Brake Assist - How does it work.

[Droverunner]

Can anyone explain how the brake assist system works... as in the type fitted to my 2012 Viano not the latest active type. I know it's effect... it's what does what in the mechanical/hydraulic/electronic sense that interests me.

 

[LostKiwi]

As I understand it the system monitors movement of the brake pedal and when it detects rapid movement applies the brakes fully. The theory is that most people don't apply the brakes as hard as they will go when performing an emergency stop
My Smart has it and when I first drove it the pedal was very low due to maladjusted rear brakes pumping the brakes to improve pedal feel created a few unintended emergency stops ...

 

[M80]

Could this be an electronic action that increases hydraulic pressure in a similar way to Brake Hold?

 

[EmilysDad]

.....
My Smart has it and when I first drove it the pedal was very low due to maladjusted rear brakes pumping the brakes to improve pedal feel created a few unintended emergency stops ...

In all the years we've had our Smart Roadster I've yet to experience it engage brake assist.

I unintentionally had it engage on my ML .... different system I know - cameras etc, but it didn't half stop me!!!!

 

[Blobcat]

As per above it monitors brake pedal pressure and duration, if it detects that you're braking hard and continuing to brake hard it looks at the wheel speed sensors and then adds further braking pressure until it detects a locked wheel (by speed imbalance) it then backs the braking off (using the ABS) but continues to brake as hard as it possibly can until the vehicle is stopped or you release the brake pedal.

A lot of W211 E-Classes suffered read end shunts when it was first introduced because the cars behind couldn't stop anything like as fast...

Depends upon the version it will also "dry" the brake discs by gently applying the brakes to clear the water from the discs if it's raining

 

[M80]

A lot of W211 E-Classes suffered read end shunts when it was first introduced because the cars behind couldn't stop anything like as fast...

I've experienced that.
I thought a dumb lass was to wrapped around my front end as I hit the anchors at about 45. It stopped about 10feet short, along with the flashing brake lights (I assume) and the hazards doing their bit. Impressive.

 

[rorywquin]

As I understand it the system monitors movement of the brake pedal and when it detects rapid movement applies the brakes fully. The theory is that most people don't apply the brakes as hard as they will go when performing an emergency stop
My Smart has it and when I first drove it the pedal was very low due to maladjusted rear brakes pumping the brakes to improve pedal feel created a few unintended emergency stops ...

That is also my understanding.

@Droverunner drive down a quiet road, suddenly lift your foot and hit the brakes, you’ll notice that it seems to react ahead of you.

It goes without saying that anything you try is at your own risk.

 

[rorywquin]

I've experienced that.
I thought a dumb lass was to wrapped around my front end as I hit the anchors at about 45. It stopped about 10feet short, along with the flashing brake lights (I assume) and the hazards doing their bit. Impressive.

IIRC sudden braking will flash brake lights and above a certain speed will also activate the hazards.

It is indeed very clever stuff.....

From my owners manual

With the help of the radar sensor system, Adaptive Brake Assist can detect obstacles that are in the path of your vehicle for an extended period of time.
If Adaptive Brake Assist detects a risk of collision with the vehicle in front, it calculates the brake pressure necessary to avoid a collision. If you apply the brakes forcefully, Adaptive Brake Assist automatically increases the brake pressure to a level suitable for the traffic conditions.
Adaptive Brake Assist provides braking assistance in hazardous situations at speeds above 7 km/h. It uses the radar sensor system to assess the traffic situation.
Up to a speed of approximately 250 km/h, Adaptive Brake Assist is capable of reacting to moving objects that have already been detected as such at least once over the period of observation.
Up to a speed of approximately 70 km/h, Adaptive Brake Assist reacts to stationary obstacles.
If Adaptive Brake Assist demands particularly high braking force, preventative passenger protection measures (PRE-SAFE®) are activated simultaneously more.

Keep the brake pedal depressed until the emergency braking situation is over.

• ABS prevents the wheels from locking.

The brakes will work normally again if:

• you release the brake pedal
• a danger of collision with the vehicle in front no longer exists

 

[Droverunner]

Thanks all. No radar on mine just the basic system.

I really want to drill down to the detail as in what provides this extra pressure.... abs pump??... the servo?? And which ecu makes the decisions?

 

[LostKiwi]

Servo provides the force. The ESP/ABS provides the logic I believe.

 

[supernoodle]

If you're interested in such things you need this. Lots of great technical detail now things work.

It's up to edition 10 now.

 

[Droverunner]

>>>Servo provides the force. The ESP/ABS provides the logic I believe.

That sounds very likely. The question is related to my thread re the surprising extra brake travel when the pedal is pressed hard at rest with engine on. I will report back finally on that thread when I get a chance to have the local MOT station roller test the brakes. But my thinking is that the brake assist could be kicking in at rest either due to a fault or quirk of design.

Odd brake pedal feel with long travel after rear disc/pad change.

This is weird... I've changed disc/pads sets probably 100+ times never having problems... and in the past given advice on forums where just the same question has been asked. But this time can't seem work out what's happening. So... 2012 Viano with genuine 58k recorded bought a few months back...

forums.mercedesclub.org.uk

 

[LostKiwi]

If you're comparing travel engine off and engine on it's quite normal for there to be more travel simply because the servo provides far more force. Brake assist should have zero effect unless you rapidly move the pedal or press far harder than you would when driving, both conditions being detected as an emergency stop.
Irrespective it should be the same behaviour before and after a pad change.

 

[Droverunner]

Yes well aware of engine on/off differences in pedal after over 80 cars owned (not being tetchy just saying) covering 45yrs. This is the first I've noticed with such an odd feel. In truth I had no reason to depress the pedal hard at rest engine with on before the pad job so it could have been like that before. As you say new pads/discs fitted well will not affect this feel/travel.

I've found this pedal feel commented on in several places over the net when extensively searching forums with Vito/Viano folks saying that's just how this model is.

If the MOT roller test come back OK I think I ill just accept it.

 

[M80]

I remember the rears being a bitch on an earlier 639. I seem to remember I improved it by using the tradition 2 person pump peddle / crack bleeder method.

But looking at WIS the procedure with a pressure bleeder is
1st o/s rear,
2nd n/s front,
3rd n/s rear,
4th o/s front.

If that's any help.