200tq ABS problem?

[Brett Dikeman]

On Sun, 9 Dec 2001, Brendan wrote:

> 5 mph down a snow-covered hill. ABS on. Hit brakes.
> crackle-crackle-crunch-crunch, car still sliding.
> I bump it up against the curb to get it to stop, disengage ABS. Depress
> brake under speed. No more crunchies.
> I'm assuming something in the chain ain't workin' or ABS wasn't designed to
> handle that situation.
> Anyone?

The "groaning"(and flickering of headlights!) is pretty normal; the type44
ABS controller is obnoxiously loud and apparently sucks a tremendous
amount of power(if you open up a dead controller some time, the entire
"cage" is a heatsink to which several VERY large power transistors are
attached.)

ABS is for preserving intended direction of travel and maintaining control
on surfaces with uneven traction...not for shortening stopping distances.
In fact, it lengthens stopping distances considerably versus a driver
which knows how to threshold brake, which is easy to learn in theory and
another matter in real world panic situations and/or varying traction...in
which case, ABS probably does better :-)

 However, all this said...ABS will get fooled by rust
between the teeth on the hub; reduces the S/N ratio from the sensor since
the peaks aren't as "peaky" and the computer has a hell of a time trying
to figure out the speed of the wheel.  I believe, but I'm not sure, that a
lister said he cleaned his up and applied a thin paint to the teeth, and
has not had any problems with the paint interfering with ABS operation.
Only someone with a scope who did 3 tests(before cleanup, after but
before painting, and post-painting) would be able to say this for sure.
Audi doesn't say anything on the subject, which can either mean they don't NOT
recommend it, or they just never thought of it.  I'm not particularly
willing to advise the list "hey, eveyone, paint the teeth for the ABS
sensor!" :-|

I think a sign of this rust problem is that ABS will kick in if you brake
hard in perfect conditions and ABS kicks in(ie, it shouldn't but does) but
this might be a sign of general sensor signal problems(like spacing), I
don't know.

Given how cheap the little caps are(few bux?), might be worth it to pull
each sensor, clean it up, replace the cap, push in until it hits the hub,
and clean up all the connectors.  I don't remember how accessible the
teeth on the hub are for cleaning.

Can't promise anything, but if the sensors have backed off too much from
the hub(I would guess this is a possibility if they haven't been touched
in 10+ years of use), that would also give the ABS controller less signal
to play with and re-settling them might give better results.

Aside from that, nothing will do better than snow/ice tires of course.
ABS won't do jack if you're running around on summer tires or baldies.


Oops...speaking of which, there's now 3-4 inches of snow, and I've got one
set of each, summers on the car right now, and very worn D60A2's
on the factory rims in the basement...arrgg.

HTH!
B