Hello,

I have pulled a factory Tach from a '67 parts car and want to bench test it to make sure it works. I was wondering if anybody knows the pulse shape and polarity coming off of the negative side of the coil.

Thanks,

About the only way to test it is to put it in a working vehicle. The signal on the negative side of the coil is very noisy. It can be a few hundred volts in amplitude both negative and positive. To make matters worse, the circuit in the OEM tachometers is basically calibrated to work. If the signal presented to the tach varies significantly from what it was calibrated to, you can expect it to operate irradically, improperly, or not at all. It's not easy to bench test.

What I tried to do is use a center tapped 24 volt transformer with a fullwave rectifier on it. This produced a 120Hz signal which corresponds to about 1800 RPMs. Not really a good calibrator but it works.

Thanks Dave

If you did want to hook it to a working vehicle and not completely install it in the dash.... you don't happen to know which posts are +, -, and coil on a factory 67 tach do you?

I tested one for my 74 Nova using a signal generator and a 12V power supply here at work. I'm going off memory but I think I used a square wave output (since the vehicles points were opening and closing a circuit) but that's about all I remember right now. I'll see what other info I have on the computer at home and post it here.
It also seems to me there was a relationship to the amount of pulses per second as to what the tach read (that would make sense to me as that is all a tach is doing is counting the pulses per second and the points opening and closing).
I'll be back.
Jim

Originally posted by SOA-Nova:
I tested one for my 74 Nova using a signal generator and a 12V power supply here at work....Jim Thats about the only real way to bench test the tach. Even then, the actual calibration in the vehicle may vary from the test calibration. But if you have the means to set up something like this, I think you'll find you need a generator capable of producing more than a 12Volt pk-pk signal. In fact, I suspect that in order for the OEM tach circuit to work as it was designed that you'd need more like a 24Volt pk-pk signal.

I got the tach to work with a normal pulse generator with a 5 volt positive pulse to the tach input. If you look at the circuitry, there is an input diode that rectifies the coil signal in the positive direction. The signal is then filtered through a simple RC circuit that eventually drives a NPN transitor. The transistor drives a RC/diode circuit that produces the pulse to drive the needle. The drive circuit is somewhat adjustable with a pot as the "R". I can see that by using some modern circuitry, one could vastly improve the ability to better tune the tach over the entire RPM range.