Direct Ignition System - Inductive Pickup RPM Reading
Number 87-6(Gasoline)-1A Date 9/87Subject: RPM READING USING INDUCTIVE PICKUP-DIRECT IGNITION SYSTEM
1987 6000, GRAND AM AND FIERO MODELS WITH 2.5L L4 (VIN CODE U,R) AND 6000 AND 6000 STE WITH 2.8L V6 (VIN CODE W)
Direct Ignition Systems (D.I.S.) will deliver two high-voltage pulses per wire during the engine's four stroke operation. This operational characteristic may make indicated RPM with an inductive pickup clipped onto a spark plug wire incorrect.
Some inductive pickups when properly used will give an accurate RPM or double reading. Verification of the equipment's accuracy should be made against the ALDL diagnostic connector RPM reading. The diagnostic "scanner" plugged into the ALDL connector running in the "open mode" will give the correct RPM reading along with other useful data.