I have a F600 with a 6.6 diesel automatic Allison transmission. I am turning it into a large pick-up. I am done painting and now reinstalling the interior.

This truck was originally a service body truck so the cargo light was covered over. I got a new to me cargo light from the junk yard and when I was installing the head liner I wired this new light

Durning the installation I mistakenly took the green wire that was there as ground and the black as power. I was correct on the black as this was the courtesy light wire but the green was constant power.

The green wire was run by someone else who was trying to make the factory dome light wiring work for a new map light dome light combo. The green wire was shoved into the fuse box with out an in line fuse link.

So i hooked everything up for testing and the new cargo light worked when the doors opened and when you twisted the head light job backwards. So I thought I was ok. Then I proceeded to install the dome/ map light combo and all heck broke loose.

First thing after I unscrewed the green wire was various buzzer an clicking started happening and it would go away if I touched the green wire to ground.

Then the tack started jumping and pegged itself at 3500 RPm and stayed there. I wasn't sure what was going on because I had to idea that the green was constant power and I had no sparks.

I started the truck with the green wire off the grounded body connection and then the truck would not shut off. I touched the green wire to ground and the truck shut off.

I disconnected the batteries and cut the connection to my cargo light. That connection was then connected to two love wires but It behaved correctly. Also as a side note when I was checking the wires to see what was ground and what was power if I hooked with the wires one way no light, the other way it light up and worked properly. Or seemed to.

So then I disconnected the cargo light and called a mechanic. He helped me figure out that someone had ran that green wire for a constant in order to run the dome/map light. I ripped out the green wire ran a wire from the right connection in the wire loom under the driver door sill and made the right connections. So all of that works right but my tach is still pegged.

I can start and stop the truck with the key and the cargo light dome light and map lights work.

I have checked the breaker that is for the fuel/gov and it tested good. That breaker should have been the one supplying the tach with juice I think.

I wanted to make sure the breaker was good so I got out a 10 amp fuse and it did not pop and the tach did not change so now I am at a loss. I also unhooked and re hooked the battery cables.

This truck is before computers, even OBD1, even though it is. '92 it is a 86 body and cab electrical. What Ido know it that I grounded constant power through a 14 gauge wire directly to the fuse box and now the tach it messed but all else seems good. Any suggestions would be great.