85 300SD - Brakes - ABS noise/softish pedal? - PeachParts Mercedes-Benz Forum

BigSkillet · #1 02-25-2011, 05:04 PM

Hey, I did a search (over 500 results) and after going through 150 or so, I think I found somebody with a similar problem. But I want to ask to make sure.

The brakes seem fine as I'm making my initial stop. Then, the last 10 feet or so, when ya put a little more pressure on the pedal, it sounds like the ABS noise (during traction loss) and the pedal sinks about an inch. Not totally soft. Not all the way to the floor. Is this probably a bad master cylinder?

Side note, one morning I started the car up and had the brick brake pedal. But I made it to the end of the street to turn around, and all was good. Hasn't done that since. Did it go into self-healing mode or is that foreshadowing of a vac problem?

Thanks everybody!

mach0415 · #2 02-25-2011, 07:16 PM

Mine did the exact same thing. Check the tone rings on the back side of the rotors. If there is road grime and grease caught in the teeth, it may interfere with the ABS sensor voltage induction at slow speeds, fooling the ABS module into thinking there is a skid, when there really is not, and therefore opens the pressure dump valve and pulses the modulated apply valve and pump motor for the affected wheel brake(s) to regain traction.This explains the funky pedal feel. Tone rings first - Clean 'em. If this does not solve it, do what I did - pull the relay for the ABS ECU and voila - problem solved!

Your hard pedal is more than likely a separate concern. A vacuum leak or weak vacuum pump would cause this. I have thought about mounting a vacuum gauge next to my boost gauge for this very reason. I put the boost gauge in the glove box, angled so the driver can see. This way, I retain the stock MB interior with a tucked away subtle goody. I rigged the gauge light to the glove box light switch, so it goes on when the switch is tripped after the glove box door is open.

BigSkillet · #3 02-25-2011, 07:40 PM

Ahhhh....that is much better news than a possible master cylinder! Thanks for passing on that info. I like your idea about the ABS ECU, too. That thought crossed my mind if my problem ended up being an ABS issue.

What kind of mpg do you get in the Club wagon? I've been trying to talk the wife into a custom van for roadtrips (we make multiple trips a year from Texas to Kentucky shuttling kids to exes houses lol). I wanted a diesel, but hard to find a diesel custom van. Usually they are ragged out ex-plumbing vans.

mach0415 · #4 02-25-2011, 08:19 PM

You can expect 15 on the hwy with a 15 passenger 5.4 L Triton. In town worse, but a great comfortable people mover.

compu_85 · #5 02-25-2011, 09:52 PM

You probably have a dirty tonewheel or bad abs wheel speed sensor giving low signal. When the car's speed gets close to a stop the signal drops out completely and ABS computer thinks the wheel is locking.

-J