TACH Wiring - Very Strange

[shhnappa]

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?

Specs:
'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.

 

[71dustar]

could it be that maybe the cover was put on wrong? that or the terminals were switched. thats my guess

 

[Swinger71]

I had the same issue with mine when I got it, I turned out to be the coil and ECU was wired wrong when the PO switched to electronic ignition.

 

[shhnappa]

I had the same issue with mine when I got it, I turned out to be the coil and ECU was wired wrong when the PO switched to electronic ignition.

So, will it do any damage, or should I re-wire it. It seems to be running fine, although I have a pinging (detonation) problem that I'm currently working on (could the incorrect wiring cause my detonation problem?). It seems to have too much advance under load, and the wrong plugs in it. What Plugs do you suggest I run in my 340 with a mild Purple cam?

 

[Swinger71]

I rewired my coil and ECU it seams to be better but that could just be all in my head thing. I would think a rewire may help but I am not sure.