Skylark L4-138 2.3L DOHC VIN D MFI (1988)
Crankshaft Position Sensor: Description and Operation
The crankshaft sensor is a magnetic switch that is used to provide the ignition module and the Electronic
Control Module (ECM) with the engine RPM and/or crankshaft position information much the same way as the High Energy Ignition (HEI) pick up coil.
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.
The magnetic sensor is made up of a permanent magnet with a coil of wire wrapped around it. The sensor is positioned .050" +/- .020" from the slotted
position of the crankshaft (refer to A charts or C charts for exact measurement). 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. 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. 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.
