Noise/sensation question: ABS or 4Matic?
 

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

Any ideas anybody?

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.

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!

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.

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.

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.........

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 :P

noise/sensation
 

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.

<The symptoms you describe are classic for warped front brake rotors.>

Classis ABS activation..
He said when he is coming to a rolling stop, he feels ABS activation..Classic ABS, for sure. A sensor is dropping the signal.
Rotors are pedal pulse , not ABS activation. And more noticed at higher RPMs, not low.

Update......

Found the sensors, but can't yet get the bloody things off (salt sucks!).

The pair of allen head cap screws holding each sensor in place aren't the star type, and they seem to be right about half way between a 6mm and 8mm size - and of course, I have no 7mm.

Odd thing is, I don't think I've ever run across an odd size metric allen head before. Am I letting rust particles cloud my size assessment, or are these actually 7mm allen heads?

From the just in case I'm totally missing the boat desk: It appears that these two cap screws must come off to remove the sensor. If that is incorrect, please - set me straight!

What is odd about 7mm ??
Allen even come in half sizes [ 4/5 5.5/ etc....]

Odd as in 'not an even number.' Just my limited Benz wrenching showing through I guess (have torn multiple Miata's down to the bare bones and built 'em back up again, and I can assure you there is not a single allen head cap screw anywhere in that play).

So I guess it's off to the FLAP place to try and procure a 7mm allen driver. Will (carefully to keep the PB Blaster off the brakes!) soak the little buggers tonight after we get home, and give it a run tomorrow.

Thanks again folks - your knowledge and willingness to help is greatly appreciated.

Maybe they are 8 and are crapped up.
A good way to clear them out is with a pick ..you can scrape into each hex corner that way. I will check on a car here ...

No 4matic to check..but a couple of standard 124 Benz and they take 6mm...

Mercedes are much harder than a miata's. They make you much work more than other car manufacturers.