Crankshaft Position Sensor: Description and Operation

OPTICAL DISTRIBUTOR SYSTEM (3.0L) (Fig. 4)

The SBEC receives its engine speed and crankshaft position signals from an optical distributor. The signals are used to control fuel injection, ignition timing, and idle speed. The timing member in the distributor is a thin disk, driven at half engine speed from the forward (left) bank camshaft. The disk has 2 sets of slots in it. The outer, "high data rate," set of slots occurs at intervals of 2 degrees of crankshaft rotation. It is used for ignition timing at engine speeds up to 1200 rpm to increase timing accuracy. During cranking and idle, engine speed changes with the firing pulses of each cylinder. The "high data rate" signal is used to trigger the ignition at the correct crankshaft position regardless of these speed changes. The inner, "low data rate," set contains 6 slots, which are correlated to crankshaft top-dead-center for each cylinder. This set is used to trigger the fuel injection system, and at engine speeds above 1200 rpm where speed changes due to individual firing pulses is small, this set of slots is also used for ignition timing.

Light emitting diodes (LED's) and photo diodes are mounted in facing positions on opposite sides of the disk in line with the slots. Masks over the LED's and the photo diodes focus the light beams onto the photo diodes. As each slot passes between the diodes, the light beam is turned on and off. This creates an alternating voltage in each photo diode which is converted into on-off pulses by an integrated circuit within the distributor. These pulses are transmitted to the SBEC.

The distributor also delivers the firing pulses from the coil to each individual cylinder through a cap and rotor. A cover between the rotor and case protects against high voltage damage to the electronic circuitry and optical system contamination.