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Pelican Technical Article:

Knock Sensors Replacement

Steve Vernon

Time:

1- 3 hours

Tab:

$50 to $100

Talent:

***

Tools:

17mm, 13mm, 10mm socket, extensions, universal joint

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

New knock sensors

Hot Tip:

Try from under the car first

Performance Gain:

Piece of mind on engine knock

Complementary Modification:

New injectors

There are two knock sensors on the M104 and they play an important part of the engine management system. The sensors detect knock or ping in the motor which is pre-detonation and can cause serious and expensive damage to the motor. If knock is detected the sensor will tell the management system of the engine to severely retard the ignition. Beside an error code you will notice a lack of power and throttle response.

Both sensors are located on the left side of the motor and are easy to replace once you get access to them. The front sensor is very easy to access from below and a little more work from above. The rear sensor is more work whether from above or below. Most people change out the sensors by accessing them from below but we will show you both ways.

Begin by locating the ground or negative cable on the battery (yellow arrow) and use a 13mm wrench (red arrow) to disconnect it.
Figure 1

Begin by locating the ground or negative cable on the battery (yellow arrow) and use a 13mm wrench (red arrow) to disconnect it. Place the cable in a manner that it cannot accidently touch the battery post while you are working; this is especially true if you are accessing the sensor from below and going to remove the starter.

Above Method- You can see and access the front knock sensor from between the water pump/thermostat and the intake manifold (red arrow).
Figure 2

Above Method: You can see and access the front knock sensor from between the water pump/thermostat and the intake manifold (red arrow).

Above Method- Reach in and disconnect the wiring harness (red arrow) from the sensor (yellow arrow).
Figure 3

Above Method: Reach in and disconnect the wiring harness (red arrow) from the sensor (yellow arrow). If your hands are large it may be easier once you have the sensor off the block.

Above Method- Use a 13mm socket and universal joint and extensions to remove the sensor (red arrow).
Figure 4

Above Method: Use a 13mm socket and universal joint and extensions to remove the sensor (red arrow). Installation is the reverse of removal.

Above Method- If you want to replace the rear sensor from above you will need to remove the throttle body (red arrow).
Figure 5

Above Method: If you want to replace the rear sensor from above you will need to remove the throttle body (red arrow). Please see our article on throttle body removal for further assistance.

Above Method- With the throttle body off you can easily access the 13mm nut and the sensor (red arrow).
Figure 6

Above Method: With the throttle body off you can easily access the 13mm nut and the sensor (red arrow). Remove the old sensor and install a new one.

Below Method- The front sensor (red arrow) can be accessed by reaching up just in front of the left engine mount.
Figure 7

Below Method: The front sensor (red arrow) can be accessed by reaching up just in front of the left engine mount. Separate the wiring connection, remove the old sensor with a 13mm socket and install the new sensor.

Below Method- To remove the rear sensor you will need to remove the starter motor (red arrow).
Figure 8

Below Method: To remove the rear sensor you will need to remove the starter motor (red arrow). Please see our article on replacing your starter for additional assistance.

Below Method- With the starter motor removed you can get easy access to the sensor (red arrow).
Figure 9

Below Method: With the starter motor removed you can get easy access to the sensor (red arrow). Reach up, separate the wiring connection, use a 13mm socket and remove the old sensor. Installation is the reverse of removal.

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Comments and Suggestions:
hrg207 Comments: OK so I unbolted one of the sensors to get a better look at the connector, the wire hoop needs to be pressed in and held so the connector can be pulled off.
October 7, 2014
  Followup from the Pelican Staff: Thanks for the follow up. We appreciate it.
- Nick at Pelican Parts
 
hrg207 Comments: Hello, I need to remove the knock sensor connectors as part of a wiring loom overhaul. I've followed your excellent guide but am struggling to remove the connector, access is still a bit restricted and I'm afraid to damage it. I assume the wire hoop plays a part in removal? Any help would be appreciated. Thanks.
October 6, 2014
  Followup from the Pelican Staff: Use a small pick to pull the wire off the connector. If you can't access it, remove the sensor, then disconnect it. - Nick at Pelican Parts  

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