Are ACC and auto emergency braking linked?
 

I had an ACC fail today at about 30mph. It didn't start slowing down at all with a car in front not moving, and I realized and went to hit the brakes just about the time pre sense warning triangle and beep happened.

So my question, anybody else have ACC fail, and secondly are they using different sensors so one may pick up a car in front but the other not?
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I beleive Pre Sense and ACC are linked

In the ACC portion of the manual, there is a box with "Warning" that states: "

• When approaching stationary obstacles such as stopped traffic, adaptive cruise control will respond with limited function.
  
• - Adaptive cruise control does not respond to people, anima ls, or crossing or oncoming objects.

"Limited function" is a nice way of saying it may not stop. Many adaptive cruise control systems will not "see" or respond to a stopped object. If it is locked onto the car in front of you and that car slows to a stop, your ACC will bring you to a stop correctly. If, however, you come up to a car that is already stopped, it may not (likely will not) stop. You need to always be watching and ready to stop manually.
My understanding is that the presense is separate from the ACC in that the ACC does not need to be "on" for presense to work. I do not know the answer in terms of which sensors are used. However, presense will only act in time to reduce your speed a certain amount (25 or 35mph comes to mind). So if you were doing 20, it would supposedly stop you in time for a stationary car, but if you are doing 40 it would reduce your impact, but not avoid it completely.

Pre sense is camera based while ACC is radar based

Eyes on road at ALL times! Yes, the Q will follow a car all the way to a complete stop. But if you are on your own and come upon a stopped car like at a red light, the Q will gladly plow right into the back of the stopped car. Sometimes, when you are following someone and they begin to slow, you begin to slow also. But if the car you are following changes lanes, you then speed back up. Even if it means speeding back up into the new stopped car that is still in your lane.

I did a dig on the Internet.
All you need to know about Audi pre sense you will find in the SSP doc Audi Q7 (type 4M) Occupant protection and infotainment starting page 18

This. But with all the safety features built into cars, people are getting more reliant on these features, and getting dumber. These features are not perfect, and better yet, people dont quite understand how they work. On this board, we learn the nuances of these features based on collective experience. Do you think the average person understands how ACC and Presense are different? No. They just assume the car will stop and will read their newspaper in the car.

Thank you for that.

Definitely was a stopped car. Had my eyes on it the whole time was just expecting it to stop. When I realized it wouldn't, that when I hit the brakes.

It seems clearly different, I'm guessing radar vs camera as someone mentioned as pre sense recognized it, but ACC did not.

No. Again, the issue is if you are coming up on already-stopped car. ACC doesn't recognize that. Pre-Sense will sense it, do a warning, pretension the seatbelt, precharge the brakes, and apply the brakes... but possibly too late depending on your speed. If the car in front of you is in the process of slowing down, ACC will slow you down and stop no matter the speed (assuming there is physically enough space/time for the car to stop).
I would bet that your Grand Cherokee will also fail to stop for an already-stopped car if you are doing 40 and coming up to a car stopped at a red light... The thing is, that we should never be in the situation to find out. If the car is already stopped, the driver has PLENTY of time to see it, assuming that the driver is paying attention. The sudden slowing of a car in front during a split-second of inattention will be handled beautifully by the ACC.