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TACHS TACHS AND MORE TACHS!

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TACHOMETER INSTALLATIONS

Tachometers operate by reading a signal from your ignition system. This is a "pulsed" signal.  Each pulse is counted and the microprocessor converts this frequency into the pointer movement you see on the dial.   Gasoline engines generally count the number of times the coil fires.  Diesel engines mainly use some sort of sensor.  The same theory applies to both types.  NVU tachometers can also read pulsed signals from some PCMs, check out our Tech blog on Interfacing the GM PCMs for more on that. 

GENERAL GUIDELINES FOR WIRING TACHOMETER SIGNALS:

- Know where to pick up your signal at.  If you are using an aftermarket ignition system, the manufacturer's instructions will label where the tach signal will be coming from.  

-  Keep tach signal wires away from speed signal wires, ignition wires, fans, pulsed pumps, anything that may either interfere with the signal or the tach wire may actually resonate a signal to something else.  

- Filtering: NVU tachometers have a built-in digital filter designed to optimize the signal coming in from the source.  You can change it based on your needs.

- Know if you have a high energy ignition system before connecting the signal wire.  Damage can result if installed improperly.

TACHOMETER WIRING SCHEMES:

Even with modern computer controlled engines being used in our favorite classics, the most common setup uses an electronic module in the distributor (sometimes points) which sends a 12V pulsed signal to the negative side of the coil where the tach reads.  

TRADITIONAL COIL INSTALLATION

This setup uses a traditional distributor/coil.  The tach reads from the negative side of the coil. The green is shown for clarity and may be a different color in your application.

ELECTRONIC IGNITION COIL

Most distributors built after 1976 use an electronic module instead of points and have a TACH OUTPUT right on the distributor.  This output generally has a 12V square wave signal,  It may be "high" or "low" voltage.  Either way it is designed for a tachometer to see.

"MSD" BOXES AND OTHER IGNITION CONTROLLERS

This is probably the most popular ignition setup for aftermarket ignitions.  The "MSD" box is a Multiple Spark Discharge system (MSD) which can generate up to 60,000 sparks (pulses)  per spark plug firing.  EVERY system like this has a tach output right on the box.  It is either a 12V or 5V square wave signal designed to be clean for a tach to read.  Notice the box has power and ground going to the coil.  This creates a high voltage feedback that will damage the tachometer input.  This is exactly why a separate output is used.

COIL ON PLUG

Below, a coil on plug setup