Noise/sensation question: ABS or 4Matic? - Page 2 - PeachParts Mercedes-Benz Forum

ramonajim · #16 07-29-2008, 06:31 PM

Well, they definitely aren't 6's (spins) and they aren't 7's (won't go in even after clearing the slot of rust). I don't have a 6.5, and neither do the two FLAPs or Sears.

So they "became" 1/4" hex with a little friendly persuasion. Figured the heads were rusted up enough I wasn't going to re-use the cap head screws anyway.....

Sure enough, the left front sensor was full of filings and crud as suggested by Arthur. Right side was only mildly dirty.

And of course, neither FLAP store had the right bolts for replacements, so I'll be trying the not so local Napa tomorrow morning. Worst case, it's off to the nearest Benz dealer (~30 miles away).

Arthur Dalton · #17 07-29-2008, 08:14 PM

If you were able to pound a 1/4" in there , then they are 6.5mm b/c that would be .255"...and a 1/4 is .250"..

If the 1/4 works , reuse them after cleaning just to see if that solves the signal dropping.
You also want to blast some brake cleaner into the sensor hole and spin the wheel to clean the reluctor segments..and a good blast of compressed air.

Here is a good read on how wheel sensors basically operate and testing them. It will give you some insight to the actual electrical working/theory and design. Te atricle was a diagnosis one, but the lead-up info is a good Teach.

http://www.picoauto.com/tutorials/diagnosing-abs.html

ramonajim · #18 07-29-2008, 09:00 PM

Quote:

Originally Posted by Arthur Dalton

If you were able to pound a 1/4" in there , then they are 6.5mm b/c that would be .255"...and a 1/4 is .250"..

If the 1/4 works , reuse them after cleaning just to see if that solves the signal dropping.
You also want to blast some brake cleaner into the sensor hole and spin the wheel to clean the reluctor segments..and a good blast of compressed air.

Here is a good read on how wheel sensors basically operate and testing them. It will give you some insight to the actual electrical working/theory and design. Te atricle was a diagnosis one, but the lead-up info is a good Teach.

http://www.picoauto.com/tutorials/diagnosing-abs.html

Thanks for the additional info. Reuse of the existing cap screws ain't gonna happen - while I could probably get them back in tight enough to do some test drives, whether the crud all over the front sensor is the root cause or not, I need to replace the screws. Don't need the car to be done today (or this week, for that matter) so I'll go ahead and get the right cap screws first and proceed from there.

Variable-reluctance sensors I understand - theoretically and in practice (one of my three Miata's uses the same technology on it's crank position sensor).

Thanks again Arthur!

Arthur Dalton · #19 07-29-2008, 09:08 PM

Yes , they are all the same "Hall Effect " principle...simple as that.
And knowing the problems that Hall Effect sensors have with low amplitude output at low rpm's, one can see where they have to be perfect , b/c if they are not, they show up as fail @ low rpm...which is what we have with your car..acting up as you come to a slow stop..[ low speed wheel rotation], resulting in the ABS module reading this "no sensor signal " as a locked/skidding wheel......

ramonajim · #20 07-30-2008, 02:37 PM

Got new fasteners (not from the dealer - nobody has 'em in stock!). Cleaned up the reluctor teeth as best I could. Right front looks pretty darn clean, the left has some rust on the teeth.

Still getting the same symptom - ABS engaging at low speed.

Help me understand something - am I fooling myself into assuming that since the ABS feels like it is pulsing the front wheels only (specifically the left front), that I have a front WSS issue? Is it possible that I have a rear wheel speed sensor issue? I haven't opened those up for inspection yet.....

I read through the article you linked above Arthur, and see that the root cause in that situation was misalignment of the sensor due to damage/corrosion. As best I can tell, the sensors are mating as designed. There is some corrosion on the mounting surface on the left wheel, but the sensor mounting bracket sure looks fully flush and parallel.

Both front sensors measure 19.98 kΩ at their respective connectors near the firewall. The article mentions 1.734 kΩ - not sure if that is what I should be expecting on my WSS's or not. Order of magnitude difference, but both reading the same, leads me to assume they're different sensors than those in the article - can you confirm or correct this? (i.e. what should I be seeing?).

Much obliged!

Once I get this settled out, the CV boots get replaced....... always another project (or ten).

Arthur Dalton · #21 07-30-2008, 02:45 PM

Once you get to that point, you really have to have a Scope...that is the omly way to see what the ABS module is reading...
You can ohm the rears and look for bad wiring at front ones by flexing the cable while watching the ohms values for change, etc...but once you are to this point , a scope is the tell.
I don't even do primary ohms/volt test on WSS...I go right to a scope wave pattern.
You may also want to test the ground side of the sensor cable to a good ground w/ohm test to be sure of a good ground at mounting ..should be trace/none..

That second section of the article was an oddity, so I would not search in that same direction...I believe you have a weak sensor and a dirty/bad reluctor..and that is why you need a scope if all other continuity an ohms test look OK.
If the ohms and wires look OK, then the sesnors SHOULD work as deisned, but you can see that there are distance spacings and magnetic properties that come into play and thew ECU will pick them up, an ohm meter wont...and a Good Scope has no problem , as you can see by the wave forms he displayed. [ and the reason why I posted the article..I thought he did a good job and is one of the better ABS ones]
There is one thing I did not mention, but is a possible with some sensitive systems..the tire sizing/pressure must be uniform or a sensor can mis-interpret wheel speed imbalance due to rolling circumference differential..this is actually quite common on ASR system and will actually bring on codes.....doubt that to be the problem, but a consideration

Just read thru the typos b/c I get tired of having to constantly edit...I can't type to save myself and I hit send before proof reading my post.. I see them later some of them are BAD ..............

