Understanding Crankshaft and Camshaft Position Sensors

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Purpose

Elimination of the distributor in modern vehicles created the need for an alternate means of monitoring engine RPM and crank position, determining the correct spark plug firing sequence, and controlling fuel injector and ignition timing. These functions are facilitated by the crankshaft and/or camshaft position sensor. These sensors monitor the speed of the crankshaft or camshaft, and generate a series of high- and low-voltage pulses, which represent the speed of rotation, to the vehicle’s computer. The frequency of voltage pulses increases as crankshaft/camshaft rpm increases.

Sensors of this type offer the added benefit of eliminating ignition timing variations due to wear and/or backlash in the timing chain and distributor gear, as well as eliminating the need for timing adjustments.

The vehicle's computer uses these signals primarily for spark timing or fuel injector control. On OBDII-compliant vehicles, the crankshaft position sensor also serves to detect variations in crank speed caused by ignition misfire. If these variations are significant or frequent enough, the vehicle's computer will log a Diagnostic Trouble Code (DTC) and illuminate the Check Engine light.

Location

The camshaft position sensor may be mechanically linked to camshaft (mounted in the timing cover over the camshaft gear, or on the end of the cylinder head), or located inside a housing that replaces the distributor. The crankshaft position sensor is mechanically linked to the crankshaft.

Operation

There are two primary types of crankshaft / camshaft position sensors in use today; the Hall effect sensor and the reluctance sensor. While the specific implementation of these sensors varies greatly between vehicle makes and models (i.e., the number of sensors used, the sensor types, the control functions affected by the sensor signals), operation of each type of sensor is essentially the same, as described below.

Hall Effect Sensors

Hall effect sensors monitor the speed of rotating components by "sensing" the passing of gaps (notches) or blades in a rotating "interrupter" ring (generally located on the harmonic balancer). The passing of the notches/blades across the sensor window interrupts a magnetic field generated by the sensor, causing the sensor to switch "on" and "off."

As a result, the sensor generates a series of high- and low voltage pulses based on these interruptions, which represent the speed of rotation, to the vehicle’s computer. The frequency of voltage pulses increases as crankshaft/camshaft rpm increases.

Reluctance Sensors

Reluctance sensors monitor the speed of rotating parts by using a magnet to sense the position of gaps (notches) in a rotating "reluctor" ring (located on the crankshaft or harmonic balancer). The passing of the notches across the magnet causes a change in the magnetic field, resulting in an alternating current (AC) output (a series of high- and low voltage pulses) from the sensor. The frequency of the AC signal increases as crankshaft/camshaft rpm increases.

Drivability Symptoms

Failure of the crankshaft or camshaft position sensor, or trouble in a related circuit, will generally result in an engine that will crank, but will not start or run. This is due to the loss of timing signals necessary for determining the correct spark plug firing sequence, and controlling fuel injector and ignition timing.

Testing

Procedures to test the crankshaft / camshaft position sensor vary, depending on the sensor type. BE SURE to use the correct procedure. BE SURE to refer to the vehicle's service manual to obtain proper specifications.

Hall Effect Sensors

NOTE: Hall effect sensors must be powered to generate a signal. It may be necessary to disconnect the wiring harness from the sensor and jumper the POWER and GROUND pins from the harness to the sensor in order to facilitate access to the SIGNAL pin for testing. Be sure to jumper the pins properly.

• Disconnect the vehicle's coil wire.
• Connect an analog voltmeter to the GROUND (-) and SIGNAL (+) pins of the sensor.
  
  NOTE: These are typical connector illustrations. Refer to the vehicle's service manual to verify connections.
  
• Crank the engine and observe the voltmeter. The voltmeter needle should "jump" as the sensor's magnetic field fluctuates in relation to the rotation of the "interrupter" ring.
  
• Disconnect the voltmeter from the sensor.
  
• Reconnect the vehicle's coil wire.
  

Reluctance Sensors

• Unplug the wiring harness from the sensor.
• Connect an ohmmeter to the sensor’s signal pins, and note the sensor’s resistance. Verify that the resistance is within the manufacturer's specifications.
• Disconnect the ohmmeter from the sensor.
• Connect an analog voltmeter to the sensor's signal pins. Set the voltmeter to the AC scale.
• Crank the vehicle's engine, and observe the voltmeter scale. The voltmeter should indicate a minimum of 20 millivolts.
• Disconnect the voltmeter from the sensor.
• Reconnect the wiring harness to the sensor’s connector.

Troubleshooting

Failure of the crankshaft or camshaft position sensor, or trouble in a related circuit, will generally result in an engine that will crank, but will not start or run. This is due to the loss of timing signals necessary for determining the correct spark plug firing sequence, and controlling fuel injector and ignition timing.

Most crankshaft/camshaft position sensor failures are the result of defective wiring, damaged connectors, or other electrical failures. Failures of this type will result in a loss of supply voltage to the sensor, or timing signal output to the vehicle's computer.

When troubleshooting crankshaft/camshaft position sensor problems, ALWAYS refer to the vehicle's service manual to determine the appropriate diagnostic routine in order to isolate the malfunctioning component.

Repair/Replacement

If you determine the crankshaft or camshaft position sensor is defective, it must be replaced. No repair or adjustment is possible.

When replacing a crankshaft or camshaft position sensor, keep the following in mind:

• Hall effect-type sensor must be properly aligned to the interrupter ring to generate a "clean" signal. Rubbing or interference between the sensor and the interrupter ring may cause idle problems, and can result in damage to the sensor.
• Reluctance-type sensors must be installed with the proper air gap between the sensor and the reluctor wheel. Refer to the vehicle's service manual for specifications.

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