A common cause of erratic Tachometer readings

A common cause of erratic Tachometer readings

Posted by Support Staff on 30th Oct 2018

The tachometer circuit Board receives a signal from the ignition system that it uses to determine the engine speed.

The signal frequency is typically in the 40-600 Hz range. (40-600 cycles per second)

Ideally the signal would look like a square wave signal the varies in frequency, like the one below.

The ignition system does some amazing things. One of those things is turning 12 volts into 50,000 volts in just a few milliseconds. This high voltage is only produced for a short time and is used to fire the spark plug. As the magnetic field in the coil collapses and the current exits to ground via the spark plug, the coil is hen energized to do it all again.

It is not uncommon for stray voltage and minor imperfections in the ignition system to cause spikes and erratic signal to carry into the tachometer signal. That is why GM installed a tachometer signal filter for several years.

A tachometer filter, traps, redirects, and dissipates the high voltage spikes and surges in the tachometer signal.

Here is one example of how the signal would look when viewed on an oscilloscope before and after being filtered.

Erratic and non-functioning tachometers can be caused by a faulty OEM tachometer signal filter.

We carry a Tachometer signal filter that is easy to install and works with most ignition systems.

Tachometer signal Filter. GM OEM compatible replacement.

Tachometer signal Filter. GM HEI replacement. MSD Compatable

This filter removes ignition spikes, voltage surges, electrical noise, causes by High Energy Ignition systems, and some after market ignition modules.


Compatible as an OEM replacement Tachometer filter for GM vehicles. Compatible with aftermarket ignitions systems such as MSD, Holley, Speedmaster.

Erratic and non-functioning tachometers can be caused by a faulty tachometer signal filter.

Provides a stable reliable signal to the Tachometer for better Tachometer response and performance.

Three color-coded wires allow fast and easy fool-proof connections for life-time protection and superior response on all Classic Tachometers.

Includes installation hardware, instructions.