Just got a V10 TDI Touareg. Have two V8s in the family also. Steep hill descent in the V10 in 2nd gear, high or low range, is troublesome. Speed increases and decreases dramatically with no driver input. On the V8, it varies only slightly (a couple mph). I am wondering if the much higher gearing on the V10 diesel is the culprit, since a smaller change in engine RPM will mean a bigger change in MPH, or if there could be something wrong with the hill descent control. And if the latter, which electronic module runs it? The ECU, the TCU, both, or neither?
I’ve not had much experience with the hill descent control other than my niece’s BMW. She complained of basically the same problem as yours. I just applied the “manual” descent control (foot on brake) and it was fine. You might test drive another Toureg like yours, if possible, and see if it does the same thing. Every vehicle make will be different because they all have different drivetrains.
I would visit a Toureg specific forum or dealer if under warranty.
Toureg’s are rare vehicles and the TDI more so.
Engine braking doesn’t work very well on diesel engines. That’s why diesel trucks have a “Jake brake” installed to alter valve timing.
On all of my vehicles, the “hill descent control” is called “The Brake Pedal”…Have you tried that??