77 F100 4.9 Auto. Trying to get the truck to run. This is a new starter solenoid. Engine will crank with key. Engine will crank with jumping terminals with a wire. No power output on S terminal. I have an electric fuel pump so no current to pump from S terminal. I have verified the pump works and wire will supply power to pump if I jump from POS terminal to wire to Pump (thru oil pressure switch, 12.7 volts). After running fuel pump with jumper the truck will start but not remain running. What am I missing?

 

Turn the key to run and check for voltage at the coil.

 

There should be an 'I' terminal on that solenoid and a wire should be attached to that. This provides 12v to your ECM while the starter is running.

 

Either of the top 2 post won't work for what you want. I would just make my own circuit for it using a relay tied into the ignition and a wire right off the battery terminal post on the solenoid.

 

You may be mixing up the terminal designations. The "S" terminal is the one with the Red w/blue wire that actually energizes the starter relay/solenoid when you turn the key to START. So it does not get power other than from the key, and only in START.
The second small terminal with the Brown wire is the "I" (or ignition" terminal and that one only has power in START as well. So as mentioned, neither of them is suitable for an electric fuel pump.

The Brown wire itself has power during RUN simply because it's connected directly to the coil's positive Red w/green wire. So anytime the key is in RUN, that wire has power, but the terminal at the starter relay does not.
Besides, with most starter relays these days (cheap imported stuff) the current capacity on that I post is pretty tiny I think. A well built one has a fairly robust connection inside the relay, but some are using a hair-thin wire to complete the circuit. Pretty sure even a small electric fuel pump would eventually fry that wire and the relay would no longer provide the extra jolt to the ignition when the starter is cranking.

Best to create your own heavier circuit for a fuel pump. The longer the wire, the larger the gauge it needs to be. I would consider 14ga to be the minimum size for a fuel pump.
You can run it off of a separate relay dedicated to the pump, or even directly off of the ignition switch. I tend to keep the load on the switch to a minimum, as I've seen plenty of failures on Ford ignition switches that are asked to do much more than they were originally.

Good luck.

Paul

 

To control an electric fuel pump, you can use the ignition circuit to power/control the primary circuit of a relay, and then use the big/secondary circuit side of the relay to power/control the pump.
Better yet is an oil pressure switch to control the relay which controls the pump.

The best way would be to use the trigger circuit of the ignition to control the pump via a relay, like how EFI works, but I'm not aware of a way to do that...

The general idear is to have power to the pump dependent on the engine turning. You want the pump to stop if the engine stops turning.