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

ramonajim · #1 07-27-2008, 10:05 PM

Gretel - our 91 300 TE 4Matic - has us pondering over a new trick.

Rolling up to a slow stop, we sometimes get a slight feeling through the brake pedal that almost feels like ABS engaging - on a small scale. ABS engagement, if it's happening, shouldn't be surface related (one, this only happens at slow rolls - speeds like 2-3 mph. Two, it doesn't matter whether the road surface is clean & dry or sandy or wet.)

So - we're wondering if what we're feeling is the 4Matic kicking in.

Figure the next step (one for tomorrow morning when the sun is up and the 151 is flushed

) is to flip the 4Matic test lever to the off position and see if I can reproduce the feeling.

Any ideas anybody?

Oracle12345 · #2 07-27-2008, 10:11 PM

You could be experiencing the abs/4 matic because if the system senses a slip in the wheel it will apply and do what you are describing even its sunny. Conditions doesnt matter in this case. if senses a wheel slip for whatever reason the abs computer will send a signal to apply the brakes and engage abs.

ramonajim · #3 07-27-2008, 10:41 PM

What is the likelihood of getting a false slip reading off of a sensor? If this is the case, what are the troubleshooting steps for finding (and correcting) the root cause?

Thanks!

Oracle12345 · #4 07-27-2008, 10:52 PM

to clarify 4 matic is mercedes 4 wheel drive and is a completely different system from the ABS. The 2 work together so I hope I didnt confuse you. A bad abs sensor will cause this problem. To see if your sensor is bad measure the voltage at the sensor or get the abs codes stored in the computer. the code will tell you what one is faulty. Also you could pay close attention to what wheel applies when it happens.

ramonajim · #5 07-27-2008, 11:26 PM

Understood re: ABS vs. 4Matic.

If you happen to know the correct voltages for the sensors, or can point me toward schematics with this info, that would be greatly appreciated.

I'm assuming the sensors are going to be located somewhere near each corner - on/near axles/CV joints up front I presume? (early unscientific 'testing' suggests that left front is the source).

Also - additional info or potential red herrings - simulated panic stops last night (dry pavement, standing on the pedal from 60mph down to 20mph) did occasionally result in brief (maybe 1.5 - 2 second) lock up of one wheel. Seemed to be left front. New General Altimax tires (less than 500 miles), dry clean pavement, straight road, 80*F air temp, probably +10 at the street, tires well warmed prior to 'test' was executed. Repeated half dozen times, got quick squeal twice.

Arthur Dalton · #6 07-28-2008, 12:17 AM

First Check:
Pull the front sensors and clean them..they get full of rust filings and don't give a signal at low rpm..this get mis-interpreted by ABS module as a skidding wheel and engages ABS system...very common.
If they are clean, then do an ohms test on each..the plugs are up on the inner fender well, each side..follow the cable up to the connectors.........

ramonajim · #7 07-28-2008, 12:28 AM

Excellent info!

If that doesn't cure things, next I'll try pulling codes - found this excellent site http://pages.prodigy.net/jforgione/MB_DM.html doing a search on Read ABS Codes here on mercedesshop.

And the darling wife says to say thanks to each of you for your quick replies - and that she will keep her "nah nah nah I was right!" dance at a reasonably low level of intensity until such time that we confirm your suspicions

rocky raccoon · #8 07-28-2008, 06:50 AM

Before digging into a complcated and usually reliable system such as ABS, I would like to offer the following.

The symptoms you describe are classic for warped front brake rotors. I had a similar problem on another of my vehicles (admittedly a non-Benz). One of the clues is that the problem develops over time. Heat generated during heavy braking is usually the culprit.

ramonajim · #9 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 · #10 08-15-2008, 10:21 AM

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

ramonajim · #11 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 · #12 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 · #13 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?