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Old 07-07-2019, 01:14 AM
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Join Date: Feb 2006
Location: Saratoga, CA
Posts: 46
W124 (1987 300D) ABS sensor troubleshooting procedure

Iíve had issues with my ABS system for some time. The dash light would come on at some point on almost every trip but generally at random (an intermittent issue) and stay on until the car was turned off. I also had some pulsing brakes when turning sharply to the right while breaking if I was going very slow (an indicator that I missed until completing the diagnosis below).

I went through the easy procedures - take off the sensors, clean the tips and get the metal fragments off, blow out the crud from the front rotors through the sensor hole while spinning them, clean the contacts on the sensor plugs, etc. but without the million dollar ABS tester it seemed challenging to diagnose further.

First, a few resources: 42-0700 on the W124 CD is the design and function. It has helpful diagrams, locations of parts, etc. 42-0720 has good test procedures but most are specified using the fancy analyzer toolÖbut it gives you good guidance on what components to check/test for which symptoms.

This solution is not likely original but I stumbled across it while I was troubleshooting the sensors and wanted to be sure it got documented because it is much easier than what I had found online as suggestions... No lifting the car, taking off the tires, etcÖ You donít even need to get dirty.

You'll need to locate the connector for each of the sensors:

- Driver side front sensor (L6/1) connector (L6/1x) is under the hood to the outside of the master cylinder, mounted to the firewall. It is a large torpedo looking coax connector that just pulls apart once released from itís mount. The FEMALE connector comes FROM the sensor, the MALE connector goes TO the N30 ABS control unit. (I.e., to test the sensor side, you test the female connector. To test the N30 side, you test the male connector).

- Passenger side front sensor (L6/2) connector (L6/2x) is under the hood and mounted above the strut mount (opposite side of the exhaust manifold area) - same torpedo shape and mount. Once again, the FEMALE connector comes from the sensor, the MALE connector goes to the N30 ABS control unit.

- Rear differential mounted sensor (L6) has the connector (L6x) in the passenger compartment under the back seat (lower seat, you donít need to remove the upper/back). It is mounted in the indentation on the rear passenger side where the lock vacuum/pressure pump sits. This is a standard 2 pin connector with a plastic guide pin mounted towards the door at the bottom. This one is different... The MALE connector comes from the sensor, the FEMALE connector goes to the N30 ABS control unit.


The first test is to put your voltmeter (VOM) on a very low DC volts scale and test the sensor wires TO the N30 with the ignition ON. You should get a very small voltage (which just is indicating to you that the wire is not shorted or broken). When I did this, the voltage from the rear sensor was very, very low and it did wobble at 0 DCV but I did get something to register. The fronts were a bit higher and steadier. All probably below 2/10th of a volt if I recall and the wobble may have just been due to the small voltage and my handheld connection of the test probes. Note that I would not expect the wiring harness to be the fault so even if this test didn't go perfectly I'd do the next test before deciding on any further course of action other than maybe reseating the ABS N30 main connector (with the ignition OFF).

Second, test the sensors themselves and the sensor cables. Put your voltmeter on the lowest AC volts option (mine was 200). For this test, youíll need leads that can connect to 2 medium sized alligator clips (2 or 3 inches long probably, but not too big - ideally with an insulating sheath over each) or 1 alligator clip and 1 banana plug, depending on which connection that you are testing. The two clips are for testing the rear sensor and you'll use one clip and one banana plug for the fronts. Iíve included pictures of both hookups.

To test for issues (including intermittent ones) with the sensor and cables: One at a time (unless you have 3 sets of voltmeters and cable apparatus), attach the voltmeter to the sensor wire (to the sensor side, not the N30). For the under the hood ones, pull the voltmeter under the hood between the hood and windshield and carefully close it (not damaging the wires) and set it there so you can see it while driving (see pic #4). Then, go find an empty parking lot where you can safely go 10 to 25 mph and also weave around and turn in tight circles. Watch the voltmeter read out out of the corner of your eye. Test all three sensor/cable assemblies.

To test the rear sensor, you just need long enough leads so that you can see the VOM while moving.

Hereís what I found:

Driver side front: When moving at 25 mph, I got about 2.2 VAC
Passenger side front: When moving at 25 mph, I got about 4.1 VAC
Rear differential: When moving at 25 mph I got about 2.1 VAC

It was always very linear - same relationship of MPH to VAC when I would speed up or slow down and it should never drop to zero or have unexpected voltage drops while moving at a steady speed. I donít know if it's by design that the passenger side front was reading higher but I assume so. A faulty cable wouldnít increase the voltage so there must be more "teeth" on the passenger side for the sensor to read.

With this test I found the culprit - When I turned hard right, the passenger front sensor would go to 0.0 VAC intermittently. This is what would trip the light for the rest of my journey.

I included 5 pics:
#1 is the female front connector (to the sensor) and my test clip/banana plug.
#2 is the same but connected to the VOM
#3 is the driver side connector (behind the brake booster)
#4 is the VOM sitting on the windshield so I can see it while driving around.
#5 is the connector under the seat under test (2 alligator clips)
Attached Thumbnails
W124 (1987 300D) ABS sensor troubleshooting procedure-screen-shot-2019-07-06-9.26.04-pm.jpg   W124 (1987 300D) ABS sensor troubleshooting procedure-screen-shot-2019-07-06-9.26.13-pm.jpg   W124 (1987 300D) ABS sensor troubleshooting procedure-screen-shot-2019-07-06-9.26.32-pm.jpg   W124 (1987 300D) ABS sensor troubleshooting procedure-screen-shot-2019-07-06-9.26.40-pm.jpg   W124 (1987 300D) ABS sensor troubleshooting procedure-screen-shot-2019-07-06-9.26.45-pm.jpg  


1987 300D - 375,000+ miles
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