Crankshaft Position Sensor: Description and Operation

PURPOSE
The crankshaft sensor is a magnetic switch that provides a signal through the Direct Ignition System (DIS) module which the Electronic Control Module (ECM) uses as reference to calculate RPM and crankshaft position.

The style of crankshaft sensor used and its function is determined by the engine application. Two types of sensors are presently being used the magnetic type and the Hall Effect type.

OPERATION
The magnetic sensor is positioned (0.050 in ± 0.020 in) from the slotted position of the crankshaft. This slotted portion is known as a reluctor. As the reluctor rotates as part of the crankshaft, the slots change the magnetic field of the sensor, creating an induced voltage pulse. The magnetic lines of flux are changed by the rotation of the slots past the sensor tip. The magnetic flux increases then quickly decreases as the slot passes the sensor. These fluctuations can be read with a DIGITAL voltmeter set on the VOLTS / A.C. scale.
The voltage amplitude varies with engine speed and application, ranging from about 0.10 millivolts A.C. to 100 millivolts A.C. depending on cranking speed and engine application. These voltages must rise above a certain positive threshold and then move through zero volts on a negative transition. The module triggers off the zero-voltage crossing of the wave from.
Correct sensor gap is critical for correct operation. Too small of a gap could result in high speed abnormalities due to voltage output exceeding the module input handling ability. Too large of a gap could cause cranking problems because the sensor output is not enough to trigger the module.

CONSTRUCTION
The magnetic sensor is made up of a permanent magnet with a coil of wire wrapped around it.

For additional testing information refer to Computers and Controls / System Diagnosis / Procedures / Diagnostic Charts / Chart A-3. Testing and Inspection