IknowHuhA6

HELP!! Need some advice and opinions.

Was using adaptive cruise control (ACC) other day and I'm used to the system being pretty quirky where sometimes it just brakes too late and quite hard. Other times it's fine.

So a week ago I had an instance where the ACC didn't brake soon enough, I thought I was going to hit car in front of me on highway so I intervened and stepped on brake myself. Suddenly the brake pedal went from seemingly obvious stop position and gave away even farther down in quick disengaging type action causing car to come to screeching halt. Very weird. It was as if the brake system has a reserve extra brake pedal range where it suddenly allows further range of travel in emergency.

Today it did it again while not in ACC mode. In an effort to make a light before it turned red, I gunned the gas but hit the brake when I realized I wasn't going to make it. So hit the brake for a fast deceleration but nothing that would cause a skidding stop. Suddenly when close to the stop light the brake pedal dropped further down from what felt as maximum stop position for slowing car, and once again the brake pedal let go closer to floorboard and car screeched to stop.

What gives? Feature I'm not aware of?

Thanks in advance.

Tinbad

Not 100% sure but sounds like you experienced the anti lock brake and traction systems of the car engage to make sure your brakes wouldn't lock up during hard braking?

IknowHuhA6

I considered this, but if this indeed is what happened, I'm not used to ABS reacting so violently. this is very dangerous as it removes the smooth gradual stop and introduces a sudden action. It's like hitting the brakes while riding a bicycle and you're stopping fine, then suddenly a pipe is thrown into the wheel as you give it a little extra pressure to make sure you make the stop.

Just want to identify if this is normal or something needs adjustment.

BillBoost37

My car exhibits none of this behavior (yet). Please ship your car to me and I'll do the same. I would be happy to go through and confirm for years that yours is working properly.

snagitseven

This doesn't sound like normal behavior. From what you describe, it almost seems like the Presense Plus Front automatic braking is kicking in at incorrect times. I would get it checked out - see if a fault code was generated. Ask if a tech can do a ride along with you and reproduce the issue.

2008A6

I'm thinking some form of Pre-sense is active during ACC as my brake pedal is extremely sensitive when i touch it to slow down or disengage ACC. Whatever it is, it's definitely a more responsive braking than under normal cruising.

snagitseven

Presense Plus is a component of ACC. That's what causes the brakes to engage when in ACC. I notice no more sensitivity when using the brakes when ACC is activated. Of course, when you hit the brake pedal, you disengage ACC until you pull the ACC stalk again.

superswiss

What you felt was the emergency brake assist. If you quickly change from accelerating to braking, the car interprets that as an emergency and goes into full emergency braking. This feature is there because most people don't stomp on the brake pedal hard enough in an emergency situation, so the braking system assists to ensure full braking. When ACC is on, the brakes are primed and a similar thing happens if you step on the brakes to assist with the car stopping.

2008A6

Thanks..that explains why they are so sensitive due to being primed while ACC is on. They grab very quickly as soon as you start depressing the brake, then go back to normal after that first press.

snagitseven

So, if that were the case, then how does that explain your second incidence when the ACC was not activated?