deliriousga

I had shaking at 80+ MPH and had done some checking and chalked it up to a small flat spot on my rear tires, but that wasn't it.

My Antilock light came on while we were on vacation a week ago so I was trying to figure that one out. To start I pulled all of the sensors out to clean all of the metal brake shavings off the magnets. They were filthy so it needed to be done. Unfortunately, my Antilock light is still on, but THE SHAKING IS GONE! I took her up to the speeds that I was getting really bad shaking at and it was smooth as silk.

If you have a problem with the car shaking and can't figure it out, try cleaning the ABS sensors. It probably needs to be done anyway. You can re-cover the wiring while the wheels are off too since the rubber coating is probably falling off, so why not?

sharkmeister85

Hmmm. Interesting. I don't have ABS and don't know anything about it, but I kind of wonder about the connection you're making. Could it be that one or more of your wheels had the lug nuts over-torqued when put on last time, causing a warp in the rotor(s), and by removing them and setting at correct torque, cured your problem?
Glenn

Randy V

Interesting, John. The weight of the metallic particles adhering to the magnetic sensor caused the wheel to become out-of-balance, and thus the shaking?

Garth S

Don't think so - the sensor is fixed in the hub carrier/spindle, and the toothed wheel responsible for the pulsed signal rotates with the hub/wheel: perhaps a glob ( tech. talk ) of magnitized particles is causing false signaling to the wheel speed differential sensing computer. However, even were that the case, the ABS has no reason to cycle unless the brakes were applied ....
I'm liking Glenns idea - or were the lug nuts lightly tightened?
One other thought is that often, wheels are torqued after the car is released from the jack: being **** about keeping the wheel 'hubcentric' with its hub, I always tighten prior to allowing the vehicle to place full weight on the tires. This assures that the conical nuts center the assembly - otherwise, one is trying to lift the chassis that last mm or two by torquing the nuts, and if not done completely, the risk is that the wheel nuts are never tight after a few miles. The signal is vibration ....

bfellows

Locking nuts not fitted on the correctly marked stud? Nearly everytime a workshop removes my wheels they always refit wrong! Wheels always feel out of balance at higher speeds until i refit correctly.

"Make sure that the colored wheel bolt is closest to the valve. Screw the lock-up wheel nut onto this bolt" to quote the manual.

deliriousga

Wasn't torqued incorrectly, I had checked that after I had the tires balanced trying to figure it out. Also, I have taken the wheels off several times since having the car and re-torqued them properly every time. Rotors are true and I've never had a problem with shaking while braking.

Here's what I tried before, but still had the problem:
1) I had the wheels balanced with the Road Force machine.
2) I checked the air pressure and torque all around. Air pressure was very low, but torque was fine.
3) I checked for a flat spot on the tires and found a small one on each of the rear tires.
4) I replaced the CV boots, inspected the joints and re-packed them. All joints were fine.

As far as the ABS connection, the sensors are magnetic. In the rear where the shaking was, there are teeth (similar to the flywheel, but smaller) on the axel right behind where the splines go into the hub. The sensors "feel" the teeth as they rotate. The sensor sends its' readings to the computer telling it if it senses the wheel moving or stopping. The computer checks the speed of the vehicle and how hard the brakes are being applied to determine what to do with the brakes depending on the sensor reading. If the car is moving at a speed > probably around 3-5mph and the ABS sensor says the wheel is stopped, the computer will tell the brake system to "let go and pump the brakes" to keep the car from skidding and help you stop the car quicker and more safely.

Here's what I think is happening:
As the brakes wear, some of the metal in the pads gets on the sensors and sticks since it's magnetic. As that builds up, the sensor has a harder time "feeling" the wheel's movement. At a higher speed, the teeth are going by so fast that the sensor (with all of the junk on it) cannot feel them moving. It checks the speed of the vehicle and how hard the brakes are being applied. Since there is always some pressure in the brake system, it determines you are barely applying the brakes so it doesn't need to stop hard. It then tells the brakes to let go and start pumping lightly since he's not trying to stop hard.

Hope that helps.

Bill51sdr

Here's what I think is happening:
As the brakes wear, some of the metal in the pads gets on the sensors and sticks since it's magnetic. As that builds up, the sensor has a harder time "feeling" the wheel's movement. At a higher speed, the teeth are going by so fast that the sensor (with all of the junk on it) cannot feel them moving. It checks the speed of the vehicle and how hard the brakes are being applied. Since there is always some pressure in the brake system, it determines you are barely applying the brakes so it doesn't need to stop hard. It then tells the brakes to let go and start pumping lightly since he's not trying to stop hard.

Hope that helps. [/QUOTE]
Not quite sure what you are getting at here, but if you are implying that the ABS system is applying the rear brakes without your input, i.e. your foot on the brake pedal, then this is an incorrect assumption. It doesn't work that way. Sounds more like a wheel imbalance you corrected by reinstalling the wheels .

