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Tech Article

Audi C5 A6 Emergency Running Mode a.k.a.. "Limp-Home" Mode

Last Updated: 10/15/2018
Parts Group: Transmission
Imagine you’re in a bad neighborhood, and the transmission on your car has just suffered a serious failure. Normally you might be in danger, have to call a tow truck, and whatever plans you had go out the window for the next several hours because you’re stranded. However, on an Audi C5 A6 with the 4.2 V8 motor, and the ZF 5HP-24A automatic transmission, you would not be stranded. Much extra engineering effort has been dedicated to helping such Audi owners get home safely.

The car’s transmission control computer gets a variety of inputs from sensors all over the car. If it determines that something is seriously wrong, it changes the transmission setting to a very conservative mode that Audi refers to as "emergency running mode" in which Park, Reverse and Neutral work as you’d expect, but as to forward motion, there is only 4th gear. In that mode, the transmission bypasses many of its internal components that might be failing.

In this mode, the car is indeed slower when starting off from a dead stop but it will gradually and smoothly accelerate and then function essentially normally. The Audi can likely still exceed 100 mph in this mode.

Audi owners informally refer to this mode as "limp-home" mode a.k.a. “limp mode” using the analogy of a dog that’s been hurt and limps home to comfort, safety and healing.

Whenever the car enters this mode, there is a loud “clunk” and a jarring motion.

The transmission control computer resets its mode every time the ignition key is turned off, so then the transmission will start out in 1st gear, but if the root cause of the problem hasn’t been resolved, then it will re-enter limp-home mode.

These cars are so powerful that they can practically be driven in this mode for a long time. The car generally behaves in a normal way; it just has lackluster forward acceleration from a dead stop or at low speeds.

The same root cause that typically triggers “limp mode” going forward may eventually damage the “reverse” mechanism too. Also, in this mode, it’s important to be gentle with the throttle. Regardless, you’re likely looking at replacing or rebuilding the transmission – and yes, we do offer that.

The symptoms that “limp” mode is active:
- On the instrument cluster, the shift lever position indicator is lighted up in reverse video so that you can’t see the position indicator changing even if you move the shift lever.
- The car never upshifts. Going forward, it starts in 4th gear, and it stays there.

One of our project cars, a blue 2000 Audi A6 4.2 V8, has been driving in this mode for more than two years. Ii shows no signs of impending doom, so it’s certainly not a short-lived mode though it is of course better to get the issue resolved, time and money permitting.

* * *

Here’s an explanation of the problem at the technical level: the automatic transmission uses six internal clutches, the most-forward one being named “A,” the next one being named “B” and so on, with “F” being at the rear.

An oil pump makes automatic transmission fluid flow through the transmission. The fluid begins in the oil pan at the bottom of the transmission, then gets sucked up through the filter, and is then routed to various places, including the radiator, the torque converter and the clutches.

Normally, the higher the engine speed, the higher the transmission fluid pressure, As I understand things, the oil pump has a high-pressure cut-off so as to discard surplus pressure. Even so, pressure generated by the oil pump is high enough to damage the “A” clutch, and so the transmission also has a protective hydraulic pressure regulator built-in.

Typically, after many years, that regulator eventually fails, due to wear and tear, and it then allows a pressure spike through to the “A” clutch, and thus damages it. After this happens, it’s crucial that any repair work should fix not just the “A” clutch but also the root cause: the worn hydraulic pressure regulator. With these concerns addressed, those components of the transmission tend to be reliable and strong for many more years. The other internal transmission components tend to be longer-lived yet, for the most part, though there are a few minor items that we find it prudent to replace if we have the transmission apart anyway.

The “A” clutch is needed for 1st, 2nd and 3rd gear, which is why the transmission goes into 4th gear when the “A” clutch fails. In 4th gear, the entire internal rotating assembly of clutches is locked together to create a 1:1 ratio of input shaft speed vs. output shaft speed. In a healthy transmission, both the “A” and “B” clutches assist with locking this assembly together; when the “A” clutch fails, then the entire responsibility falls on the “B” clutch. When the car is gently driven, the “B” clutch can handle the strain for a long period of time – as one of our project cars has proved. When the car is not gently driven, the “B” clutch will eventually fail too, and then the car doesn’t move forward any more.– as a different one of our project cars has proved.
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