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Old 01-29-2008, 09:29 PM
Jeremy5848's Avatar
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Location: Sonoma Wine Country
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W124 ABS -- learning the hard way as usual (DIY w/pix)

My '87 300D has had an occasional problem with its ABS since I've had it. When pulling up to a stop, the brake pedal sometimes shudders a little as if the wheel is slipping (which it isn't). A mechanic said that the right front wheel sensor tested bad (don't know what kind of a test he performed).

Last Sunday (1-27-2008), I took the opportunity to get most of the ABS parts (brain, hydraulic unit, both front wheel sensors) from an '87 300E in the Windsor Pick and Pull (about $100 for all).

Today I installed both wheel sensors. I learned the following, which I pass on to you so that you don't make the same mistake that I made (these are MINE, you go make your own mistakes!).

There are three items that pass through a hole in the inner fender -- the ABS sensor cable, the brake pad wear sensor cable, and the metal hydraulic line to the brake itself. Sealing the hole in the inner fender is a single large rubber grommet. The grommet has a cut to allow it to be slipped over the brake line but the two rubber cables are built as part of the grommet -- the only way to take them apart is to cut either the wires or the grommet.

When I was at the P&P, I said to myself, "I don't need the brake pad sensor wire," so I cut it and brought home the ABS sensor with the attached grommet. I planned to make a new cut in the grommet and fit it over my brake pad sensor cable (and that's what I ended up doing).

My mistake was in not taking both wires as part of the grommet. The brake pad wear sensor cable is easy to remove, so I should have removed it rather than cutting it. When I did the installation, I discovered that the rubber covering of the left wear sensor was cracked worse than the ABS wire -- I should've replaced both at once. Now I've gotta find another W124. (The complicated assembly of wires, plugs, sensors and grommets is what causes the factory price for this thing to be $170.)

Others here have discussed measuring the sensor with an ohmmeter -- the DC resistance is about 1000 Ohms. I also discovered that, if you connect the sensor cable to your VOM and use the AC voltage scale, you can spin the wheel by hand and generate between 1 and 3 Volts. No voltage means a bad sensor. The exact voltage is a function of how close the sensor is to the metal teeth on the inside of the wheel cavity -- it may not be critical, as long as there's enough for the 'brain' to measure. (While you're in there, make sure the cavity and 'teeth' are clean.)

Results -- I've only driven the car a few miles with the new-to-me sensors, and the brake pedal shudder has not happened. I will want some more miles before I'm ready to say it's gone for good. If not, I still have the 'brain' and hydraulic unit to swap.

Jeremy

Photo captions --
1. Wheel sensor before removal
2. Wheel sensor removed, toothed wheel can be seen through hole
3. Ordinary banana plugs work great to connect the wheel sensor plug to your VOM. One goes in the center hole, the other slides into the outside ring, both fit snugly so you don't have to hold them in place.
4. The wheel sensors are attached with Allen head cap screws. Use blue Loc-Tite and a 6mm Allen wrench.

Attached Thumbnails
W124 ABS -- learning the hard way as usual (DIY w/pix)-img_2451.jpg   W124 ABS -- learning the hard way as usual (DIY w/pix)-img_2454.jpg   W124 ABS -- learning the hard way as usual (DIY w/pix)-img_2447.jpg   W124 ABS -- learning the hard way as usual (DIY w/pix)-img_2456.jpg  
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Old 01-29-2008, 10:42 PM
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Join Date: Dec 2004
Location: Southern Illinois
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That some good information, Thank you for sharing....gene2

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