Electric tach

[AHawks]

Ok I need some help. I have a electric car tach and the stock points ignition on a cb550k1. Is there a way to hook this up? I have no problem making an adapter, but none of the adapter schematics online seem to work. Thanks in advance.

 

[Big R]

Been a long time since I've looked at electronic tachs, but what kind is it?

 

[AHawks]

Its an old mallory. It has a red wire for 12v source. Black for ground. White for lighting and green for signal. There is a 4-6-8 switch on the back. I have made an adapter with IN4005 diodes off of each negative coil wire to the signal wire. the tach will read fine until 2000 rpm. Anything over that and the needle drops.