ABS

[John Hess]

----------
From:  Jens Stobernack[SMTP:jstobern at acs.ucalgary.ca]
Sent:  Wednesday, January 29, 1997 10:00 AM
To:  diy_efi at coulomb.eng.ohio-state.edu
Subject:  Re: ABS




On Wed, 29 Jan 1997, Mike Turner wrote:

>
> I was one of the people affected by the server glitch and never 
received any
> replies (maybe there were none.....), so I'm going to ask again....
> >>>I know this is an EFI group but ...... What type of sensor does 
Bosch ABS
> use for rotation? Hall effect? Proximity? Something else? Does 
anyone know
> what the minimum speed for ABS to be engaged is? Has anyone hacked
> one of these?<<<
>

	I can only speak from VW experience. There is a toothed wheel
on the brake hub of each wheel. It spins with the wheel. An inductive 
pickup sensor seats mm from the wheel, connected to a static part of
the car. I think the ABS is always on. Even at slow speeds on ice you 
can engage it.

Hope this helps,
Jens

Unfortunately, on very slick surfaces such as icd, all four wheels 
lock up at once.  Generally the ABS systems work on a speed 
differential algorithm.  If a wheel is going appreciably slower than 
another (the others), then that wheel is considered locked and the 
brake system is modulated.  If all four are locked at the same time 
(at least at very slow vehicle speeds), the system sees the vehicle as 
stopped and is not activated.  This is why ABS is generally found to 
be ineffective on ice.