the flyin' scotsman

With no antilock what is normal braking?

80/20 front/rear bias?

jstadter

A few thoughts about the ABS light and effectiveness of the rear brakes...

When I had an ABS light on my S4, the problem turned out to be the ABS relay. That's obviously an easy swap, but before you drop the money for a new relay, pull all the wheels and clean the toothed ring and ABS sensors at each wheel. The sensors are magnetic, so over time they can accumulate tiny metal particles that might trigger the ABS light.

Regarding the hydraulic portion of the system, you might consider changing all flexible lines in the brake system. Typically these lines fail on the inside by becoming delaminated, so you won't necessarily see any wear or bulges on the outside. Usually a delaminated line will act like a clogged artery by letting fluid get to the calipers when you apply the brakes but then not letting the fluid escape back into the system when you let off the brakes. The result is that the brakes tend to drag or stay applied. That's obviously not the symptom you're having with the rear brakes, but it may explain the abnormal wear on the fronts.

Jim

Imo000

You don't need to modulate the breaks just don't push it hard enough to lock up. When it does, letting it out and aplying just enough pressure so tey don't lock up again. This is easier said than done but with enough practice and epreciance, this can be done. However you are right, having ABS is best for the masses.

dr bob

Malcolm--

The rear pads still do some work, and even though they don't appear to wear as much as the fronts they should be changed together. With the resurfaced rotors, new pads will stop better than used pads. Every bit of stopping that the new rears contribute is work the fronts won't have to do. Or it's extra capacity that the fronts can contribute. In the big picture, pads are cheap. You have a month or two of winter during which to resurface the rotors and swap in new pads. Seems simple to me.

If you haven't done so recently, take the time to flush the brake fluid and replace with new. Moisture accumulates and you have the whole month of winter for the moisture to do its dirty deeds to your hydraulics.

Alan

If you want to achieve maximum possible braking you do need to start to lock them up and get there quickly... that was my point - then its down to how you keep them right at the edge. Otherwise ABS has you beat - even in the dry in a straight line.

Alan

the flyin' scotsman

Good thoughts Jim:

I have a replacement relay en route so we'll see if that works.........the one on the panel is the original ~ 20yrs old. I'll also clean the rings.

Re. the lines I agree; once again these are original and 20 yrs old. I had thought of brake drag especially when the brakes are hot. The car freely moves in the garage when the brakes are cold so the next time I arrive home from a spirited run I'll push her around. Another drag consideration would be the caliper pistons themselves; we'll see.

the flyin' scotsman

You are one funny guy Bob ........"the whole month of winter"

I just finished flushing the brakes yesterday and inspected the brake system at the same time. Thats when I noticed the wear on the fronts pads. I have the wheels off enough during the season that I have not seen any wear on the rears...........perhaps the pads have turned to wood?

Bottom line is I need a set of front pads for the remainder of the 'no snow' season..........then I'll contemplate a complete refurbish. After all I have that "whole month"

123quattro

I work at Bosch doing ABS/TCS/ESC calibration.

Alan

Good - so what are the functional differences in the Bosch system from late 80's early 90's to todays systems? true 4 way system? different algorithms...? faster?

Alan

Rich9928p

ABS is the way to go for 99.999% of braking. The areas of dispute that I read about were mostly in mud and heavy snow where locking a wheel permits a wedge of material to build up in front of the tire and helps during the last 5 MPH of the stop - in the early Audis I recall they even had a switch to turn off ABS for those who wanted to.

This is my gripe - people aren't trained to use ABS. When you have a chance on a rain slick or snowy parking lot, drive and stomp on the brakes and get used to the ABS pulsing feeling, try steering. ABS CANNOT WORK if you don't stomp as hard as possible on the pedal and keep it down, DO NOT LET UP or breaking is reduced. Teach your wife, kids, sig other, friends to use ABS. Too many people let up on the brake pedal too early.

