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Re: E30 ABS

Hi All,

I think I have my answer...

I'm responding to the list, and cc'ing the folks who responded to me, in
an attempt to kill many birds with one e-mail...this is long, so please
delete if you're not interested in a discussion of the merits of ABS.

First, Alex responded with some points regarding the general merits of
ABS:

On Fri, 3 Jan 2003 [email protected] wrote:
>
> Date: Fri, 3 Jan 2003 15:27:34 -0600
> From: [email protected]
> To: Scott Ruffner <[email protected]>
> Cc: BMW Digest List <[email protected]>, [email protected]
> Subject: Re: E30 - '86 325E - ABS Question
>
> "Scott Ruffner" <[email protected]> wrote:
> > although I believe it is coming on when it shouldn't.
>
> When?

This was a fair question - the answer is clearly: it's coming on when the
wheels are turning and NOT sliding (ie, they ain't locked up).

> > The brake pedal feels "crunchy" or "brittle" like it's vibrating
> > at a high frequency, which I am guessing is what it feels like
> > when the ABS activates.
>
> Yep.

And, I'll just say, yes, it is exactly what I was feeling...mother nature
cooperated this weekend and I got to do some controlled skidding in snow
this weekend....more on that in a moment.

> It's simple: press until the ABS starts massaging your foot, than press
> _harder_.
> Contrary to popular believe, ABS is not a binary input device. The harder
> you press the brake pedal with ABS pumping, the more aggressively does it
> work.

Unhh...I don't this this is true.  I'll look at things in the manual, but
I don't think the ABS ever generates any more "pressure" on it's own than
what you generate by applying pressure to the brake pedal...the ABS cycles
the pressure DOWN from what you are applying, until the wheels start
turning again, and then gradually increases back up to what you are
applying, until the wheels lock up again, and starts the cycle over.  If
you are applying enough pressure to lock up, then any pressure beyond that
is ignored.  The ABS will cycle on lockup.

> If you easy the pressure on the brake pedal or just continue to brake as
> hard as you did before the ABS kicked in, you will wind up with longer
> stopping distances than you would have had before ABS kicked in.

Well, maybe...depends on how good you are with braking...when ABS first
appeared in police cruisers there was a rash of police rear-endings where
they found their net stopping distance was increased by ABS.  This is
pretty well documented, and I have personally found it to be true.  The
point of ABS is to provide better control for those who will instinctively
just keep pressing harder the more they skid out of control.  You can't
steer nearly as well when you're sliding.

> Based on your description, it sounds like ABS is working as it should.
> What leads you to believe the ABS is activating prematurely?

Again, the fact that the wheels were not locked up.

>
> > I notice this most when the car is cold, and I'm making a low-speed
> > turn while braking.
>
> Cold car => cold tires, cold brake pads and cold rotors => potentially
> lower coefficient of friction. Between crappy tires and pads, all my E30
> cars used to drop into ABS mode long before an E36 would.
> When was the last time you bled/rebuild your brakes? If one of the pistons
> is sticking, it could trigger premature ABS activation.

Well, sure, these things might contribute to the degree they cause the
wheels to lockup - bald tires certainly are less sticky.  I'm quite
certain these have nothing to do with the problem, but for the record,
I've got 4 brand new tires (like 1.5 months old) - the problem appeared
about one month before putting them on, and did not change after they were
installed.  The brake pads are also new - replaced 6 months ago (and I
typically get like 40k on a set of pads - I am not a leadfoot), and bled
the brakes then.

> Doubly so since ABS is a really good thing in the vast majority of the
> situations. The only time it extends your stopping distance is when
> braking over loose turf (tires can't dig in).

Well...I'd expand that to say any situation where you don't have good
traction...see my comments above about how it works.  Just so nobody has
any doubts...ABS increases your stopping distances, but improves your
control.  If you have the presence of mind to unlock non-ABS systems on
your own, you get back the steering, and the stopping.  Especially if you
apply pressure right to the edge of the slide.  This is a question of
skill, and skilled driver can beat ABS.

Next, Robert wrote with good ideas specific to my issues:

On Sat, 4 Jan 2003, Robert Duckworth wrote:
> The ABS triggering sounds like a buzz and you will feel that vibration
> in the pedal.  The system self-tests when you first move the car after
> every start.  Typically when you are backing out of a parking space, but
> also when you are moving forward after pulling off (the first time you
> apply the brakes).

