TACH Wiring – Very Strange

OK, I have installed hundreds of Tachs over the years, pretty simple actually, 3 wires (4 for lighting), one to ground, one to 12V switched, and the third (green) to the (-) side of the coil. Can someone please explain to me why I get no tach signal unless I connect it to the (+) side of the coil, very strange. I have also checked it with my diagnostic equip, and I only get the signal from the (+), so that rules out a defective Autometer Tach. Now, it all works fine, but it shouldn't, so that is my question, do you know why?

'68 340 Cuda
Updated ign to MP Elect. Ign
Coil seems to be original (Should that be changed???)

Car purchased 1 1/2 months ago, as is, so I'm finding things out day by day.

Thanks for your help.
Author: admin