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Tech Article

Audi C5 A6 Quattro 4.2 V8 Transmission Fluid Low

Last Updated: 07/28/2020
Parts Group: Transmission
This article discusses the transmission for the Audi C5 A6 with the 4.2 V8. It's the ZF 5HP-24A transmission, as made from 1999 to 2004, with the main focus being on how the transmission behaves when being driven with very low fluid -- not that I recommend ever doing this, but I hope it's useful to you to be able to recognize the symptoms, so that you can top up the fluid.

If you're not here for the technical analysis but would rather buy a used or rebuilt unit from us, with warranty, then please select the link below: The information herein is based on my own car.

Last night I drove one of my 2000 Audi A6 4.2 V8 cars, and the transmission shifted hard from second to first. When accelerating gently from stop, the transmission slipped slightly. When accelerating hard, the transmission slipped a lot.

My tech and I hypothesized about multiple causes. We finally concluded that perhaps the fluid was very, very low. We decided to check the fluid level.

Checking the fluid on these transmissions isn't as easy as pulling a long dipstick and staring at a wet high-level mark on a rod of steel or plastic. On this type of transmission, the ZF 5HP-24A, the entire transmission is sealed. This has many benefits, including cleanliness. It also prevents a clueless owner or helper from dumping in the wrong type of ATF. The correct ATF for this transmission is a special, expensive, thick-viscosity, honey-colored fluid. The typical red ATF alternatives such as Dexron / Mercon etc. are chemically more slippery, and are not approved by ZF nor -- specific to this type of transmission -- do I use these, nor do I recommend or condone anyone else doing so.

Checking the fluid on these transmissions requires getting below the transmission oil plan and removing the large Allen-head fill plug, then trying to add fluid until the transmission doesn't accept any more, as evidenced by the fluid running back out the fill plug. It's a simple yet ingenious design that makes the transmission essentially impossible to overfill.

On these transmissions, the fluid level is optimal when the motor is running and the fluid temperature is between 30 and 50 degrees Celsius. This is normally attained after a room-temperature-cold engine has been idling for about ten minutes. A more precise approach, however, is to plug a laptop computer into the car, to monitor the exact fluid temperature.

So last night, while I was crouching below the transmission oil pan, my tech started the engine. While I was pumping fluid into the transmission, she monitored the fluid temperature. Indeed, the fluid had been low. The transmission took in several quarts of fluid.

Why had it been so low? It turns out there's a massive leak at the oil plan gasket. So, now we know that. And, we also know what "very low fluid" symptoms feel like on these ZF 5HP-24A transmissions.

Fortunately, a subsequent test drive confirmed that all was well again; the transmission behaved as a healthy transmission should.
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