I have a Sun tachometer, Model Mach 451-1, brand new, still in the original box that a buddy gave me and I want to use it in my '65 442. My problem is I have an HEI distributor in the 455 motor, and the tach is of probably 60's vintage and has a "trigger" terminal on the back of the unit. The tach has three terminals, ign. switch, ground, and trigger terminal; I hooked the trigger terminal up to the HEI and no workie. What do I need to make this animal work? Am I lacking something? I remember from the old days that there was a "sending unit" for the older tachs, but later model "one-piece" tachs did not use them. I've had no luck online finding any info on it.