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Front left wheel rpm is intermittent - ABS unavailable

2157 Views 10 Replies 3 Participants Last post by  Ivan Kocher
Hi all,

I have a 2008 ML320 (164.122), were ABS/ESP says it is not available. I already replaced ALL rpm sensors, but front left wheel is still strange.
I have a iCarsoft scanner, and if I read ESP -> running values -> rpm sensors, I can see that all four report speed, *but* front left speed is strange, goes up and down as the wheel turns, while the other three wheels show a 'constant' speed. I have done the test at multiple speeds, from 10km/h to 40km/h.

If I reset the errors in ABS/ESP, and start driving the car, the front right wheel brakes get engaged like crazy, kind of expected, as ESP sees left wheel "stopping".

Currently, I am thinking that the magnetic ring might have some damage. The car was bought used 2 years ago, and was until one month ago that the ABS/ESP & front wheel problems started. I also discovered that it had a frontal collision of some type, cause I discovered a few mis placed/ broken parts while changing the rpm front sensors. Has anyone seen something like this before? I know that the wheel bearing comes with the magnetic ring.

Any suggestions?

I already read the thread " ML320 CDI ongoing self braking problem??? ", but does not look like my problem.

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Yes it is correct, and expected (I think). If ESP detects left wheel is not spinning, it will lock right wheel.

When I turn on the car, there is no ABS/ESP error. I pull out of the garage, and no issues. I start to drive and I can hear the brakes being applied on the front right wheel. This lasts for at least 300m.
Then, as soon as I pass 25~30km/h the "ABS unavailable" shows up, and there is no more right wheel breaking. I have not stopped the car at that point to check if disc is hot, but for just ~300m I guess not. It is not braking continuously, it pulses. I guess the software logic figure out that a wheel sensor is not OK after a while.
If there is debris on it, do you know if I can clean it without taking the bearing out?

I already talk to my mechanic, and I will proceed to order the bearing, plus the discard-able nuts, sort of $100 order.

One thing that picked up the mechanic attention it the high tightening torque for the shock absorber fork and wheel nut, @ 250Nm, as per WIS/ASRA AR33.30-P-0620GZ. He mentioned this system is pretty similar to Honda CRV, and in that one torque is around 180~200Nm. Can anyone verify that the torque is OK at 250Nm, or is there a typo in the WIS/ASRA document?
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I do not want to be pendantic, but looks like the pdf is for a w163. Also on pag 4 of your pdf says 490Nm for the axle nut ...

I agree that the 250Nm is pretty hi.
Follow up

As a follow up with results.

After changing the bearing, life is good again, yeah. That is almost $220 later ...

The original bearing's magnetic ring was full of crap, stuck to it, mainly oxides. After "cleaning" (ie scrubbing the finger against it) the ring looks pretty OK.

I was wondering... is there a magnetic ring cleaning procedure anyone knows about? ASRA has nothing. If one removes the speed sensor, thru the hole you can see the strip, but if you try to clean it, crap might fall into the internal part of the bearing. There is no access to clean it, and where the ring goes is not sealed from the environment. I think this is a bug in the design, really. My suspicion is that the other three wheels might also have crap there, and as time passes, more will accumulate and start causing the same issues. Also, I do not think that removing the bearing for cleaning is an option.

My w210 uses reluctance sensors, to which metal shavings and oxides do not stick. How is the wheel rpm sensor setup in other cars?

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