After Market Tach Signal fix

So, you got yourself an brand spanking new aftermarket gauge and speed reads fine but suffers erratic rev read out despite following the installation instruction to the letter of the words?

I found an easy fix for it.

For most conversions and reviving old motorcycles not always the original gauges are available and we need to resort to an aftermarket gauge.

Tach signals are usually taken from 12V input into ignition coils which is fairly universally used in many make of motorcycles however, each company takes different ways to use the signal to convert it into a visible indication.

However, for newer motorcycles, the signal maybe handled by CAN-BUS which you will have to ignore the CAM-BUs signal and wire directly to the coil input. This is usually exactly the way your gauge installation manual says.

The figure above is the typical set up for 90's and 00's bikes with few exceptions. Single coil can power 1 cylinder to 2 cylinders depending on ignition timing but for Tach signal only needs one cable to work.

Not so much with single cylinder engines, but for multi-cylinder engines such as 2, 3 and 4, you have 2 anything from 2 to 4 coils being energized and that generates a lot of noise.

That can transmit through air and cables giving false pulse and if it is consistent, it can be used but noises are transmitted with random amplitude.

That is why you see your aftermarket tach jumping up and down and can't tell what speed the engine is running at.

The Graph above shows what the voltage on the coil 12V input voltage. The ignition controller basically grounds the coil power until the spark is needed and the moment the ignition controller energizes the ignition coil, you get a voltage spike momentary and it provides high voltage in the secondary coil.

However, while that is going on, between firing of each cylinder you have other cylinder firing too and that sometimes can be detected by the Tach.

That is how you end up with erratic tech read out.

You can search lot of enthusiast forums etc. They build circuits using 555 timer chip etc...needs a lot of work. For me, simple is better.

So I searched my electrical junk box to see if I can make it work with what I had.

What I used was 1 x diode, 2 x R100k resistor, 1 x Zenner diode, 1x 0.1uF capacitor and ferrite filter all from Amazon.

This needs to be installed away from the source of noise so that no additional noise is introduced,

What this aims to do is basically to ignore the spike and noise from other coils and resistors are just there to prevent overloading the Zener diode. The red line is the desired voltage more or less.

Red wire to Coil input, Brown to Tach signal and Blue to the ground...

That is it...