Bill Ball

I love the statement ""Make sure that the colored wheel bolt is closest to the valve. Screw the lock-up wheel nut onto this bolt". The red paint on the end of the lug ("colored wheel bolt") is long gone on 3 of the 4 wheels on my 89. Oh well.

deliriousga

The sensor doesn't spin, just sits there, so I don't think it was a weight/balancing issue.

Here's another thing that happened when the ABS light came on that makes me think this was the problem. On my way from Miami to Cocoa Beach, if I got to the 90mph range, the brakes would apply on their own and off and the car would pull back. Slow down under 80 and it went away. When I checked everything out, there was too much brake fluid in the resevoir causing high pressure when I it got hot. (That one was my fault for bleeding the brakes at 4:00am.) I bought a turkey baster, parked sideways on a hill and sucked out enough fluid to get it down to the range it was supposed to be in. Voila, problem solved. The ABS reads the pressure in the system to see how hard the brakes are applied. Since the pressure was high, it thought I was trying to stop and couldn't tell the wheel was not locked up so it helped me out.

In my previous post I talked about the ABS system applying the brakes lightly (sorry Garth, I went to lunch during that and didn't get it in until after your post). Since it only senses pressure in the brake system, and there is a "range" that the fluid can be in, it cannot assume a lower pressure is no braking at all so it thinks you are just lightly pressing the pedal and starts pumping lightly at several times per second. It assumes you are braking lightly because if you were close to the bottom of the range for fluid and actually lightly pressing the brakes, the pressure in the system could be close or the same as no braking witht he fluid close to the max of the range. At 80+mph it doesn't take much to affect the ride. That would also explain why it got worse as you go faster. It thinks you're trying to slow down a little and the faster you're going the more pressure it takes to slow down a little.

mark kibort

Hey, ive been chasing a HUGE problem like this with my BMW 328. (yes, i know,the dark side) anyway, every once in a while, the wheel will start to shake and get worse and worse without touching the pedal. then, if you brake, it gets real bad like warped rotors! if you pull over to check the brake rotors, they are smoking hot!!! all this, and no brake input. ABS light is not on though, but ive also tried pulling ABS fuses. had this problem intermittanly for 3 years now, and just last month, i wore the pads to the metal, replaced the pads, pushed the pistons way back in the calipers and installed the new pads. bled the fronts, and off i drove.
didnt think that would be the issue, but it fixed the issue.

Im thinking, there was some air in the line that would expand under heat, maybe near the ABS controller that when hot, started to apply pressure to both the front brakes causing them to get hot and start oscillating. and they would shake pretty bad. one time, i looked out the window and you could see the wheel going back and forth!!! (and the sterring wheel would be moving back and forth too)

Ive also heard the others in the BMW say it could be a ABS controller issue too. (ie $1200 to replace)
i dont know how the ABS put ON the brakes, because thats not its job, but its been a pretty bad issue for my BMW, that is now seemingly fixed. (after 45days, no vibration , smooth as silk, 100miles per day driving)

MK

deliriousga

It does work that way. You're dealing with a pressurized system that has a valve for releasing pressure when it locks up and one to put the pressure back so you can still stop. Here's a snip from a website that explains how the system works: "When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second."

Check this out for how the system works: http://auto/howstuffworks.com/anti-lock-brake1.htm

deliriousga

Sorry, after reading the posts I see where I've not explained the braking at high speed very well. I don't think it's "putting on the brakes" as in the computer saying to brake. Like you said with the pressure building up, I think that pressure is what applies the brakes as the ABS system opens the valve to allow the pressure back to the calipers. Even though it's a very light pressure, it's still enough to make the pad contact the rotor. Then it opens the valve to release the pressure again because it thinks it will lock up. It does this over and over several times per second.

Bill51sdr

Well then, using this logic, perhaps your rear rotors are warped and should be machined or replaced. The ONLY time I have ever had an ABS system kick in without my input was when I had a master cylinder failure on a previous ABS equipped 928. The ABS system was the only thing that kept the wheels from locking up completely until I got the car off the road.

mark kibort

Nah, i dont think its a warping issue. (maybe after extreme heat) but, mine are fine unless the ABS does it thing like Bill was saying.

the pressure builds. then, you can still modulate smooth braking at high speed for a while, then the heat starts to build after 10 miles or so, then it starts to shake. both front wheels start to shimmer. it gets worse and worse and you pull over, coast to a stop after not even touching the brakes and they are 400 degrees!!!!! I then started to push the brakes like i was bedding pads and then pulled back on the brake pedal (physically) it would then be fine and not vibrate , although big vibration if i used the brakes .

i buy bills assessement.

Mk

Bill51sdr

Another thoought here... since your ABS light is on that means the system is disabled and is not functioning. This could explain the lack of vibration if your theory that the ABS system is causing it is correct.

Mark, perhaps you should try disabling the ABS system in your 328 (fuse?) and see if it clears up the problem with that car next time it occurs.