I purchased a Sunpro Tach for my '79 F-350. I hooked it up exactly as they stated in the instructions doing a quick check. Red to positive, Black to negative and Green to the "Tach Test" side of the coil. When I start the truck, the Tach shows zero. Could I have just gotten a bad Tach? When there is no power to the Tach the needle is below zero, when red is attached the needle jumps to zero, but that is it, nothing more after starting.

 

hmmmm maybe a faulty tach. but im no electrical wizard

 

If you measure the voltage across the wires that are providing power and ground to the tach, do you get 12 volts? Yes, you need a multimeter for this ("wires look OK" does not work).

If that checks out, then connect a test light between the GREEN wire at the tach and ground and make sure it flashes while the engine is cranking (not when it's running, it will blink too fast and look like it's stuck on).

 

Yea, chances of getting faulty tach is slim. Likely option is bad wires. Do what fmc400 said to do.

 

The red and black are connected straight to battery, and I get 12 volts, the coil shows about 5 volts. With a test light on the green wire the test light is solid not flashing,when cranking.

 

It sounds like you've got the GREEN wire of the tach going to the positive (BATT) terminal of the coil and not the negative (TACH TEST). The trigger signal going to the tach should either be (up to) 12 volts or zero as the module switches the coil. That's why the test light is supposed to blink as the engine cranks. If you simply see it stuck on as the engine cranks, that almost sounds like you've got the tach triggered off power to the coil (BATT). On a Duraspark II system, this point will be 6 to 7 volts with the key in RUN.

Please check the wiring again. I assume the engine runs fine otherwise? A test light can stick on when connected to the TACH TEST terminal in certain cases, but the engine won't run under those circumstances.

 

Also,,, make sure you set in on 8 on the back,,,

 

The switch is on 8. The top of the plastic connector says "tach test" with an arrow on the side where I have connected the green wire. Could the wiring be backward? Some wiring was butchered by the previous owner. Will the truck even run with it backward?

 

It's not possible for the engine to run with the coil backwards because the BATT terminal is common to both the primary and secondary windings. Both the coil and horseshoe connector are keyed for this reason.

A test light connected to the GREEN wire of the tach needs to blink as the engine slowly cranks before the tach can ever operate. If you move the test light to the actual TACH TEST terminal of the coil (rather than at the tach connection itself) do you see the same behavior?

 

If I put my test light on the tach test connector on the coil it lights up dim. While cranking, the light brightens and stays solid. If I put the test light on the other connector, it flashes while cranking. Also, if I pull the plastic connector off the coil, I get zero voltage on either line with a volt meter. If I turn the key to "on" the tach test side has a steady 12 volts the other side has nothing. The truck starts and runs fine.

 

Resolved! Thanks for all the help, the wires were reversed on the coil. When I put the 460 in, I connected the wires without checking. The previous owner had cut off the curved connector from the wires and had them put on the old coil with nuts (I believe it was an aftermarket coil because it had threaded studs for the two wires) I just marked each wire with a + and - when I took them off and then spliced them into the cut wires on the connector on the coil of the 460. I did not clean off the grease to see the colors, but I did tonight and the green was spliced onto red and the red spliced onto green. The truck started and ran fine with them like that??? Oh, and now my tach works!!!

 

Glad to hear you got it worked out. Sounds like the tach was reading the coil's power signal and not the switching signal.

It's not completely clear to me if the actual signals on your setup are really swapped, or just the colors are swapped but end up making it through correctly. You'd need to look at the whole system to be sure. I have never tried running the engine with the primary side of the coil backwards, but just from thinking about it I do not think it would work (I could be wrong). It might be worth checking things over to make sure everything's set up correctly.

Power (connected directly to 12 volts in START, or through a ballast resistor in RUN) must run to the '+' side, and the GREEN wire of the ignition module must run to the '-' side. Here is a wiring diagram: