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Emergency brake feature in the vacuum assist... Posted by magalar [Email]  (#1325)  [Profile/Gallery]
(more from magalar) on Mon, 19 Dec 2016 09:22:43
In Reply to: Re: AWD 9-3 2008 6cyl, aero- LOCKING BRAKES in slush/cold,, St0nemason, Sun, 18 Dec 2016 08:25:26 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Hi everyone,
Most cars have a feature build-in the vacuum assist that triggers full-on braking power when some conditions are met. This system is there to help shorten braking distance by exploiting all the features of the car even if the driver doesn't press on the brake pedal with enough energy.
It works usually when you depress the brake pedal rapidly but not strongly enough to trigger ABS. If the target is met, then the vacuum brake booster pulls on the brake pedal strongly to achieve maximum braking power. This system is usually mechanical (it's a valve in the brake booster, or something like that) and is independent from the ABS pump. At least, it was like that when I was at Renault and VW in 2000-2005.
So you may want to look at the vacuum booster for any leaks/rust/problem, and also at the brake pedal itself. If i remember well, the initial brake pedal position is important to trigger the Emergency Braking Assist feature (called MBA in Saab WIS).
From WIS : The brake servo unit includes the Mechanical Brake Assist function which provides more powerful vacuum assistance during lower pedal pressure than previously available, This has been accomplished with modified control valve calibration for regulating vacuum/atmospheric pressure in the brake servo unit. At pedal pressure of 500 N, an output force of 4200 N is achieved compared with a normal force of 3000 N.
From your symptoms, I guess this is where the problem is. MBA pulls the pedal strongly to the floor if you jump on it and try to release it shortly after while keeping braking.
For the anecdote, the first MBAs were too sensitive and were prone to full-braking in sporty conditions. They were supposed to release if the driver remove its foot from the brake. Now, their intervention is much more subtle, I don't remember when was the last time I noticed it working. I did a few days of ice racing with the XWD the last winters, and never suspected its operation.
Good luck !
magalar
Montreal
, Sun, 18 Dec 2016 12:36:55, Mon, 19 Dec 2016 09:22:43 <-- Viewing This Message, Mon, 19 Dec 2016 09:31:12, Mon, 19 Dec 2016 13:21:35
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