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Knock Sensor Testing and Replacement
 
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Pelican Technical Article:

Knock Sensor Testing and Replacement

Nick Czerula

Time:

4 hours4 hrs

Tab:

$300

Talent:

*****

Tools:

Set of sockets (10mm, E12, E10), flathead screwdriver, torque wrench

Applicable Models:

Mercedes-Benz E320 (2003-05)

Parts Required:

Knock sensors, knock sensor fasteners, intake manifold gasket

Hot Tip:

Work with a cool engine

Performance Gain:

Remedy knock sensor fault codes

Complementary Modification:

Replace breather hoses

Engine knock or detonation in spark-ignition internal combustion engines occurs when combustion of the air and fuel in the cylinder starts off correctly in response to ignition by the spark plug. But one or more pockets of air or explode outside the area of the normal combustion front, causing a shock wave and a collision, resulting in the knocking sound. The shock wave creates the characteristic metallic pinging sound, and cylinder pressure increases dramatically. Effects of engine knocking range from non-damaging to completely destructive.

Pre-ignition in an engine is the event wherein the air / fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is caused by an ignition source other than the spark plug, such as hot spots in the combustion chamber, a spark plug that runs too hot, or carbon deposits in the combustion chamber.

W211 6-cylinder models utilize two sensors to monitor engine knock. The knock sensors are mounted to the engine block, beneath the intake manifold. The knock sensors usually fail setting a check engine light and a knock sensor fault code. When a knock sensor fails, the ignition timing on your engine may be held in a retarded position until the fault is remedied, therefore reducing engine power and the chance of detonation. The sensors are replaced in pairs. You have to replace the fasteners also. In this tech article I will go over how to test and replace the engine knock sensors on your Mercedes-Benz W211.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Testing knock sensors: To test the sensors, you will have to access the DME electrical connector. See our tech article on DME replacing. Working at the electrical connector, you will backprobe the connector with the ignition ON. There are four wires at the connector, two for each sensor. One is a ground, the other is a reference voltage used for circuit integrity. To test the bank 1 knock sensor, backprobe Pin 4 at the DME, violet wire. To test the bank 2 knock sensor, backprobe pin 17 at the DME, yellow wire. Double-check the wiring for your vehicle. This is what it was for my subject vehicle. Here's the quick test. Use a DVOM. Connect the black lead to battery negative and then connect the positive lead to the sensor wires. One wire should read close to 0 volts, as this is the sensor ground. The reference voltage wire should read around 2.5 volts, for a good sensor. A bad sensor will either short the reference voltage to ground, or not pull it down to 2.5, therefore giving a reading of about 5 volts.

This photo shows the location of both knock sensors.
Figure 1

This photo shows the location of both knock sensors. The green arrow points to the right side or Bank 1 knock sensor. The red arrow points to the left side or bank 2 knock sensor. Disconnect the negative (-) battery cable. See our tech article on battery connection notes. Remove engine covers. See our tech article on removing engine covers. Remove the intake manifold. See our tech article on intake manifold removing.

Now it's time to replace the knock sensors.
Figure 2

Now it's time to replace the knock sensors. First disconnect the electrical connector by squeezing the wire retainer and pulling the connector straight off the sensor.

Working at the knock sensors, remove the E10 knock sensor fastener.
Figure 3

Working at the knock sensors, remove the E10 knock sensor fastener.

Then remove the knock sensor from your engine.
Figure 4

Then remove the knock sensor from your engine. Be sure the mounting surface is clean (red arrow). Then install the new knock sensors finger tight. Then torque the knock sensor fasteners, 20 Nm. Reassemble the engine and check the DME for fault codes.

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Page last updated: Mon 3/27/2017 02:46:00 AM