I have read many of the tach related post here, but I can not find what signal is driving my tach.

I have a 88 Trans-am 305 TPI. I have heard 4 pulses / rev at 12 vdc, can anyone confirm this - I do not have a o-scope

 

You are right, 4 pulses per crank revolution (8 sparks per camshaft revolution). The pulse is nice and square 12V with some kickback from the coil so make sure you use a good filter if you want to use the signal for anything besides a tach.

It should be the white wire behind your instrument cluster.

Hope this helps.
Lou

 

If it is a dc signal, what kind of kickback are you referring to, I am making a digital cluster with a lcd and all kinds of neat stuff.
Do you have any suggestions for a filter?

 

When the coil ground is disconnected (rising edge of the pulse), the coil produces a high-voltage spike. You need to filter that out. There are numerous filters online if you search, but I always use a opto-element anyway so that takes care of it.

Enjoy your project, I know I WOUL! :-D

Lou