ACC playing up

[ed30James]

So a few weeks ago I had a bright red something flash up really quickly whilst using acc.
Since then it has flashed up with brake on two occasions. Seems odd as it never had before and isn't that ACC'S job?

Also now it seems to be more aggressively accelerating and braking.

Anyone else had any issues and is there a reset or some such or do I need to book it in for a checkup at the dealers?

It is a 2017 ed40.

Thanks

[fredgroves]

The big red symbol is a collision warning system. Either triggered by the anti-collision system or by ACC when it needs more than a little braking - eg when you bowl at 70mph into a queue of stacking traffic on the motorway. ACC can only apply a certain percentage of braking, if it needs more than that you have to do it yourself. I'm guessing its to avoid liability for VW... but it also is mooted as maybe being because ACC only uses rear brakes and to stop in a hurry you need all 4....

If ACC is accelerating and braking harder, is your car in sport mode or individual with ACC set to sport?

[I wanted a GTi]

The big red symbol is a collision warning system. Either triggered by the anti-collision system or by ACC when it needs more than a little braking - eg when you bowl at 70mph into a queue of stacking traffic on the motorway. ACC can only apply a certain percentage of braking, if it needs more than that you have to do it yourself. I'm guessing its to avoid liability for VW... but it also is mooted as maybe being because ACC only uses rear brakes and to stop in a hurry you need all 4....

If ACC is accelerating and braking harder, is your car in sport mode or individual with ACC set to sport?

There is no way ACC only uses rear brakes surely. I usually travel at around 80mph on the motorway when the conditions are right and the odd time you get Billy no mirrors pull out in front of you and the ACC takes care of all the braking and the force it brakes with could not be done by rear brakes alone. The car will dip in the front to show that the front brakes are being used.

Besides all that can you imagine how dangerous it would be to have rear only braking?

[fredgroves]

Actually, whilst I read that, I'm not sure its correct.

However this (Audi) SSP has a lot of details about how VAG ACC works:

http://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwis0oSL5ZLWAhWKtxQKHXNYCKQQFgg4MAM&url=http%3A%2F%2Fforum.a8parts.co.uk%2Fattachment.php%3Fattachmentid%3D4812%26d%3D1348136382&usg=AFQjCNEo9Q-ydkio-1sZ0BaxhAbccMeplw

Its all in there but in particular:

If the distance is less than desired, the vehicle is decelerated by reducing power, changing gear and if necessary applying the brakes. In the interests of ride comfort, maximum possible braking is restricted to approx. 25 % of the maximum deceleration potential of the brake system (full braking).The control action is designed to assist the driver and thus indirectly contributes to greater road safety.In certain traffic situations, active braking by the driver may still be necessary.

The resulting "brake or die" is the warning the OP has seen.

[Excession_OCP]

A little OT I know, but I was recently watching the Carwow review of the new Polo on YT and Matt mentioned that it was very impressive that the car could be specced with ACC considering that it had a manual parking brake (handbrake) which got me wondering,  why would a car with a manual parking brake struggle with ACC. I came to the conclusion that in the Golf, ACC braking is achieved through modulation of the electronic parking brake. This also tallied in my mind with the sensation I get when manually braking with the ACC turned on - it feels like braking++ which might be explained by the fact that I was augmenting the modulated EPB with the regular braking system. Anyoo, just my two pennethworth :-)

[fredgroves]

The most remarkable thing about vag acc is that it's the only one on the market for manual gearboxes... Nobody else does it.

[I wanted a GTi]

I know it's a while since I've had a car with a regular handbrake but I was brake tested a week ago whilst looking at the muppet in the car in the other lane who nearly ran into me and the braking force that the car made on my behalf was much greater than what I remember the handbrake could do without locking the rear wheels.

[Micky1]

The acc actuated all 4 wheels if it needs to slow down. It may well drag the rear brakes whilst in cruise control to keep the speed in check. The BMW's do this as well. I have had the big red brake thing up. Great little reminder to keep your distance!

[Talk-torque]

Are we not talking about 2different systems here? I've had the "brake or die" flash up without acc. If I was designing an adaptive cruise control system, I wouldn't want the brake lights flashing, like those muppets you see, whose foot must be constantly flicking from one pedal to the other, so it might make sense that for minor speed control, to keep the speed as close to the set point as possible, the system just uses gentle application of the electronic parking brake. Obviously, for the anti collision system, with or without acc, you need full braking power, with brake lights!

Don't know any of this for fact, but it makes some sense to me.

To the OP, as said in previous answers, there is adjustment available in the menu for acc distance to the car in front. If you feel that there is something wrong, that cannot be accounted for by adjustment, or driving style, then, as a safety related system, you should get it checked out by your dealer asap.

[fredgroves]

As far as I know, it doesn't use the parking brake. Its main mechanism is just backing off of the throttle. Acc does illuminate the brake lights though, you can spot other cars using cruise control on the motorway