C 2500 Truck 2WD V8-350 5.7L VIN M 4-bbl (1986)
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Engine Speed Sensor: Description and Operation
On all models with full function systems except V6-229 (1983 - 84 VIN code 9) engines, the ``R'' terminal on the 7 terminal ignition module provides
reference pulses to the ECM indicating engine RPM and crankshaft position.