How to Test a Tach?

[bobswei]

Is there any way to test a tachometer, while it's "out of the car"? The one I have seems to be in perfect condition but want to make sure it works.
Thanks

[Early_S_Man]

What year and model car/engine is it for? Date code and pic of the back of the tach would be useful, too!

[bobswei]

Thanks for the response. Just as I hit submit, I thought, I should include year and model.
It's out of a 68' 912. Here is a pic.......

[304065]

That tach may have come from a '68 but it is date coded 10-65 and says 4-zyl, which means it's a green letter tach with a silver bezel from a 1966 model year 912. A '68 would have had a black face with white letters and black bezel ring.

As such, the /1 terminal accepts a 24 volt square wave, which should ideally come from a function generator. I posted the circuit diagram to the best of my ability here. Warren posted an inexpensive diagram for testing the later tachs which accept an 11 volt square wave, but these are coded after approximately October of 1970.

That one looks neglected, rusty like it's been in the rain. If the outside looks like that the internal VDO movement is probably rusty. . . I would source a correct '68 912 tach before spending $150 to have that one repaired.

[bobswei]

Thanks,
I'm not even sure what a "function generator" is so maybe I'll just put this on the back burner for now. I was just going through some parts I have to see what's good and what's bad.

Bob