i have a 1987 f150 with the straight 6 300. I have converted it to carbureted but the in tank fuel pump is still wired through the computer. Any idea how to bypass the computer for it so i can take the computer out

 

What are you running for the ignition?

 

Wire the relay to switched power and you're good.

 

What wires on the relay go to wat the diagram shows 5 wires but i only have 4

 

I have a red wire with a fuse in it and a yellow, a black, and a tan/ blue may have been light green. The yellow wire it a little bigger than the rest i think the black goes to ground

 

Big safety issue if your fuel pump runs after an accident.

 

It never fails to amaze me that when people follow that ill advised route of swapping out EFI in favour of a carburetor that they would leave the most failure prone component in place. That would be the in tank electric fuel pump!

But enough of my anti carburetion ranting. I would hope that if your truck had the external high pressure pump that you at least took that out of the equation and have dealt with the fuel line pressure issue in a safe manner. Dual tank switching becomes an issue without in tank pumps, but if you insist on going old school, you really should eliminate the in tank pumps, add the mechanical pump driven off the cam and use a fuel tank selector valve that is mechanically controlled.

There is a way to control an electric fuel pump with a relay controlled by an oil pressure switch. That would be the safe way to handle such a setup, but there are some considerations such as a bypass when cranking to enable the engine to start. It does become somewhat complex electrically and beyond what I can explain in one simple post, but there are examples out there. I could sketch it out on paper, but that does not easily make it onto the internet. Maybe I can scan it and post later. Hmmm...

 

I hope your truck has a manual transmission

 

Pretty simple actually. I had one on my Bronco before I "upgraded" back to a carb and mechanical fuel pump*

Get the oil pressure / fuel pump switch at a NAPA or something. It has a terminal that connects to the starter solenoid, one that connects to 12V positive and one that connects to the fuel pump relay. Of course the relay needs a ground, a +12V and a wire to the fuel pump (which also needs a ground).


* I had upgraded my '71 Bronco to fuel injection with a Holley ProJection system. I had issues with that , and not much budget to fix it, so I went back to a carb "for the time being", knowing that it would be years, if not decades, before I'd get around to getting fuel injection back on it.

 

These trucks have an inertia switch and removing the PCM won't affect that.

 

True, but the computer still provides a signal to the fuel pump relay that is dependant on seeing PIP pulses from an engine that must be turning. A stalled engine for any reason does not need a pressurized source of fuel and a ruptured line would just keep on delivering that fuel until something, either the inertia switch or the computer signal turns it off. Call it redundancy, but it is an important feature.

On to reconfiguring the relay to be controlled both by the ignition switch and an oil pressure switch, I would keep both the EEC power relay and the fuel pump relay. Substitute the oil pressure sender for the signal from the computer to the relay coil. The relay has 2 coil connections and 3 contacts. The coil gets ignition switched power from the EEC power relay. This same power should be jumpered to the normally closed contact of the relay, (which was unused with EFI). The fuel pump was powered from the normally open contact via the inertia switch. This must be moved to the common contact after removing the switched ignition power that previously was connected here. The contact vacated by the fuel pump wire now needs a circuit to the crank signal at the starter solenoid. The old style solenoids with two small posts would be best to make this source. Function is similar to the ballast resistor bypass in the Duraspark or points ignition used with carburetors.

The way this would work is that the relay becomes an electric fuel pump disable. When the ignition key is turned on, power is applied to the NC contact and the + side of the fuel pump relay coil. There would be no oil pressure and the oil pressure sender would complete the relay coil circuit to ground and the energized relay would open the circuit between the common and NC contacts while closing the circuit between the common and NO contacts. The fuel pump motor is powered by voltage delivered from the common contact via the switched ignition power at the NC contact and through the inertia switch in the set position. With the relay energized, the power is interrupted and the pump does not run. When crank voltage is applied to the NO contact of the energized relay, that power is applied to the fuel pump through the common contact and the pump runs, enabling the engine to receive fuel and run. When the engine starts, oil pressure builds and opens the oil pressure switch, deenergizing the fuel pump relay which now supplies switched ignition lower to the fuel pump through the NO and common contacts. Turning the ignition off removes power from the fuel pump, stopping the fuel flow. A stalled engine will lose oil pressure, which will energize the relay, stopping power to the fuel pump and stopping the fuel flow. Impact will trigger the inertia switch, shutting off power to the fuel pump and stopping fuel flow, even with a running engine.

Whole point of the exercise is to shut off the fuel flow if anything goes wrong. Not feeding the flames becomes a good idea.

 

I have wired the relay so its on the key but still nothing my check engine light and such stay on when the key is out now too. I also checked voltage at my fuel tank switch and theres a lone wire that doesnt go through the switch with 12v the one that feeds the switch has 7v but on the switch side of the relay i have 12v any ideas? And also i put a jumper on the 12v wire and touched all the terminals to see if i could hear it and all i heard was static in my radio???