor is Eriksson Industries out of Old Saybrook. When I went there, the owner took me for a tour. They do all ZF trannies and some others such as SAAB. They have quite an impressive and professional operation going on there.

This is the whole transmission on the bench, unfortunately I don't have the factory fixture to mount it to the bench

1st step, removing transfer case

Factory repair manual

Side view with no transfer case, showing case bump for worm shaft

With the transfer case off, you can remove the end cover which covers the worm gear. The worm shaft can stay in the case (pressed in and attached to pinion in front). The differential then has to be taken out before the front pump. The left side axle flange just unbolts and then is pulled through the case Then the differential cover is pulled.**Make sure you realize that the big heavy differential will fall out unless you have this sideways! Remove the differential assembly and then remove the tube that the shaft passes through. Then the front pump can be pulled, be careful to remove the two metal plugs on the bottom to access the lower screws. These screws are torx head and strip out very easily, an impact driver might be a good choice to pre-loosen them. You can see that 3 or 4 of them stripped for me and the heads had to be drilled out to remove pump body.

front view of pump

Not too happy about drilling these out!

Differential removed on side

On bench in pieces

Once I removed the front pump, I disassembled tranny, I didn't replace other clutches or seals as I was just going after reverse gear. This tranny had all forward gears working fine and all clutches looked good. Some of these seals can be very difficult to change without some special tools. They were left alone. Unfortunately the "F" clutch which was destroyed was the absolute last thing in the case to remove and every last bolt had to come out. The F clutch housing bolts to the inside of the case through the back of the case with flat head bolts which also strip very easily, an impact driver should be used to loosen them. There is a big bearing that has to be removed before you can get to the bolts. This bearing got destroyed in the process and was replaced. I replaced the F clutch steel and fiber plates and the F clutch piston. You can see the F clutch piston rubber had come apart all over the place, seemed very brittle like it had been overheated or was made of inferior material. I then bolted the housing back to the case and reassembled transmission. As you can see, not many parts were necessary to fix just reverse, the biggest expense was the special Audi transmission fluid which is crucial and very expensive from Audi. It is a clear fluid and about $15 a quart (ZF distributor only had it in bulk barrels!)

Many parts in an automatic transmission!

More parts

F clutch housing

The crumbling source of the problem!

Parts that were replaced

Uh oh! spare parts, where does this go?

When I had the whole tranny together and before I put on the rear worm gear cover and the transfer case, I had one spacer left and wasn't quite sure where it went! The manual didn't really have a good picture and there was another thrust washer that looked similar to it, so I didn't know which one was which! Finally after a half an hour I managed to determine the correct placement of these spacers and finished the assembly. Driving the vehicle, reverse works great and everything else shifts smooth. Good experience, but I'm happy its out of here!