How do crankshaft speed sensors work?

Written by edmund gary
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How do crankshaft speed sensors work?
Crankshaft speed sensors in modern engines help maintain optimal performance. (engine image by goce risteski from Fotolia.com)

Crankshaft speed sensors, otherwise known as position speed sensors, are electronic control devices which are used in automotive internal combustion engines. This component sends crucial information to the engine control module (ECM). Crankshaft speed sensors are used to measure the speed of the crankshaft rotation. The information from a crankshaft speed sensor is used to control the engine management and ignition timing systems. These devices appeared on engines when distributors were eliminated in the early 1990s.

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Engine Management Control

Basic timing information is sent from the crankshaft speed sensor to the ECM. The engine RPM (revolutions per minute), timing and firing order are determined by the information which is received from the crankshaft timing sensor. The information tells the ECM how fast the engine is running so the ignition can be advanced or retarded accordingly.

Spark Plug Timing

The ignition pick up and trigger wheel of the distributor have been eliminated from new engines. The crankshaft speed sensor serves the same purpose as the ignition pick up and trigger wheel, which is to adjust the spark timing of the spark plugs. The ignition coil is connected directly to the spark plugs.

Crankshaft Position (NE) Signal

The signal coming from the crankshaft speed sensor is known as the crankshaft position or NE signal. The NE signal is used by the ECM to determine crankshaft position, engine RPM and engine misfire. Combined with the G signal from the camshaft position sensor, the cylinder under compression is identified. The ECM uses this information to determine the firing order. The NE signal has a periodic gap, which represents a missing tooth in the timing rotor of the sensor. The gap is used by the ECM as a reference point for the crankshaft position.

Magnetic and Hall Effect Crankshaft Speed Sensors

The magnetic type of crankshaft speed sensor uses a magnet to generate an alternating current signal. The magnet senses the notches in the crankshaft, and the sensation changes the magnetic field. The frequency of the pulses is calculated by the computer as rotations of the crankshaft. The Hall Effect type of crankshaft speed sensor uses blades, which are fixed on the crankshaft. The blades disrupt the magnetic field in the Hall Effect window. The sensor switches on and off. This produces a signal which the ECM reads to determine the crankshaft speed and position.

Crankshaft Speed Sensor Problems

The engine will not start or will stall after a few minutes of driving if the crankshaft speed sensor fails. These faults can be traced to the wiring harness. If the vehicle is equipped with a Hall Effect sensor, a poor ground, a disruption in the sensor supply voltage or a lack of return circuits can result in the loss of a signal to the ECM.

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