Check knock sensor: Function, fault symptoms and testing instructions for the workshop

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

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

OE-No. Manual Vehicle Identification Universal Parts

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

E-mobility in everyday workshop activities: technology and requirements

Electromobility is changing workshop processes: find out how workshops handle high-voltage systems, new diagnostics procedures and targeted qualifications

Reading Time: 16 minutes

Basic Knowledge

Starter damage assessment

Starter faulty? Spot typical causes such as overloading or misfiring and find out how to repair damage effectively.

Reading Time: 1 minute

Basic Knowledge

Alternator damage assessment

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

Reading Time: 1 minute

Basic Knowledge

Intake-air temperature sensor - checking & faults

Discover how to check the intake air temperature sensor for defects and how to replace it efficiently.

Reading Time: 3 minutes

Basic Knowledge

TPS sensor – function & troubleshooting

Learn how to check a throttle potentiometer for defects and recognize the most common symptoms.

Reading Time: 2 minutes

Basic Knowledge

Coolant temperature sensor testing

Instructions for testing the coolant temperature sensor with a multimeter - diagnosis and repair for your workshop.

Reading Time: 3 minutes

Basic Knowledge

Lambda sensor testing and troubleshooting

Find out how to check and measure lambda sensors - with step-by-step instructions for fault diagnosis.

Reading Time: 14 minutes

Basic Knowledge

Air-mass sensor - Function & troubleshooting

Learn how to check a mass air flow sensor, detect defective sensors and replace them professionally.

Reading Time: 2 minutes

Basic Knowledge

Idle speed controller & actuator - function & check

Everything you need to know about testing and troubleshooting idle speed regulators: function, typical symptoms and repair tips for your workshop.

Reading Time: 3 minutes

Basic Knowledge

Camshaft position sensor - function & troubleshooting

Learn all about the function and testing of the camshaft sensor and possible symptoms of a defect.

Reading Time: 2 minutes