Car knock sensor — function and troubleshooting

Car knock sensor

10037926a_AM0_white_inclined

The knock sensor monitors the combustion process in the engine. Its signal helps the engine control to prevent knocking combustion and therefore protect the motor/engine control. On this page we will provide you with various information, including how a defective knock sensor becomes noticeable, the causes that may lead to its failure, and how it can be tested in the workshop.

The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for suitably qualified personnel only.

Car knock sensor function

Car knock sensor function — Knock sensor

The knock sensor is located on the outside of the engine block. It is intended to record knocking noise in all engine operating states in order to prevent engine damage.

The knock sensor "listens out for" the structure-borne vibrations from the engine block and converts these into electrical voltage signals. The signals are filtered and evaluated in the control unit. The knocking signal is assigned to respective cylinder. If knocking occurs, the ignition signal for the respective cylinder is adjusted in the "late" direction until knocking combustion no longer occurs.

Check the knock sensor

Check the knock sensor: Example OK

TROUBLESHOOTING:

Read out the stored fault code ✓
Check the sensor for correct fit and tightening torque ✓
Check the electrical connections of the sensor wiring, the plug, and the sensor for correct connection, breaks, and corrosion ✓
Check the ignition point (older vehicles) ✓

Testing with the multimeter

Check the wiring to the control unit by checking continuity and short circuit to frame for every wire to the control unit plug.

1

Connect the ohmmeter

Connect the ohmmeter between the knock sensor connector and the removed control unit connector. Setpoint: <1 Ohm (circuit diagram required for the pin assignment of the control unit).

Check knock sensor: Setpoint — Setpoint: <1 Ohm

2

Check the pin against ground using an ohmmeter

Check the respective pin at the wiring harness connector against ground using an ohmmeter and with the control unit connector removed. Setpoint: at least 30 MOhm.

Attention:

A connection pin can act as shielding and thus have continuity to ground.

Check with the oscilloscope when the engine is warm

1

Connect the test probes of the oscilloscope

Connect the test probes of the oscilloscope between the control unit pin for the knock sensor and ground.

2

Briefly open the throttle valve

The oscillogram must produce a signal with considerably enlarged amplitude.

3

Signal not clear

If the signal is not clear, tap gently against the engine block near the sensor.

4

Knocking is not detected

If the knocking is not detected, this indicates a defective sensor or circuit.

Observe the tightening torque during assembly. Do not use spring washers or washers.

Not helpful at all

5

4

3

2

1

Very helpful

Spare Parts Finder

Manualy Oe-No. Universal Parts

Top manufacturers

Other manufacturers

Spare Parts Finder

Simple vehicle identification Also determine spare parts with OE numbers Detailed product information Find wholesalers near you

You might also be interested in

Basic Knowledge

NOx-Sensor - Mercedes Benz

If a fault is diagnosed in the NOx sensor when troubleshooting an exhaust gas purification system, this can present a small challenge for a workshop.

Reading time: 7 minute

Basic Knowledge

Checking an alternator regulator

Function and design of the overrunning alternator pulley explained - including installation instructions for the overrunning alternator pulley on the alternator.

Reading time: 12 minute

Basic Knowledge

Media pump - design, function and troubleshooting

Find out more about the function and system integration of the medium pump as well as testing and repair options.

Reading time: 7 minute

Basic Knowledge

Function and assembly of the alternator freewheel clutch explained

Function and design of the overrunning alternator pulley explained - including installation instructions for the overrunning alternator pulley on the alternator.

Reading time: 2 minutes

Basic Knowledge

Alternator damage assessment

How to diagnose damage to the alternator: from corrosion and short circuits to bearing damage.

Reading time: 1 minute

Basic Knowledge

Starter damage assessment

Faulty starter motor? Discover typical causes such as overloading or misfiring and how to repair damage effectively.

Reading time: 1 minute

Basic Knowledge

Check multi-function regulator

Find out how to check the multifunction regulator, measure the generator voltage and ensure that the regulator is functioning correctly.

Reading time: 4 minute

Basic Knowledge

Checking the alternator

Detailed solutions for problems with the alternator - from testing to troubleshooting, everything for your vehicle workshop.

Reading time: 8 minute

Basic Knowledge

Checking the starter - Causes of failure, troubleshooting

Everything you need to know about typical starter motor faults and how to rectify them - with instructions for testing and repair.

Reading time: 9 minute

Basic Knowledge

Service work on the charging system

Step-by-step instructions for servicing the charging system, including visual inspection and signal testing on the controller.

Reading time: 6 minute