Jeep Wrangler/YJ 1987-1995 Repair Guide

Engine Speed Sensor (Crankshaft Position Sensor)

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The engine speed sensor is attached to the flywheel/drive plate housing. This sensor detects the flywheel/drive plate teeth as they pass during engine operation and provides engine speed and crankshaft angle information to the ECU. The flywheel/drive plate has a large trigger tooth and notch located every 90 degrees and 12 smaller teeth before each Top Dead Center (TDC) position. When a small tooth and notch pass the magnet core in the sensor, the concentration and subsequent collapse of the magnetic flux induces a small voltage spike into the sensor pickup coil winding. The higher voltage spike indicates to the ECU that a piston will be at TDC position 12 teeth later. The ignition timing for the cylinder is either advanced or retarded as necessary by the ECU, according to the sensor inputs.

 
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