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I had my car in for the timing belt replacement (OUCH! that hurt...), and the mech asked if he could have a go at diagnosing the transmission hunting. Sure, I said, knock yourself out, knowing that plenty of thought has gone into this problem.

Well, the mech put it on the monitor to check the hunting behavior. What he found was that the brake switch kept showing up. This caught my attention because the cruise control hasn't worked either. The cruise, as we all know, disengages when the brake is tapped. Now I'm wondering whether the torque converter lockup also releases when the transmission gets a signal that the brake has been applied. If so, perhaps some electrical leakage through the switch leads the tranny to *think* the brake has been applied, and therefore the TC lock should be unlocked.

Or maybe not. I freely admit I'm no expert at this. But I thought the theory was worth bouncing off youse guys.

tozoM8

Defective brake switch will burn up your TC and the fluid.

tehA8PIMP

During vagcom, the brake switch malfunction code is common... I was told it was harmless, just an error from when the pedal was presssed.

Today I cleared an engine bank efficiency error code, what the heck is that?

bA8

THEN the HUNTING CAME BAAAAAACK. It's something else.

PaulW

<ul><li><a href="http://www.audipages.com/Tech_Articles/auto_transmission/brakeswitcherror.html">http://www.audipages.com/Tech_Articles/auto_transmission/brakeswitcherror.html</a</li></ul>

CalG

In reading Bosch literature, the interplay between the engine management, the ABS system and the transmission control system is quite complex. Since the electronics and "brains" for the command signals are past their "supported" service life, we are likely not able to do much with the intellegent(?) parts.

Leaving us with what? Exchanging or modification of hard parts?

My observation of many VAG-COM logs of Transmission control suggests two aspects as symptom.

1st, An abrupt command to signal the TCC into the "controlled slip mode" at times that may not be the best for "drivability and "comfort". Yes, the TCM is directed to slip, then again directed to lock up, repeat until the word "hunting" appears in you mind.

2nd the limited range of Pulse width modulation available to the TCC while in the control mode.
Perhaps a greater range of fuid pressure control could accomplish the performance task that the command to enter controlled slip mode is designed for.

The first aspect is that which suggests we search for the "defective component" such as the TPS, or the brake swith or the Oxygen sensors etc. (It migh be the wheel speed sensors ???;-)

Stop the command and the "hunting" is a non-issue

The second aspect is one that benefits from Tozo's modification or the replacement of the Valve body with a later model unit.

If one had the the complete control scheme understood, and the ability to monitor the related signals in real time, certainly the cause could be discovered.

And if the TCM e-proms are doing exactly what they are programed to do?? Though these controllers are "flashable" Audi is not offering upgrades, and there is no aftermarket.

Hmmmm.......

A challange to be sure. I for one would like to know entirely the logic of the Audi DSP transmission control. Though it is contained within the Bosch controller, the programing resides with Audi.

As said, the brake pedal needs to be pressed when entering diagnostics mode in order to clear that fault. That "symptom" is a non issue. The mechanic needs to look deeper.

If the brake switch is sending a defective signal, the brake lights will flash on as well. There are not two switches in one

CalG