Brakes on 124's
 

I'm sure this has been covered, but I can't find it with search. In checking out a 124 - 300e I get a pulse when braking that would normally make me think I've got a disk that needs replacing or maybe a grabby caliper. I'm told it's just the ABS. Well fine but there must be a problem with a sensor.
Do they go out ferquently or just need to be cleaned? Can I just unplug one side and test the brakes or will I mess something up if I do? Can I unplug one side and then the other or must it be both at once?

I think if you leave one unplugged, it will trip the ASR light.

I would just clean them both really well, and see what happens. You have identical tires on all 4 corners, yes?

It seems like there's about a dozen things that cause pulsating brakes on these cars, sometimes very hard to find. ABS is not a prime prospect. You can confirm that it is not ABS by pulling the ABS control unit. The brakes will work fine (as well as they do now) without it, confirming that the problem is elsewhere.

I have been through at least 5 sets of rotors on my 400E and I never could figure out why my brakes still pulsate. I just live with it. I may try to pull the ABS, but doesn't my car have some sort of electric pump as a primary for power assist (vacuum secondary)?

If you've done everything else, and you have the upgraded 25 mm rotors, it's probably the lower control arm bushings.

I bought the car in 1997 with 78,000 miles on it and it was doing it then. Would the lower control arm bushings wear out that fast? It uses brakes for a 1992, even though it's a 93. Maybe Mercedes had some left over rotors and calipers they wanted to use up before they changed over. I didn't know there was an upgrade for the brakes. Is it an aftermarket product or is it available through Mercedes? Thanks.

Sometime around 93 they went from 22 mm front rotors to 25 mm front rotors. If you have some reason to believe yours are for a 92 you probably have 22 mm. All you have to do is buy the later 25 mm rotors and put them on. No need to change the calipers but the earlier pads probably won't fit - need to either sand them down or go with the thinner later pads. Try a search - I believe there's several threads on the subject. That would be my next step, then the lower control arm bushings.

The more I read the more I think this is almost the normal condition, but may be dirt one one of the senders. Is there anything that can wear? Not bushings on this car. To the guy switching rotors - this is exactly what it seems like at first, but when you switched rotors did it go away for a time and then come back? What did the old rotors look like?
It's just not possible to go through rotors like that unless the pulseing is so bad as to wear them very quickly. I think it all relates to the sender causeing the pump to act badly. This is not so apparent under hard, high speed braking - right - more in just normal stop and go traffic?

Exactly. It's not noticeable at high speeds if I really got on the brakes--only when rolling up to a red light at low speeds. I get about 18,000 miles per set of front pads, so I change rotors about every other pad change. With the new rotors installed, it does go away for about 20 miles, but it always comes right back. I always buy the rotors and pads from the dealer and I don't think I would have gotten five bad sets. As for the condition of the old rotors, I never checked to see if they were warped. They looked okay-just worn where you would expect.

As an update - the sender seems the problem. I did a quick down and dirty fix. Just sprayed cleaner in the area. I can't even see how it would have gotten into the sensor, but I now only feel a slight pulse at like 5-0 mph when stopping. It was apparent from 30 or so when just lightly (only) touching the brakes. I'll just plan on doing a proper cleaning when I do pads.

I'll look into that. Thanks.

Thanks, Deanyel. The reason I know that the 93 rotors don't fit is that I couldn't even fit my existing caliper back over the 93 rotors once they were installed, not to mention the pads. The 92 calipers (at least on my car) are too narrow and will not allow 93 rotors to fit.

Crash9, I'm glad to hear you resolved the problem by spraying cleaner onto the sensor. I will give this a try. I was looking at the part and I can't see how the cleaner would get into the sensor either.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=28A16M0UW28A17104S&year=1993&make=MB&model=400-E-001&category=N&part=ABS+Speed+Sensor

Interesting info which may be applicable..
http://www.picotech.com/auto/articles/diagnosing-abs.html

Excellent article! It must have taken this guy a week to put this together. I'll plan on pulling my WSSs and checking them and the cover plates for corrosion next time I'm under there. Thanks.

Yes, it is well written in the sense that a diyer can understand. He has other systems diagnostics that are as good.

One can see from this article how a Scope makes ABS/ASR sensors reading
the way to go..anything short of seeing the signal as the computer sees it leaves too much guesswork. One can also now understand why there are so many slow speed/coming to a stop abs activation faults. Inductive sensors loose their sensitivity and output at low speeds and that is where a system fault will first pop it's head.