I am installing a factory tach in place of the clock in a 65 Chevelle convertible and did not receive a wiring diagram. On the back of the tach it is marked + / - and S , I assume that the plus goes to the distributer side of the coil, the minus to ground and the S to the switch, am I assuming correct on this? Thanks for your help

Is this a factory tach? Where did you get it? This is how i think it should be wired, + goes to a power source with the key on (You can use the IGN terminal at fuse box) - goes to a good ground and "S" is the sending, would go to distributer side of the coil, which would be the negative side. I would be sure of the wiring before connecting any wires.

That was my original thoughts also, seems to make sense that the S would be " sending " and the plus and minus would be self explanatory. It is supposed to be an original GM NOS tach and it was in a Box with a GM part number, it is brand new and never been installed before, but no diagram came with it, however it was wrapped in buble wrap which I doubt that they had in 65 but either way it looks original and fits fine.

The factory wiring is only two wires, one for a keyed hot and one to the coil. The unit is grounded by being bolted to the instrument panel pod. Only the two mounting studs to the left and right sides are used, the one in the middle isn't connected to anything.

I will look up the diagram and post it when I get home later tonight.

Thanks, I would appreciate it very much

I was wrong, the wiring uses the center stud and the one to the right (when looking at the rear of the tach).

http://heartland.chevelles.net/RCstorage/tachwiring.jpg