I need a 12 volt hot under the hood to run a vacuum switch. Only needs power when engine is running. Can I just run it from the ign terminal on the Ford starter relay mounted on the firewall? I'd put an inline fuse before the switch to protect the circuit. Much easier than running a wire from fuse box inside the truck.

Seems like a no brainer, but I just want to make sure I am not going to mess up the ignition hot to the coil.

 

Nothing on the starter solenoid is hot only when the engine is running.

 

Not the starter solenoid but the starter relay, on the firewall. The ING terminal is hot anytime the ignition is turned on. It feeds the coil.

 

Solenoid = relay

The "I" terminal is hot only when cranking.

 

Thanks Ross. Your post got me thinking about my wiring and I went down with the meter to do some checking. The original wiring in the truck had a wire running from the "I" terminal to a resistor block and then to the coil. When I re-wired I left that wire in place. When I went to a Pertronix ignition, Pertronix told me to eliminate the resistor block, which I did, but I just attached the wire from the relay "I" to the coil. Thus, the "I" terminal on the relay is hot when the engine is running because it's back feeding off the coil.

I guess the "I" is only hot during cranking to give the coil an extra boost for starting which I am not sure I need. I am going to eliminate the wire from the "I" to the coil.

Guess I'll be running a wire from the fuse box for my vacuum switch.

Thanks again.

 

Normally, there is a ballast resistor in the wire from the ignition switch to the coil. The wire from the "I" terminal runs past the ballast, directly to the coil, to give higher voltage for starting. So the old wiring on your truck was wrong. Sounds like you're in good shape now! You don't need the "I" wire with a Pertronix.