I'm new to the site, and a first time restorer of a 1979 F-100 Custom with an I-6 300CID/4.9L. The truck sat for 10 years in a barnyard, and has had some parts scavenged. Specifically, the primary wiring/ignition system. I'm trying to find a reliable wiring digram for the primary. Haven't been able to find any answers yet on the forum (but that could just be ignorance..... Any help out there?

 

Welcome to FTE.

Your truck would have had Duraspark II, which is a straightforward, reliable system. Power is supplied to the positive terminal of the coil (BATT) through a 1.35-ohm ballast resistor (implemented as a resistor wire) when the key is set to RUN. When the key is turned to START, the resistor wire is bypassed and the coil is powered from the "I" terminal of the starter solenoid.

The negative terminal of the coil (TACH TEST) is switched by the ignition module (GREEN wire). The pickup module (a Hall Effect sensor) connects directly to the ignition module through two wires (ORANGE and PURPLE). The ignition module grounds at the distribtor via the BLACK wire.

Lastly, the ignition module receives hot-in-RUN power and hot-in-START power from the ignition switch. Unfortunately, the color coding for these two signals in inconsistent among manufacturers and diagrams on the internet. The diagram below is backwards of the original implementation, but represents how most aftermarket modules are designed.

 

Very helpful - and especially your notes about the wiring differences. I understand that I need to be very careful to ensure my wiring is correct before I apply any voltage - bad things can happen if I mess up some wiring. Any suggestions re any pre-testing I could do to verify I have it right before connecting it up?

Thanks!!

 

To be extra cautious, you could hold off on connecting the 2-connector pigtail for the ignition module as a last step, and look at the voltages coming from the 2-connector pigtail from the truck and make sure they're what you expect under different positions of the key. Then you could look at the resistance between the RED and WHITE connections coming from the ignition module, and make sure they're no short between either conductor and ground. This is called a "smoke test" - it just makes sure that if you were to apply power to these connections, there wouldn't be a dead short.