Here is what I have done to install tach on my 1978, 302, automatic, F-100:

1. switch on back set to 8 cylinders
2. red wire for lights hooked to instrument panel fuse
3. red wire for power to 12V when engine starts
4. black ground wire to battery negative
5. green wire to "tach test" side of coil

When I pull out light switch, tach lights up, but when I crank engine - no movement of needle.
Where have I gone wrong?

Thanks for any help.
CB3

 

What brand tach?

 

TomTop Technology

(I know - a cheapie.)

CB3

 

"Tach test" is the negative side of the coil, correct?

 

You say "crank" Does the engine actually fire up and run? I'm asking cause my tach won't read till a few seconds after the engine fires to life. If I unplug the fuel line for example and just run the engine on the starter even though the coil is firing the tach won't read.

 

It is my assumption that the "Tach Test" side is the negative side. It has a dark green wire coming from that side already.

And the engine cranks & runs. The tach will light up when the light switch is pulled. But no reading from the needle - not even a twitch.

I have poked my green wire from the tack down into the "tach test:" side, and even poked it up underneath thinking that I might not be making contact with the coil post, but still nothing.

What is generally the method for attaching the tach wire to the coil? Splicing into the wire already there?

CB3

 

Yes, the TACH TEST is the negative (-) side of the coil.
Yes, it is a green wire.

If you pull the horseshoe connector off the coil and peek at the female terminal in there, you'll see it's difficult to make another wire connection inside the horseshoe. Best bet would to splice a wire in the circuit where feasible.....my opinion, of course.

 

I think you have something there! I will give that a shot tomorrow after I pick up some connectors with which to splice the wires.

CB3

 

Don't throw a crimp tap on the green wire. Either open up the harness and splice one in correctly, or open the actual female spade connector and run both wires to the connector