'05 Resolution - FIX ABS problem 1988 300SE

[Ron Brooks]

So far I've had my car to three shops and no one can determine the problem. Any suggestions would be appreciated. Here are the symptoms on my '88 300SE:

At speeds above say 15 mph, the ABS always works perfectly. The problem only occurs at speeds below 10 mph and nearly always it's just prior to reaching a full stop, say below 5 mph. The ABS will just activate regardless of road conditions just prior to stop. It happens about 30% of the time. It makes no difference whether I have been driving for a while or just started it up. There's no pattern to it either. It can do it 4 stops in a row and then not hapen again for two days.

Any suggestions?

Thanks,

Ron Brooks

[Ted_Grozier]

Not sure how the ABS sensors look on this car but on others there is a toothed metal "cage" that gets pressed over the hub. This cage creates a hall-effect eddy current in a fixed electromagnetic near the cage's perimeter.

These cages are easily damaged when removing the hub for wheel-bearing work (and other types of ABS rings can get damaged by small stones, etc). If a cage loses a "tooth" it can make the wheel appear to stop at low speeds when it has not actually done so.

My first guess would be to inspect every single tooth at both front wheels. If those check out, move to the back, where you might have two sensors or a single rear sensor in the differential that's unlikely to get damaged.

Good luck and happy new year.

Ted

[Mike Richards]

Ted offers some sound advise. I've been through this same thing on a '91 300-SEL.

The problem was a loose front wheel bearing. The gap between the electronic parts Ted mentioned is apparently pretty fine.

Elevate the front of the car, then do the 6 o'clock - 12 o'clock tug on each front tire. One might be loose. If you don't feel comfortable tightening the 126 front wheel bearings, find a shop with a dial indicator and experience using this device.

[MikeTangas]

I would also pull the ABS pick-up sensor at the wheels and clean them with brake cleaner. The sensors are magnets and are probably completely covered in fine metallic dust. Might even be a good idea to pull the front rotors and really clean the ring gear, which is mounted to the back side of the rotor, the spaces between the teeth of the ring gear are probably chock full of metallic dust as well.

Pretty sure your car has one sensor at each front wheel and a single sensors at the rear differential. Mine is set up the way, yours may vary. I wouldn't start looking at the rear sensor until everything up front had been checked and cleaned.

[Tx Benz]

Sounds like a pattern failure of ft wheel speed sensor.
Unplug the underhood connector, check sensor for continuity while flexing
the sensor wire along the length in the wheel well(There are two wires at the wheel,pad wear and wheel speed, easier to check with wheel off )
Easier with an assistant to hold the meter on the plug while you flex the wire.

The sensor wires develop an open spot from constant flexing over the years.
Replace faulty sensor, problem solved.

We've done a hundred fronts, 2 rears(in diff, almost never a problem).

[donbryce]

Can the sensor wire(s) be cut at the sensor and new wire(s) soldered to the sensor lead(s) and connector plug?
If only the wire(s) are bad, why go to the $$ expense of replacing the whole sensor unit, unless there is something special about the wire?
My electrical diagram shows a 'coaxial connector' between the sensor and the control unit, and from this connector to the sensor, a pair of wires (left is 1 Gray/1 Green, right is 1 Red/1 White) with a caption 'wires are twisted'.
(I'd go out and look for myself, but the baby is in storage right now. I'm having an intermittent ABS warning light problem with low speed anti-lock similar to th OP's)

[Ron Brooks]

You replied, "would also pull the ABS pick-up sensor at the wheels and clean them with brake cleaner." Where are the ABS pick-up sensors? Do I pull the wheel off and access them? What do they/it look like? Do I just follow the wires running to the wheel that are not for the pad wear indicators?

Sorry for the silly question but I really have no idea what they look like.

[Strife]

If one were to find the break in the wire and replace it (with special high-flex wire), I would re-twist it. There is a reason it's twisted. This tends to cancel out electrical noise. Using a straight set of wires could lead to extremely weird gremlins, like your ABS going out when a trucker keys his CB radio, etc. The signal from a Hall effect sensor is very, very weak.

[moi]

it is on the back side of rotor almost in the center.one wire going in and one bolt holding in place.The sensor is like a 1" long cylinder after out.It should come out without to much effort after removing bolt.Then all you need is to clean it and reinstalled.