Tachometer testing .#2

Thread Starter


Joined Apr 7, 2022
So bare with me guys n gals, I'm a mechanical engineer that occasionally plays with electronics.

Let me explain what it is I am doing/what I am attempting to accomplish. So I am attempting to bench test the tachometer for a 1969 Gto, it just came back from the builder who "made" it compatible with electronic ignition systems (very reputable company). Now, most people would simply install it and assume all will be fine. Not me... I need to see it work infront of me on the bench before I spend the time installing the tachometer and removing it if it does not work correctly (entire dashboard removal).

So now that I have explained what I am doing, let me explain what I have done thus far. So i am aware that my ignition system is going to produced a 12v square wave at 50-1000Hz. So i have a cheap sunpro tachometer that i use for testing (https://www.summitracing.com/parts/sun-cp7903) and a cheap amazon signal generator to produce a squarewave signal (https://www.amazon.com/KKmoon-Frequency-Generator-1Hz-65534Hz-Amplitude/dp/B07FNLR6LX/ref=sxbs_sxwds-stvp?crid=3VXY81RY2KL5&cv_ct_cx=signal+generator&keywords=signal+generator&pd_rd_i=B07FNLR6LX&pd_rd_r=015ae4e1-db24-43cc-945a-615509be2c7f&pd_rd_w=T8PF8&pd_rd_wg=jPnjl&pf_rd_p=a6d018ad-f20b-46c9-8920-433972c7d9b7&pf_rd_r=2M2N0KYEFKGYJK9BQE5M&psc=1&qid=1584734926&s=industrial&sprefix=signal+ge,aps,173). With the sunpro I attach the positive and negative leads to a small 12v battery, then the positive signal lead from the generator to the signal lead and the negative lead from the generator to the negative power lead from the tachometer.

Now when i do this same procedure with the gto tachometer i get nothing, not until i move the 12v lead from the aux. power source to the output of the signal generator does it work, but that procedure does not seem correct to me.

So the conclusion I have come to, is that i need a single powersource on the gto tachometer, and the cheap generator does not have a 12v dc output. and if that is what i need to test this accuratly can someone point me in the correct direction of where to obtain this!

Hello fellow 69 GTO owner! I have a similar situation, a hood tach that is non-operational. The car has a newer GM HEI distributor. The original points style distributor with a separate coil was replaced with the newer GM HEI one, by a previous owner. So I want to troubleshoot this, and want to know how to test the hood tach to see if it's OK, and if so, then I can move on to the next step. I did learn that older style tachs will not work properly with newer style HEI type distributors, and some kind of filter or other mods that may be needed. Were you able to determine a proper test procedure for your in dash tach of your GTO?


Joined Nov 6, 2012
If You can find and post a Wiring-Schematic for a '69 GTO, it might be possible to help You,
but without knowing how the Tach was originally connected,
it's just a total guessing game.

DNA Robotics

Joined Jun 13, 2014
It should need +12 volts, ground and negative side of the coil.
Try it.
Or try it on a car with points or regular electronic ignition.
Even if you have to park next to it and use jumper wires.


Joined Sep 30, 2009
Straight unECM HEI modules have a tach output pin that isn't always used. Have you looked at your HEI module to see if it has one?


Joined Nov 6, 2012
From the vague memories that I have of Factory-Tachs back in the'60's,
there were all sorts of strange "one-off" designs that were in no way "universal",
and would only work with the specific wiring scheme, of that particular model of Car.

I would seriously consider using the guts out of a modern "Stepper-Motor" driven Tach,
( virtually all Gauge manufacturers have gone exclusively to Stepper-Motors, even the cheap stuff ),
rather than trying to figure-out the strange solutions that used to be used back in the day.

Modern Tachs are better in every way compared to the "old-school" stuff.