factory tach hookup?

[kdrodworks316]

I installed my cluster with a factory tach in it, just need to know how to hook it up. I am hooking it up to a 230 6 cylinder with stock ignition. anyone help me out with some pics?

[jonzcustomshop]

this might help?
they are talking about hooking up to an hei though,but maybe there is some useful info in there.....


A tachometer works on averaging the voltage spikes of each ignition event. Each time the ignition fires, it puts out a signal in the form of a square wave, based on the voltage supplied to the ignition. It takes this signal from the primary side of the coil, so it is based on the level of the battery voltage.

So, the way that the tach works is it takes an average voltage during a wavelength, so the more ignition pulses that are inside one wavelength the higher the average voltage is. This average voltage can be directly related to RPM, so your tachometer is basically a voltage meter.

Ok - since most points systems operate on a steady 9 volt supply, you need a 9 volt ignition pulse on a points tach. Your tachometer was built to read signals that have 9 volts as an input. HEI operates at 12-13 volts, so in order to work with the 9 volt system the tops of the square wave need to be clipped off at 9 volts. This is done using a Zener diode, which clips the top off of each square wave.

Clipping the tops of the waves causes some trash in the signal, so there is usually a filter to remove what is called Gibb's Phenomena. It usually is some combination of a capacitor and resistor. This also eliminates some of the flyback voltages to/from the coil.

Anyway, it clips the pulse voltage to 9 volts, averages it across a wavelength, then outputs it as a voltage for the tachometer to read.

If you had trouble following that, you could read this Adobe Acrobat document written by a MG Sprite guy:http://www.nonlintec.com/sprite/Sprite_Electro nic_Tach.pdf

Remember, you asked for it.


www.nonlintec.com/sprite/Sprite_Electro nic_Tach.pd...

Or go to www.nonlintec.com/sprite and then select the Electronic Tach.

The host page introduced some trash into the link, so we will try posting it again. If that link above doesn't work, I google'd "tach filter", if you plug that into your home computer you will get that link when you do a search (on the first search page).

The Sprite does it slightly differently, it uses a counter and a couple of op amps. Most of the tach adapters sold by MSD just use the zener in combination with the filter, to reduce component count and cost.

A filter for tach needle bounce (doesn't include the zener) :
http://www.camaros.net/techref/electrical/ta chfilt...

[LILRED66]

Jon:

That is a very in depth and informative narrative of the tach hookup to an HEI. Thanks for sharing.

[jonzcustomshop]

I should give credit:
joe231 gave a link in another post from a member vaughn on another board.
I just cut and pasted that narative..... I wish I knew that much about auto-electric!