Standard ABS (prior to traction control and stability control) ONLY works off of master cylinder pressure, and the control logic has three states

- pressure increase (up to 100% of master cylinder pressure)
- pressure hold
- pressure decrease

The ABS fail safe mode is all brake pressure from the master cylinder is passed through to the brake calipers, there is zero difference in fail safe between a car with no ABS and one with ABS. It is the owner's responsibility to keep the fluid flushed and clean such that the ABS valves aren't clogged and do things that they shouldn't.

The ABS system used in Porsche 928s is a 3 circuit system, the front left and front right wheels have their own braking circuit. The rear two drive wheels are handled as one "linked together" circuit.

The major benefit of ABS is that the steering wheels are able to provide steering traction under maximum braking - such that a driver can steer out of a problem, or better maintain directional control. If the brakes are locked, the car goes in the straight line direction dictated by physics. Secondly, if different wheels brake over different surfaces (split mu) directional control is maintained, otherwise the car would be put into a spin.

I had the fun of being a passenger in a car on the Bosch ABS test pad in Germany and feeling/observing the difference with ABS on and off - it is amazing.

Bosch's early ABS systems used a custom chip (internal name Bayreuth) and the system was fine tuned by fitting appropriate size fittings in the circuits - I believe that our 928s use this ABS system. Bosch employees joked that one guy had a "magic ***" and he was the one who had to go to Sweden in the cold winter to tune each car type's ABS dynamics. Later Bosch ABS systems went to microcontroller based systems (two tandem microcontrollers, shut the system down if they disagree) - which could implement much more sophisticated control logic.

Every he-man thinks that he can brake better then the ABS - I say BS to that, ABS can compensate where no driver can, because we don't have 3 brake pedals on the floor, and even if we did how do you know when one wheel has an impending lock up and how quickly can you modulate pressure? I strongly suggest that anyone who has ABS keep it working. There is a need to clean metal off of the wheel speed sensors/tooth assembly and to inspect connections and wires for breaks and corrosion. The ABS system is very reliable.

StreetJustice928

Im having a problem with my ABS light, I put a tester on all the sensors and only one reads a tiny bit off. Would that effect pedal pressure, it seems since the light when on, my pedal depresses to the floor to get proper braking, and sometimes there is a small rubbing sound when i turn left....Im confused.

123quattro

The stuff we are working on now is for MY09 and MY10. This algorithm is at least two generations newer. There are 3 and 4 channel versions of it depending on how cheap the manufacturer wants to be. Only a few trucks are 3 channel, everything else being 4 now. The new stuff, including Conti and TRW, is way better than the old ABS systems. A lot more functionality, can be much finer tuned to the individual vehicles (brake development for a vehicle is generally two summer and two winters to optimize control), and can do surface detection for ice, snow, sand, and gravel. Newer programs may also recognize grade and react accordingly.

Rich9928p

What do you mean by the sensor reads "a little off" ... are you using an oscilloscope? What are you reading from the sensor?

The ABS controller needs to see sharp enough pulses from the sensors to use as inputs for the computations. If the pulses aren't a sufficient "shape" then the controller throws the ABS light. In most cases, all it takes is removing the wheels and blowing the crud off the sensors and harmonic toothed rings. A continuity test will tell if a wire is broken or a connection may be corroded.

UKKid35

Thanks Rich for confirming what I suspected, but didn't actually know for sure.

I have a couple of questions about ABS faults, because I really want to get mine working:

Car on axle stands, spin right front wheel, ABS fault triggered, as expected
Spin left front wheel, no fault triggered, but 1VAC present at the engine bay connector
Spin either or both rear wheels, no fault triggered

I pressume there must be a wiring problem between the engine bay and the ABS unit to explain the front left (or the signal isn't clean enough)

But what about the rear wheels, pressumably the ABS can't fault if you speed off spinning the rear wheels, so is it possible that I only have one problem with the front left to fix?

Alan

Wheels slipping under traction (faster than both fronts) is a different case than wheels locking/slipping under braking (slower than both the fronts)....

Alan