Arthur Dalton · #22 07-30-2008, 08:45 PM

<>

Is that a decimal typo ??
Spec for front on that chassis is .85-2.3K
Rear is .6-1.6K

ramonajim · #23 07-30-2008, 11:08 PM

Quote:

Originally Posted by Arthur Dalton

<>

Is that a decimal typo ??
Spec for front on that chassis is .85-2.3K
Rear is .6-1.6K

Ah - not a typo, but was an error on my part. Went back out and double checked my tracing of the cable to the connector - my 19.98 kΩ readings were on the wrong side of the connector

Actual readings are within the range you gave, but over a spread:

RF = 1.023 kΩ
LF = 1.611 kΩ

Is there any chance this amount of difference between the two could be a factor?

As of tonight, getting the left front axle out has moved up the priority list - the inner CV boot has progressed from "gee that looks fatigued" to "has an actual split" so Gretel is going to stay as parked as possible until I can get the boot replaced. At that point, I'll do a proper clean up on the reluctor teeth.

Arthur Dalton · #24 07-31-2008, 12:35 AM

I don't think so....
....in spec.

ramonajim · #25 08-08-2008, 12:05 PM

Update: got the boots!

Quick plug for Phil at Fastlane - this guy rocks! Called him up since I was confused by the photo in his catalog.

Wanted to make sure I wasn't ordering more than I needed - text read that I'd need an inner kit and an outer kit, but the photo showed two boots in the outer axle kit. He checked, and concluded that the outer kit contained everything. Placed my order, then got a follow up email - seems on further checking, I did in fact need both kits, so Phil was going to ship both - at no extra charge. I would have been perfectly happy to hear "I was wrong, sorry, you need to order the inner kit separately" but Phil wouldn't have it. "I said you only needed the one, so I'm not gonna charge you more" was the gist of his reply.

Anyway - now I'm ready to pull the axle to replace the boots, and (back to this thread

) clean up the WSS related bits.

Any pointers - link to a step by step guide, with photos, per chance?

ramonajim · #26 08-14-2008, 11:35 PM

Got time this morning to replace the CV boots on the left front.

Found the teeth on the axle that the WSS reads to be truly crapped out with rust - something like 30% of them had reduced or non-existing gaps between the teeth (rust growth). Seemed kinda odd, since the other surfaces in the vicinity weren't bad at all.

Judicious application of a wire wheel cleaned 'em up just fine.

Now, the original problem (ABS kicking in on slow speed stops - down at 2-3 mph) is happening on damn near every stop instead of some to most.

My guess - since I believe the ABS is looking at wheel speed differentials - is that I just upset the balance between two almost equally crapped up set of teeth. So, tomorrow's mission will be to pull the right front axle and clean those teeth up.

Will also trace back on the rear WSS's and check the resistance on those.

Any other thoughts, observations, suggestions appreciated.

Arthur Dalton · #27 08-15-2008, 10:21 AM

Turn the steering from lock to lock a couple of times.

ramonajim · #28 08-15-2008, 01:27 PM

Quote:

Originally Posted by Arthur Dalton

Turn the steering from lock to lock a couple of times.

Did that this morning. Cleared up the half of yesterday's observations (the half that I forgot to mention): yesterday, every time the ABS would engage on a slow stop, I'd get the 4matic light to come on as I pulled away.

Today, after going lock to lock with the steering wheel 4 or 5 times, the ABS still engages on every stop, but the 4matic is not engaging on take off.

Any other suggestions Arthur, others?

It's not looking like I'm going to pull the right front today to clean that side up - is this even worth doing, or am I barking up the wrong tree?

Also - any hints as to where I would find the connectors for the rear WSS's?

Thanks again!

michael cole · #29 08-15-2008, 02:59 PM

heres what i did trying to track down an identical problem.connect a DVM to each wheel sensor at the coaxial connectors in the engine compartment.attach meter to windshield with duct tape.drive car at slooow speed and observe voltage readings.the sensor with the lower reading is the problem wheel.rear sensor is either good or bad but you can measure it too by finding the connector under the rear seat.the air gap between the sensor and axle teeth is very critical.but there is no adjustment.possibly the teeth are worn down

ramonajim · #30 08-15-2008, 03:24 PM

Quote:

Originally Posted by michael cole

heres what i did trying to track down an identical problem.connect a DVM to each wheel sensor at the coaxial connectors in the engine compartment.attach meter to windshield with duct tape.drive car at slooow speed and observe voltage readings.the sensor with the lower reading is the problem wheel.rear sensor is either good or bad but you can measure it too by finding the connector under the rear seat.the air gap between the sensor and axle teeth is very critical.but there is no adjustment.possibly the teeth are worn down

Did you disconnect the sensor, and just read voltages (resistance maybe?) from the sensor side of the connector, or did you figure out a way to measure voltages with the sensor connectors still connected?