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Early 1990s Tachometer Drive Board

There is a design flaw in Chrysler’s early-1990s tachometer drive board. Part number P4375410, it’s used in most early 1990s MoPars and plugs into the instrument panel circuit board.

The flaw is that it is connected both to ignition and battery power so that it draws down your battery with the engine off. If you look at the picture, R10 is a 1K-ohm resistor that, because of its value, keeps Q1 turned on all the time.

That produces a voltage at the emitter of over 11 volts, which is enough to cause current to flow through the Zener diode CR4. It will draw between 90 and 105 mA of current with the engine off. This also self-heats the diode, and since it’s used to create a crude 5V power supply, will affect the accuracy of the tach.

Tachometer schematic

If you leave your car unused for 4-6 weeks, this can draw the battery down enough to cause a no-crank situation.

The solution is simple: Cut the circuit trace to the battery feed and jumper it with a wire over to the ignition feed. The board will still function exactly as before, but draw no power with the engine off.

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