Tachometer Output from Coil Negative - 1N4004 Diode Rev Counter Schematic

This document is a simple electronics schematic showing how to create a tachometer (rev counter) output signal from ignition coil negative (-ve) terminals, typically when converting to a coil-per-cylinder or aftermarket ECU setup. It instructs connecting a 1N4004 diode to each coil's negative terminal (Spark output): two diodes for a 4-cylinder engine, three for a 6-cylinder, and four for an 8-cylinder. The diode outputs are joined and fed through a 15-18V Zener diode (1N5325B or 1N5355B) to the rev counter. A caution box notes this method is ONLY suitable for rev counters that were originally fed from the coil negative terminal, not for tachometers driven by the ECU. A parts table cross-references DigiKey and Farnell part numbers for the components: 2N2222A transistor, 1N4004 diode, 1K resistor, relay, 1N5355B Zener, 2N5551A transistor, and a 10K potentiometer. A footnote warns the part numbers need verification and some components have minimum order quantities of 5 or 10. The document itself is generic and undated; it was uploaded to a 1993 Ford Probe gallery, likely for use in an ECU/ignition conversion project.