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Purpose

This article discusses the purpose and function of the Camshaft Position Sensor (CMP), and describes some common functional issues and their effects on sensor operation.

Introduction

The Camshaft Position Sensor (CMP) is an electronic component used in an internal combustion engine to monitor the rotational speed (RPM) and position of the camshaft. The Engine Control Module (ECM) uses this information for spark timing, fuel injector control, and other engine parameters.

The sensor is most commonly located near the primary pulley, flywheel, transmission bell housing, engine cylinder block, or the camshaft itself.

Four different types of CMP are currently in use:

• Inductive
• Hall effect
• Magnetoresistive
• Optical

The inductive sensor has the simplest design and is the most commonly used type of sensor. It is a passive device comprised of a body, collar, magnet, and coil, and features an O-ring and collar swaging to shield the coil from pollutants.

Both the Hall effect sensor and magnetoresistive sensor are small-sized sensors that integrate the detection element and processing circuit on a single chip. These sensor types offer the added ability to recognize static (non-varying) magnetic fields.

Of the four sensor types, the optical sensor delivers the most accurate edge detection.

The two most commonly used CMP sensor types, the inductive sensor and the Hall effect sensor, are discussed in the following paragraphs.

Inductive Sensor

Operation

Inductive sensors monitor the speed of rotating parts by using a magnet to sense the position of gaps (notches) in a rotating trigger wheel. The pin pole core of the sensor is comprised of a solenoid winding that encloses soft iron. The solenoid winding is connected to a permanent magnet. The pin pole core is positioned directly opposite the trigger wheel. An air gap separates the sensor from the trigger wheel.

Through the pin pole core, the permanent magnet generates a magnetic field that reaches the trigger wheel. As the trigger wheel rotates, the passing of the notches across the pin pole core cause changes in the magnetic field, The magnetic flux is stronger when the sensor is aligned with a tooth on the trigger wheel, and weaker when it is aligned with a gap. These changes in the magnetic flux produce a sinusoidal AC voltage in the solenoid winding. The frequency of the AC signal increases as camshaft RPM increases.

Schematic diagram of CMP Sensor

Output Waveform

Output voltage of CMP Sensor

Benefits

• High reliability
• Durability

Hall Effect Sensor

Operation

Hall sensor operation is based on the Hall effect. The sensor monitors the speed of rotating components by "sensing" the passing of gaps (notches) or blades in a rotating trigger wheel. The Hall Effect Integrated Circuit (IC) is positioned between a permanent magnet and the ferromagnetic trigger wheel. The IC is separated from the trigger wheel by an air gap. The trigger wheel rotates with the camshaft.

As a trigger wheel tooth crosses the sensor axis, it alters the strength of the magnetic field that is perpendicular to the Hall Effect IC. The passing of the teeth/notches across the sensor window interrupts a magnetic field generated by the sensor, causing the sensor to switch "on" and "off." This generates a voltage signal (in millivolts) that remains unaffected by the relative speed of the sensor and the trigger wheel. The electronic components integrated within the sensor process this signal and transmit a rectangular pulse signal output (high/low) to the ECM. The frequency of the signal increases as camshaft RPM increases.

Schematic diagram of Hall Sensor

Output waveform

Output waveform of Hall Sensor

Benefits

• Smaller size
• Higher reliability than inductive sensors
• Higher position detection accuracy

Common CKP Issues and Effects

The ECM utilizes data from the CPM to determine the ignition timing (spark advance) and perform fuel injector control. When there is a malfunction in the sensor or an open or short circuit in the wiring, various operational issues can result due to the loss of timing signals necessary for determining the correct spark plug firing sequence and controlling fuel injector and ignition timing. These issues include:

• Engine stalling
• Poor acceleration
• Shifting problems
• Poor fuel economy

Under worst case conditions, failure of the CMP, or trouble in a related circuit, may result in an engine that will crank, but will not start or run.

In addition mechanical or electrical failure, CPM faults can also be attributed to scratches on the magnetic surface sensor, the accumulation of foreign material on the sensor, exposure to high temperatures, and damaged wiring.

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