Optical Distributor
The SMEC receives engine speed and crankshaft position signals from optical distributor. These signals are used to control fuel injection, ignition timing and idle speed. The timing member is a thin disk, mounted on the distributor shaft and driven at 1/2 crankshaft speed. Disk has 2 sets of slots on its surface. 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 firing pulse of each cylinder. The "high data rate" signal is used to trigger ignition at correct crankshaft position regardless of these speed changes. The inner, "low data rate" set contains 6 slots, which are correlated to piston TDC for each cylinder.
This set is used to trigger fuel injection system and operation at speeds greater than 1200 RPM where speed changes due to individual firing pulses are 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 LED's and photo diodes focus light beams onto photo diodes. As each slot passes between diodes, 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 SMEC. Distributor also delivers firing pulses from coil to each individual cylinder through a cap and rotor. A cover between rotor and case protects against high voltage damage to electronic circuitry and optical system contamination.