82 F100 Tach help
 

Hi, I have an 82 F100 that was a base model off of the line with a 3.8 V6 with no factory tach installed. I now have a 302 V8 with a HEI coil top distributor. I used the factory wiring and CDI module from the 3.8 just to keep installation simple. I have the red power wire and the green tach wire ran to the HEI. Besides that and the stock electric choke wire that is all I am using of the factory wiring for the engine. My problem resides in getting a factory tach to work with my set up. I used advice from others and placed the tach in the stock location where the "blank" was used on the instument cluster. I used the printed circuit sheet that came with the tach. Now, when I turn the key to "on" the tach needle just goes all the way over to the other side as if that were 0. I have started the truck thinking maybe just a glitch but made no difference but, when I do rev the engine a bit say 2000 to 3000 rpm's the tach needle starts to bounce a little as if it was trying to read back wards for a couple hundred rpm's. Does anyone know how to wire the tach up straight to the distributor and get it to work that way? Am I missing something else that may be a simpler fix? I do have a Hayne's and Chilton's repair manuals with wiring diagrams and I am having trouble tracing the wiring to the tach through the circuit board, any help there would be much appreciated as well. Thank you.

Hi everyone, I think I have narrowed it down to the tach itself being the problem. I used my hand voltmeter and checked continuity between the top and bottom post of one side and compared it to the other side. Looking at it upside down, the right side that has actual ground has very good continuity and the left side is only a third of what the right side read. I took apart the tach and tested the coils that moves the needle and it checked out good. So, now I believe it is in the circuit board of the tach. Where could I find a new factory replacement tach online? I have searched and come up empty handed. Thank you.