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ABS Wheel Speed Sensor Replacement
 
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Pelican Technical Article:

ABS Wheel Speed Sensor Replacement

Steve Vernon

Time:

3 hours3 hrs

Tab:

$40 to $200

Talent:

**

Tools:

6mm, 5mm Allen, blue Loctite

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

ABS wheel speed sensors

Hot Tip:

Check for fault codes before replacing

Performance Gain:

Working ABS system

Complementary Modification:

New brake pads

There are three ABS or wheel speed sensors on the W124. There is one on each of the front wheels and one located on the rear differential. The sensors are magnets that read the gaps in a metal toothed ring on the wheel hubs and rear differential to monitor the speed of each wheel and the rear dif. The sensors are expensive and just one of the components in the ABS system so keep in mind that other problems can illuminate the ABS warning light. Be sure to scan and read fault codes before you start replacing expensive parts.

While you may be able to perform this work while lying under the car with the wheels on, if you are just starting to work on your own car or like a little more room to work I recommend doing this job with the car safely in the air. Begin by jacking up and supporting the car. Please see our article on jacking up your Mercedes. Remove the front and rear wheels.

Front Sensor: The front wheel speed sensors are located on the forward edge of the steering knuckle (red arrow).
Figure 1

Front Sensor: The front wheel speed sensors are located on the forward edge of the steering knuckle (red arrow). You can see in this picture how the magnet in the sensor has attracted metal filings from wear of components around the area.

Front Sensor: Remove the sensor and its mounting plate by unscrewing the two 6mm Allen head bolts (red arrows).
Figure 2

Front Sensor: Remove the sensor and its mounting plate by unscrewing the two 6mm Allen head bolts (red arrows).

Front Sensor: The bolts came with blue Loctite on them and can be difficult to remove.
Figure 3

Front Sensor: The bolts came with blue Loctite on them and can be difficult to remove. If you are reusing the bolts make sure to clean them up with a wire brush or wheel and put a small amount of Loctite on when reinstalling.

Front Sensor: With the sensor removed from the knuckle you can see all the metal filings, dirt and debris on the end of the sensor (red arrow).
Figure 4

Front Sensor: With the sensor removed from the knuckle you can see all the metal filings, dirt and debris on the end of the sensor (red arrow). Sometimes giving the sensor a good cleaning will cause it to function properly again.

Front Sensor: Disconnect the cable where it attaches to the strut and chassis (red arrows).
Figure 5

Front Sensor: Disconnect the cable where it attaches to the strut and chassis (red arrows). Remove the rubber pass through grommet where the wiring enters the engine compartment (yellow arrow).

Front Sensor: The sensors have two connections.
Figure 6

Front Sensor: The sensors have two connections. The driver side sensor connections are located by the master cylinder and the fuse box (red arrows). Disconnect the wiring and pass it down and out through the grommet. Installation is the reverse of removal.

Front Sensor: On the passenger side they are located on the firewall between the battery and the overflow reservoir (red arrow).
Figure 7

Front Sensor: On the passenger side they are located on the firewall between the battery and the overflow reservoir (red arrow). Disconnect the wiring and pass it down and out through the grommet. Installation is the reverse of removal.

Rear Sensor: The rear sensor is located on the left side of the differential.
Figure 8

Rear Sensor: The rear sensor is located on the left side of the differential. Remove the 5mm Allen that holds the sensor in place as well as the 5 mm Allen holding the wiring bracket to the diff (red arrows). Make sure to clean around the sensor opening and the Allen bolt.

Rear Sensor - Make sure to completely seat the Allen key in the bolt (red arrow); you do not want to strip this bolt.
Figure 9

Rear Sensor: Make sure to completely seat the Allen key in the bolt (red arrow); you do not want to strip this bolt. It is also a good idea to have a drip pan below the differential as some fluid from the diff may drip out. Follow the cable up to where it enters the interior of the car and disconnect the wiring and install the new sensor. Depending on whether you have a sedan or wagon the sensor will be located under the seats or the rear compartment. Installation is the reverse of removal.



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Comments and Suggestions:
Bill Ben Comments: I have a question about the ABS system in my Mercedes. I have a 1995 E300 Diesel. The ABS light has come on. I was hoping to use the Obd1 diagnostic system to troubleshoot this, however, I am unable to find the ABS fault codes for my car or even which number diagnostic pin to use. I have the 16 pin Obd1. Can you help here ?
October 5, 2016
  Followup from the Pelican Staff: You will need to read the fault codes. Benz has a special tester and test kit for testing. I would see if a local shop can read the codes for you. - Nick at Pelican Parts  
Ather Comments: But if there's no fault massage and it's not work and I have one of the sensor damage the abs will not work?!
July 8, 2016
  Followup from the Pelican Staff: ABS will not work with a damaged sensor, correct. - Nick at Pelican Parts  
nik Comments: Wait but so where does the sensor come out for the e280 wagon? Under a seat or where?
November 12, 2015
  Followup from the Pelican Staff: Which sensor? What year vehicle? - Nick at Pelican Parts  
nik Comments: E280 wagon from 93 : you guys don't even have the diff speed sensor for sale, apparently it's a rare part
November 11, 2015
  Followup from the Pelican Staff: I am sure we can hunt one down for you. I’m not the best with part numbers.

Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need. - Nick at Pelican Parts
 
nik Comments: Depending on whether you have a sedan or wagon the sensor will be located under the seats or the rear compartment

which is it for which version??
November 6, 2015
  Followup from the Pelican Staff: I would need to know what vehicle you are working on to check. I may have that info. - Nick at Pelican Parts  

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