Brake Assist Off After Full Stop

Hello everyone,

Today, we had an Audi A2 in our workshop. The customer reported unusual driving behavior, describing it as a "jerking sensation in the pedals."

I went with him to inspect the problem. I couldn't reproduce his complaint. I told him I wanted to test the ABS. I did so, but at that moment, the ABS activated, and there was a "popping" sound (like a hose detaching), and the brake pedal "kicked" back at me. At the same time, the brake assist function was lost.
So, I checked to see if any hoses were disconnected. Everything was okay from the BKV (brake vacuum pump) to the vacuum pump. I disconnected the hose and, with the engine running, held my finger over the end of the hose -> it was being sucked in nicely, so the vacuum was okay.

Now, my opinion is that the ABS (Anti-lock Braking System) failed during hard braking. My experience with ABS failures is zero.

What is your opinion on that?

Best regards, Basti.

Could you test whether the BKV maintains the vacuum?
If not -> Defective.

Hi!
With many years of professional experience, I can say that I have never encountered a defective vacuum pump. In the past, with very old diesel-powered VWs, there were sometimes problems with the membrane vacuum pumps.
My neighbor once had his ABS module completely filled with water, which then froze in the winter, causing a complete brake failure. This was because the ABS module on his Audi was located in the engine bay, the drain was blocked, and the ABS module filled up with water. It was just a poorly designed system.

Regarding your problem: I'm not familiar with it, I haven't encountered it before. Usually, there are no issues with BKV.

I'd like to take a look.
Specifically, when the vacuum system draws air out of the ventilation system, air needs to be replenished somewhere. As far as I know, the air normally enters the ventilation system from the footwell of the car, and there's a small filter located there.
Take a look at this and see if it's okay.

Update:

After replacing the ABS control unit, the car brakes normally again. What was then noticed was that the car started to activate the ESP uncontrollably when cornering. After some research, a wheel alignment was performed.

Everything is okay now.

Thank you for the tips.

Basti.

Basti1605 wrote:

...the car started to activate the ESP uncontrollably during cornering. After a short investigation, a wheel alignment was performed.

If you experience problems with the ESP activating uncontrollably, in addition to a wheel alignment, it is also advisable to perform a zero calibration of all available sensors used by the ESP.