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Replacing Your Mercedes-Benz Top Dead Center Sensor
 
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Pelican Technical Article:

Replacing Your Mercedes-Benz Top Dead Center Sensor

Steve Vernon

Time:

2 hours2 hrs

Tab:

$130

Talent:

**

Tools:

8mm socket and extension, Philips-head screwdriver, zip ties

Applicable Models:

Mercedes-Benz W126 (1981-91)

Parts Required:

New TDC sensor

Hot Tip:

Always disconnect the battery

Performance Gain:

Better control of fuel injection and engine timing

Complementary Modification:

Change your motor oil

Your W126 TDC (Top Dead Center) Sensor monitors your engine's ECU when the motor is at top dead center. This helps facilitate the proper fuel injection and spark function. If the sensor is not working then that car can have a multitude of problems, from rough running all the way to not starting at all. The sensor is basically a small magnet that generates a signal from a small attachment on the vibration dampener. Each time the vibration dampener, (which is attached to the crankshaft) passes TDC the sensor lets the ECU know and that helps set the timing of the spark.

The sensor is located under the oil pump on the front of the engine, and while it is not difficult to change the power steering pump must come off and be moved to the side. The front of the engine can get quite dirty, especially if you have a leaking power steering pump so have some rags on hand. You do not need to remove any of the drive belts as you are only going to loosen the pump enough to slip the belts off and then move the pump out of your way.

There are three 13mm nuts holding the power steering pump on. Loosen the two 13mm nuts attaching the pump to the front plate. With these loosened but not removed, loosen the 13mm nut on the lower driver side of the pump and then use a 13mm wrench from the back of that bolt to turn the toothed bolt in its gear. It is the same system as the other pumps. Turn the rear bolt counter clockwise to loosen the pump. You can now remove the belts and finish removing the 13mm bolts. Keep the pump upright and secure it off to the side.

With the power steering pump moved to the side you can see the TDC harness running down between the oil pump and A/C bracket. It is secured to its mount on the front of the engine just off to the side of the vibration dampener by an 8mm bolt. Use an 8mm socket on an extension and remove the bolt. If you are just replacing the sensor and there is nothing wrong with your timing replace the sensor in the same position. Setting the timing requires special tools that will not be covered in this article.

Remove the sensor and follow the harness through the wiring looms, you will need to clip off a few zip ties so be ready to replace them. Follow the wiring until it arrives at the diagnostic plug on the left side of the engine. Remove the plugs two retaining screws with a Philips-head screw driver.

With the plug out of its mount you can replace the TDC harness from the bottom.

Installation is the reverse of removal.

PLEASE NOTE: The sensor in the pics below is only used for backwards compatibility with scan tools or testers. THERE IS A CYLINDER ID SENSOR ON THE TRANSMISSION SIDE--IT IS THE ONE THAT SENDS THE INFO TO THE DME. PLEASE MAKE SURE YOU REPLACE THAT ONE!!

Loosen the two 13mm nuts (red arrows) attaching the pump to the front plate.
Figure 1

Loosen the two 13mm nuts (red arrows) attaching the pump to the front plate.

On the lower driver side of the pump use a 13mm wrench to loosen the front bolt (red arrow), and then working from the back turn the toothed bolt in its gear (yellow arrow).
Figure 2

On the lower driver side of the pump use a 13mm wrench to loosen the front bolt (red arrow), and then working from the back turn the toothed bolt in its gear (yellow arrow). It is the same system as the other pumps. Turn the rear bolt counter clockwise to loosen the pump.

With the power steering pump moved (yellow arrow) to the side you can see the TDC harness (green arrow) running down between the oil pump and A/C bracket to the sensor (red arrow).
Figure 3

With the power steering pump moved (yellow arrow) to the side you can see the TDC harness (green arrow) running down between the oil pump and A/C bracket to the sensor (red arrow).

The sensor (red arrow) detects when the engine is a top dead center by a small metal nodule (green arrow) mounted on the vibration dampener passes it.
Figure 4

The sensor (red arrow) detects when the engine is a top dead center by a small metal nodule (green arrow) mounted on the vibration dampener passes it.

It is secured to its mount on the front of the engine just off to the side of the vibration dampener by an 8mm bolt (red arrow).
Figure 5

It is secured to its mount on the front of the engine just off to the side of the vibration dampener by an 8mm bolt (red arrow).

Use an 8mm socket on an extension and remove the bolt.
Figure 6

Use an 8mm socket on an extension and remove the bolt.

This photo illustrates the dampener end of the sensor (red arrow).
Figure 7

This photo illustrates the dampener end of the sensor (red arrow).

Remove the two Philips head screws (red arrows) holding the diagnostic port in place.
Figure 8

Remove the two Philips head screws (red arrows) holding the diagnostic port in place. The green arrow shows the wiring harness for the TDC sensor.

With the plug out of its mount you can replace the TDC harness (red arrow) from the bottom.
Figure 9

With the plug out of its mount you can replace the TDC harness (red arrow) from the bottom.

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Comments and Suggestions:
jack Comments: Just a fyi this is not the correct sensor that controls firing sequence and tdc info back to the ECU. In the application of M116 and M117 all this sensor is used for is TDC diagnostics when connected to a MB factory timing comparator device.

The correct TDC sensor used for ignition timing and ecu feedback is located on the top of the engine at the back above the flywheel. That sensor plugs into the EZL and provides the actual firing feedback.

Replacement of the sensor mentioned above does absolutely nothing to the normal operation of the vehicle and is used for diagnostic purposes only. You can clearly see it doesn't actually plug into anything, and just snaps into the bottom of the diag port on the driver side fender, so that the pins are accessible from the top
November 3, 2016
  Followup from the Pelican Staff: Thanks for the additional info. We appreciate it. I will have the article updated.


Yes, as noted in my 2014 reply to someone:

The sensor in this article is only used for backwards compatibility with scan tools or testers. There is a cylinder id sensor on the transmission side. it is the one that sends info the the DME.

- Nick at Pelican Parts
 
350sdlvr Comments: Does anyone have info on the tachometer sensor or sender?
October 7, 2016
  Followup from the Pelican Staff: I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
 
thad Comments: Does this sensor have a name other than cylinder id sensor, or are you referring to the crankshaft position sensor.

thanks
October 20, 2014
  Followup from the Pelican Staff: No, this sensor is the TDC sensor. - Nick at Pelican Parts  
chad Comments: thad im with you on your question I need an answer
October 17, 2014
  Followup from the Pelican Staff: The sensor in this article is only used for backwards compatibility with scan tools or testers. There is a cylinder id sensor on the transmission side. it is the one that sends info the the DME. - Nick at Pelican Parts  
thad Comments: Could you explain exactly how this magnet lets the ecu know each time the vibration dampener passes tdc, because I did not see where it is connected to the ecu to pass this information to it, or does it not need to be connected to the ecu to accomplish this.

thanks
October 10, 2014
  Followup from the Pelican Staff: The sensor in this article is only used for backwards compatibility with scan tools or testers. There is a cylinder id sensor on the transmission side. it is the one that sends info the the DME. - Nick at Pelican Parts  
kelvin benz Comments: mercedes crank position sensor
February 20, 2014
  Followup from the Pelican Staff: Thanks - Nick at Pelican Parts  

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