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Tech Article

Audi C5 A6 Quattro 4.2 V8 Transmission ZF 5HP-24A Input Shaft Broken

Last Updated: 09/24/2017
Parts Group: Transmission
One of our 2000 Audi A6 Quattro 4.2 V8 cars has a problem that puzzled us greatly. The engine was running and yet the car would not move in any forward gear, or reverse. It wasn’t even trying. It wasn’t as if it were slipping; there were simply no externally detectable signs of anything happening inside the transmission. This is the 5HP-24A transmission made by ZF.

With the engine running, we looked at the torque converter through a gap in the bell-housing. It was indeed turning.

Again with the engine running, we disconnected a transmission cooler line at the radiator. Fluid spurted out at high volume and high pressure. This meant the pump inside the transmission was working, which meant it was turning, which meant the torque converter was turning the oil pump. It also meant there was a decent amount of fluid inside the transmission.

We checked for a bad output shaft, at the rear and on the sides, in front. The flanges seemed fine and were not spinning.

We checked if moving the gearshift lever had an effect at the selector switch on the side of the transmission. It did.

We drained the fluid and tested it for water content. We saw none.

We removed the pan. The magnets looked okay. The filter looked okay.

We checked if moving the gearshift lever had an effect at the selector switch inside the valve body. It did.

We replaced the entire valve body hydraulic system, speed sensor and solenoids. We put everything back together again and filled the transmission up with fluid, the proper way. Still no improvement.

We checked the internal transmission speed reading. Even with the engine at 2,000 rpm, the reading was zero. Aha! So, nothing was spinning inside the transmission. Indeed that is how it felt to us.

We emailed someone wise, who explained that a snapped or sheared input shaft would have such an effect. The input shaft is part of the clutch “A” drum, which is a typical point of failure if the pressure regulator in the lower front housing of the valve body isn’t renewed after 15 years or so – ideally sooner.

Several aftermarket manufacturers offer an input shaft and clutch “A” drum. Quality varies. Some of it is substandard enough that snapped input shafts are not so rare an event any more.

So, our conclusion is that the input shaft is broken. As a result, we have to remove and repair this transmission. We emphatically intend to replace the input shaft and clutch “A” drum with an original, high-quality made-by-ZF part.

* * *

We removed the transmission today, from the car that didn't want to move at all, in any gear, even while the engine was being revved. We discovered that the input shaft had indeed been sheared clean off.

From a conversation with the previous owner, I know that he liked to drive the car hard. So, he probably put a lot of torque on the shaft, which explains to some extent why it snapped: under the massive amount of torque generated by the A6 4.2 V8 engine. However, that input shaft should be able to handle such torque. These transmissions are also used in the S6 4.2 V8, which generates much more torque yet, so certainly with an A6 engine, a ZF input shaft should not have failed.

From the mess that my tech saw while removing the transmission, I conclude that this transmission had been out of the car at some point prior. Indeed, the ID plate shows it's type "FBC" which wasn't standard on the 2000 model year of the car that we're working on. So, I'm guessing that the original transmission had failed, and then someone had bought this replacement transmission from a junkyard, perhaps in badly-rebuilt condition. I say this because when I compare the details of the snapped input shaft to a true ZF input shaft, there is a slight difference. Both are hollow but the snapped shaft has a larger inside tunnel, hence would be weaker, all other things being equal. Conclusion: the input shaft that snapped isn't an original ZF unit but an aftermarket part.

Before becoming infatuated with ZF original parts, I'd bought an aftermarket input shaft more than a year ago. I like the seller, Cascade Transmissions in Oregon. The owner cautioned me that there are multiple aftermarket input shafts, and their quality varies greatly. The issue comes down to, essentially, a made-in-the-US shaft (one manufacturer only, as I recall) vs. a made-in-China shaft (several manufacturers), with the former being much better quality. I bought the made-in-the-US shaft, and it's still on the shelf in my shop, so I compared it to the sheared-off input shaft. The chamfering is markedly different, so the shaft that had snapped is not the made-in-the-US shaft. So, my best guess is that whomever had rebuilt this transmission had used a cheap made-in-China shaft with inferior quality.

Assuming I'm correct then this is a very solid object lesson: a rebuilt transmission is only as good as the quality of the parts that were used in the rebuild.

Ignoring, if possible, all the silicon rubber goop that someone had smeared all over the sad transmission in this picture, let's look at the input shaft as it normally looks:

Here is the input shaft that had sheared off. The picture shows the transmission end of the picture.

Here is the input shaft with the broken piece being held close to it.

Here is the piece, by itself, seen from front and back, respectively:

Here is a healthy input shaft, by contrast, extending all the way to the clutch "A" drum.

Here is the sheared-off piece placed next to a healthy input shaft.

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