Reading signal to Tach

[randywebb]

I have a 1973.5 tach that is being run off of a msd box. I'd like to be able to read the signal with a data logger. My data logger can read voltages to several nV or it can do some pulse count stuff.

Can anyone tell me what the signal from the 1973.5 will look like? (BTW, to make it more complicated... it is a hotrodded 3.2L motor with a 964 dual drive dist. that has the SC inards installed... the tach signal IIRC, _is_ coming off the msd box tho.)

A search found only this by john_cramer:
- 1971-1973 Tachs. These appear the same externally but have been altered internally to be driven off the high-impedance circuit between the points and the Bosch CDI box. In the early 3-pin CDI boxes, there is a 33 ohm pullup resistor in the trigger circuit that would allow only around 425 mA to flow in the circuit. The tach must be sensitive to 11 volts and 425mA, which is why the later CDI won't drive the earlier tach off this circuit-- the earlier tach needs higher voltage.

Should I be looking for a square wave?

[Zendalar]

Hmm, does this mean that my "early" tach is not broken afterall?

I mean, i installed this Allison/Crane XR700, and from which I took the tach signal directly from the -pole. Since the install, the tach has not worked.

Is it getting too much of something, or too little? Somebody earlier said The system now is sending square wave, and earlier with the CDI box it was sine wave..

[304065]

Randy,

MSD boxes output a 12V square wave with a 20% duty cycle. In truth, one tester has measured the duty cycle and found it to be variable with frequency.

http://www.dainst.com/info/circuits/msd6a_02.pdf#search=%22msd%20duty%20cycle%22

As you can see, the tester found that at 10hz, or 200 RPM for a 911 engine, the duty cycle was around 10%, and roughly linear up to 22% at 400Hz, or 8,000 rpm.

I think the Allison boxes, and most other aftermarket stuff, also output a square wave.

Of course you understand that there is no output signal from the tachometer itself. A late CDI tach post 1972 is "looking" for an 11V square wave with about 60% duty cycle. The duty cycle should be relatively constant as long as the six little bumps on the distributor shaft are the same size all the way around. With 38 degrees of "dwell" for the CDI ignition, times six ignition events, that means that the points are closed for 228 degrees of CRANK rotation, or approximately 63% duty cycle. To make the math easier to understand, if you had a 4-cylinder 356, these use 45 degrees of dwell, times four cylinders, or 180 degrees of CRANK rotation, voilla, 50% duty cycle. The difference between these measurements helps explain why Porsche went to a CDI in 1969, BTW.

[randywebb]

Thanks John. Now I just have to do some programming...