Hmm...I didn't get this at all...I still don't.  I do get the test light
that blinks to check the brake lights until the first time I hit the
pedal.  In fact, the first time or two I apply the brakes, I generally
don't get the ABS.  I always apply them before releasing the
"emergency"/parking brake.  The ABS light lights briefly, then goes out
(quickly)  when I first start the engine.

> Now, the fact that it seems to occur when you are
> moving to a stop or a slow turn would indicate a fault with either a
> sensor or the wheel sender rings.

Which is what I think is going on.

Now for some really good diagnostic suggestions:

> I would do two things in this order:  1.  Pull the sensors and clean all
> the accumulated crap off them at each wheel. They're magnets and collect
> metal shavings effectively.  While you have them out, I would take some
> brake cleaner, spray into the sensor holes while spinning the wheel and
> then stick a small brush in there and clean the sender rings as best you
> can.  Put it together and test drive it.  If this does not remedy the
> problem, check the sensors for resistance.  Simply unplug the sensor
> connections and probe the sensors side with a VOM.  The rear connectors
> are under the rear seat and the front are usually on the inner wheel
> arches.  The resistance should be around 1.05 mOhms as I recall.  What
> you are looking for is a sensor that is out of spec with the others in
> terms of resistance.  It's useless to spin the wheel and look at the
> signal unless you have a scope.  Don't be shocked if all the resistance
> values are within spec.  In that case, probe the sensors while you flex
> the cables in the wheel wells and look for break in the conductors.
> Should you find a bad sensor, your only choice is to replace it with a
> new part as the system is very intolerant of resistance differences and
> will generally not work with a repaired sensor.

Excellent - my plan of action for some quality garage-time next Saturday!

> HTH
>
> Bob Duckworth

Most Certainly Does!

And Rob provided some confirmation of the behavior I expect from a proper
system:

On Sat, 4 Jan 2003, Rob Ellis wrote:
> I've never had my ABS work in the rain.  The only times I've felt
> it was when I was driving through a parking lot completely
> covered in ice.  See if you can find an icey lot and force the
> ABS to engage.  Then you can see if your problem is from the ABS
> or not.

Well, as I said at the outset, Mother Nature cooperated this weekend and
dropped 2 inches of fresh, slick, snow.  I took my E30 up to an empty
parking lot, and did some controlled sliding.  Afterwards, for comparison,
I took my wife's 99 Subaru up to the same lot and tried the same things.
First off, the sensation I had was most definitely the ABS.  So I do have
a problem with the sensors causing false triggers.  I'm hoping that brake
cleaner will do the trick, but if not, I'll follow Bob's excellent
procedure.

> Also, I'm sure you've heard this before, but I'd like to suggest
> that you NOT release pressure on your brakes if your car skids,
> unless the ABS is deactivated.  If you pump your brakes, then
> there are two people pumping the brakes... the car and you.  It
> will take a lot longer to stop the car, and when you're skidding,
> time is important.

And again, I disagree...especially after my experience with the car in the
snowy lot.  The ABS is a new thing to me and my family - this is the first
car I've ever had with it (my other cars are a motorcycle, an old Triumph
and an old pickup truck).  I found the ABS did not get me out of the slide
or back in control.  Letting off the brakes did, and then I was able to
bring the car down to a stop faster while having steering control.  I
tried this both with and without using the ABS repeatedly and got
consistent results.  I have not tested myself in dry pavement slides - I
may find that the ABS is better than me at this.  I'd just as soon not eat
up my tires, but maybe I'll give it a try when these are almost shot.
However, in bad weather situations (ice, snow and rain) I'll stick to my
own brake control.

I was surpised by the ease of breaking the rear end loose under
acceleration.  That torquey motor really does it.  I've got the Dunflop
SP-Sport-A2's on the car.  I love the tires, and they're significantly
better than the Daytons the last guy got put on at the dealership.
Overall braking and handling on the Snubaru was better in the snow, which
is not a huge surprise.  The latter is AWD, and so engine braking and
control is better.

Thanks for all who responded.

Scott
==================================================================
Scott Ruffner
Computer Systems Senior Engineer       Computer Science Department
[email protected]                University of Virginia
(434)982-2219                    http://www.cs.virginia.edu/~jpr9c

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