[joeydurango]

Okay, update time. Yesterday I finally had time to dig into this a little more. I checked out the back of the tach more closely and it didn't have a ground wire run at all - I believe stock they relied on grounding through the bezel.

I ran a new ground wire, and no change.

I checked that I had power to the fuse block - yep. I checked that I had power to the tach - yep. So, now I know the tach is getting switched 12v, and that the ground is good. The back of the entire cluster looks nice and clean, in excellent condition, for what it's worth. There are three options I can see:

1) The distributor is faulty. This is a new HEI distributor I dropped in there about 18 months/3000 miles ago, so I doubt this as a cause. Also, this problem existed with the old distributor.

2) The tach is faulty internally somehow. As I said, in excellent shape, but it could happen, I guess. Don't know how they work inside.

3) The wire from the distributor to the tach is faulty.

To begin checking these possibilities, I guess I'll start the truck and see if there is power to the tach at the brown wire a) when the tach is cold and not reading, and b) when the tach is warm and reading properly.

Finally, my latest question: What should the brown wire read at idle? 12v? Or does the voltage go up with RPM? Maybe I need to rev the engine to get a reading on my multimeter? I am no electrical genius.