Garth S

... getting more interesting ... " brakes is brakes - and then there's ABS" - the ABS is brought into action when an activating parameter of its algorithm is triggered. Otherwise, it just sits there monitoring normal driving, including braking. Limit signals are also used, like 'above X mph', etc., but the primary activation is a frequency comparator of the signals fed in from the four wheels.
The teeth on the rotating disc create a frequency pulse voltage in each hall effect sensor. As long as each wheel is sending in an identical frequency within a +/- allowed limit, no matter how hard the brakes are applied at any speed - the ABS will not activate. If one wheel grabs ( reduces the frequency of the input signal), braking force is reduced to that wheel by a rapid series of valve activated pulsations: depending on the system, the pressure is not reduced - it is the duration of the applied pressure. This is sustained until the wheel in question falls within the velocity ( frequency) envelope of the other wheels, even under maximum braking: at that point, the ABS again goes into stand-by mode as normal braking continues. IIRC, this is how the deal works in antiquated ABS fitted to 928s - the newer systems connected with lateral accelerometers, engine torque output, traction controls, ..., start to get complex.

Jim bailey - 928 International

To add to Garths comments the 928 two channel system if one wheel slips it reduces the pressure to TWO wheels. And ABS reduces the pressure to the brakes it never ever adds pressure. The power brake booster is what helps you apply the brakes and sometimes decides to do it for you ! It has the POWER ! ABS makes the decision to reduce the braking.

deliriousga

I don't think the light coming on has anything to do with the stop in shaking. The light came on over a week ago and it still kept shaking.

I I don't know why, but even though the ABS light is on, the system still works somewhat. This weekend I locked it down a few times to see what would happen. Only the front-right tire locks up, the rest are fine (I'm going to test the signal in that sensor tonight). Yesterday morning I cleaned out the sensor on the wheel that would lock up before going to a family birthday party to see if the light would go away. On the way home from the party it still shook (the shaking has always come from the rear). I let it cool down when I got home and cleaned up the other three sensors at about 11:00pm last night. This morning on the way in to work I got to the 80mph range and was ready for the vibration but it didn't come. That's the first time I ever got to that speed since owning the car without vibration so I was shocked and I hit the gas to see when it would start. Got to 100 and no vibration at all.

I know this sounds weird, but there are so many threads I've read where people have spent hours, days and mucho $$$ replacing axles, springs, shocks, bushings, bearings, tires, etc. and still have the problem. Since you can't see the sensors I don't think most people would even think about cleaning them until the ABS light came on and they did a search here trying to fix that problem. I figured I'd say something just in case so they may be able to stop pulling their hair out. I've done the same on my Intrepid (God be with me) as I've replaced the entire suspension, axles, struts, tie-rods, bearings, you name it on that thing and still get vibration at 70+mph. I'm going to clean those soon and see what happens there.

mark kibort

tried that and it didnt help. some how,the pressure is building up in the system and not releasing, even with the fuses removed.

just noticed its doing it again now!!! (Today) you guys jinzt me!!!!

actually, you know that faint vibration when you are at a stop but in drive (D), and you can make the rotors vibrate? when i release the brake pedal, it still drags a little and makes that vibration for a second. on the freeway, its starting to make the vibration again, but maybe the rotors didnt get hot enough to start the surging. also, i pulled back on the pedal and that may have helped.

maybe need to bleed the entire system again, not just the fronts!

MK

mark kibort

try and see if you can get on the freeway and this vibration starts happening. then, pull over , coasting to a stop, and check the rotor temps. (spit on them if you dont have an IR meter) if they are above 210degrees, you got this weird ABS or pressure build up issue. if not, the vibration is due to something else.


sorry to hijack this thread with my BMW 328 issues!

MK

Jim bailey - 928 International

Mark disconnect the vacuum to the brake booster (block the hose to the intake so it runs OK ) and see if the brakes release. Be carefull you will NOT HAVE POWER ASSIST.

mark kibort

I think i did this when it was vibrating badly, by turning the engine off, pumping the brakes and was waiting to see if the shaking quit, and it didnt!! there is some pressure, caused by something in the ABS, and i dont know what it is caused by! especially since when the rotors are vibrating you would think that there may be some warping (although stoptech says there is no such thing as warping, only different areas of the rotor with grip and no grip), that the warping would push the pads back in the calipers. this is not the case. those pads dont retract until you actually unscrew a bleeder line.

puzzeled!

MK

deliriousga

Same issue, different car, right? Sorry to hear it has come back. Bleeding the whole system may help. I know my ABS acts up pretty badly if condensation occurs. I've had times when the brakes would let go and chatter as the ABS is releasing the pressure over and over on a hard braking in the Intrepid and Boxster. Bled the whole system and it went away completely. That's when I decided I need to bleed the system every time I change the pads.

I know mine's not warped rotors. It stops very smoothly. Even if I was in the high speed range where the shaking occurred, I could hit the brakes and the vibration went away immediately. If the rotors were warped, I don't see how it would suddenly go away when I haven't taken them off or had them machined. I'm going to try and get it up to speed again on the way home and see if it was a one-time occurance or not. I'll hit the brakes and see if I get any vibration.

Jim bailey - 928 International

Mark.. Disconnect the vacuum line because if the valve inside the booster which senses that you are appling the brake pedal fails it THINKS you are stepping on the brakes and helps push in on the master cylinder. As soon as you start the engine the vacuum can start to apply the brakes. Also if the pedal is misadjusted no freeplay it can apply the brakes, or if the mastercylinder does not retract far enough....... But more often than not the booster fails in such a way that it decides to apply the brakes.

deliriousga

It's like falling in love with the shark all over again. It's sooooo smooth at the upper end now! Scary thing is I used to be able to tell when I hit 80 and now I can't feel it.

I tried picking up speed and hitting the brakes hard several times to see if there was any vibration from warped rotors but nothing. Just smooth braking. I didn't get a chance to check the output of the individual sensors last night so I may try tonight. I spent the evening in front of the television with my favorite tool, the scratch brush, cleaning all my fuse contacts instead

Getting close to feeling comfortable enough with the Shark to go to a DE event and learn how to really drive it.

SMTCapeCod

THere's an interesting diversity of views on abs functioning and faults. Bumping up an older thread and linking this one:
https://rennlist.com/forums/928-foru...ml#post2432459

Ducman82

the ABS pump modulates the pressure you insert in to the system, it does not create it own. you obviously touched/moved something else while cleaning that sensor. but i sure would get that fault figured out. that light would dive me bonkers.

mark kibort

I was noticing something at the race track..... haven't bled my fluid in a few weekends and I was getting a little soft pedal at the end of qualifying. at some of the long high speed braking zones, I was getting the ABS light to come on if I dove too quickly on the brakes and there was lock up for a moment. released and went easier and the light went off upon release and re application at a slightly less aggressive push with no issue. in the pits, I changed the fluid and got a nice pedal. in the race , that didn't happen again. wonder if air in the lines can cause ABS failures???

mark kibort

by the way , my shaking on the BMW328i was the master cylinder.. changed it out and its all fine.
I think this OPs issue would probably be a balancing issue, that was changed when the wheel was mounted differently or something else changed.... all the magnetic material the pads could stick to the sensor couldn't add enough force to do anything balance wise..... or he has a master cylinder issue too, and that would be determined by the rotors warping and getting cooked by pressure the system was producing to clamp the pads on the rotor , independent of the foot pressure. with out high temp rotors found during the shaking, its a wheel and tire issue.