[Tigers] Auto Meter tach

[Rollright at aol.com]

Hello,
 
Well, the tach project is finished. Now I'd  like to test it on the  car 
before installation. 
 
The easy part is 12 Volts and Gnd.
 
What can I hook the third "signal" terminal to?
 
And finally, when I pull out the old tach, how do I hook up the new one.  
Yes, I know 12V and Gnd. I'm concerned and confused about the third  terminal.
 
Can anyone help?
 
Jim  Armstrong