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1985 Ford F-150 Pick-up Won't Start - Fuel System
Question: Fuel pump(s) in tank(s) do not run. Injector pump runs. Suspect fuel selector valve is bad (again). Fuel selector is getting power but does not send power to fuel pump in tank. Am I right? And if not, where is the fuel pump relay?
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Technican: The fuel pump relays are on the drivers side fender. They will shut down the pump, even with a good selector valve.
Owner: There's only one relay on driver's side and it is on firewall and doesn't appear to be connected to the wiring harness that leads to the fuel system. Is there an alternate location for the relay and how do you check it?
Technican: Which engine do you have? The relay typically has 4 wires. One is hot all the time, one is hot with the key on. One goes to the load (fuel pump), And the last one is the trigger. The trigger should be a ground supplied by the fuel switch or computer. To test the relay, use a test light to ground to check each of the wires. Make sure you have your 2 power wires first, then backprobe the ground signal wire with the key on and make sure the relay clicks. It should also operate the load (fuel pump). If the load has power, but is not operating, then the pump or wiring from the relay to pump, or from the pump to ground is faulty.
Owner: I have a fuel injected 302; I'll test after work today.
Technican: There are 2 relays. The first turns on the computer. (Does your check engine light turn on when you turn the key on?) The power goes from the main relay to the fuel pump relay. The fuel pump relay sends the power to the pump. Both relays are 5 pins, but only 4 used. Tan/light green is the trigger wire for the fuel pump. It can be grounded at the diagnostic connector under the hood. If it clicks with the key on, then you can find the location. Red is the output power from the main relay. This should be 12v with the key on. It also goes to the fuel injectors.
Hope this helps,
Owner: Emissions light comes on with key; so does relay under dash on side of computer. Relay on firewall comes on for 2-3 seconds running injector pump then shuts off. Probed as directed and found what you said but the only way to keep this relay on is to leave backprobe in place on trigger and that still doesn't run pumps in tanks. The same 2-3 second power goes to the selector valve on red wire or brown wire with white stripe, depending on position of selector switch in cab. Ohmed out inertia switch, it checked good.
Now what?
Technican: Does this truck have a fuel pump in each tank? Some had a small pump in the tank to provide fuel to the valve and main pump. If your tank pump has failed, the injector pump is not able to suck the fuel up out of the tank, but needs a low pressure fuel supply to keep it primed and flowing.
Your controls are all working up to the selector valve. Check your in-tank pumps. You should have fuel flow with the hose disconnected with the selector switch in either position.
Owner: Truck has pumps in both tanks. Cannot verify any power going to tank pumps with selector switch in either position.
Original problem was no fuel at fuel filter or selector valve.
There is one wire at the selector valve that has alternating power on it (on, off, on, off; like a turn signal). Is this what is supposed to power the tank pumps?
Technican: Sounds like the selector switch has failed. Wow, just about what you thought in the beginning. The switch directs the power to the proper pump and also switches the selector valve so that the fuel is routed from the proper tank to the main pump. The selector switch should have power coming in on one wire and directed to the tank on 2 other wires. And 2 more wires to the valve. MAKE SURE TO TEST PROPERLY....It may only provide 2-3 seconds of power to tank pumps as well! (This is a safety feature to prevent fire incase of accident. The pumps turn off 3 seconds after the engine does.)
Owner: Just to confirm: the selector switch in the dash sends power out on two separate wires; one to the selector valve and one to the tank pump, right? So the tank pump is supposed to receive power directly from the selector switch not via the selector valve?
Owner: one more thing- I just remembered my selector switch only has four wires... not five.
Now what?
Technican: I have the wiring diagram in front of me. You have proved the switch good, because it is switching the power to the selector valve. The pulsing signal should be the signal to the fuel gauge. Both tank pumps are fed power directly from the selector switch on the red wire to the rear and the brown/white to the front. The selector valve switches the fuel gauge only.
You can manually turn on the pump relay as in earlier testing to test the tank pumps. (disconnect your injector pump so that it will not burn up)
If you have power at the tank pumps, make sure your ground is also good. You could have 2 bad tank pumps...been there, done that.
Owner: Ok- disconnected injector pump and triggered relay. Found front pump had power and grd but inop; assume bad. Switched and heard rear pump running but no fuel coming out of line disconnected at selector valve (all lines were disconnected at selector valve at this point). Assumed tank was empty (gauge has been inop for years). Poured in 5 gallons of gas. Still no fuel at selector valve. Reconnected all lines at selector valve; still no fuel at fuel filter mount.
Now what? Can a pump run and not pump fuel?
Technican: You may have a clogged fuel line. Or the line may be ruptured inside the tank. Or the pump motor may be spinning, but the impeller frozen (bad pump). Or, if it is wired in reverse, it will pump nothing. Sounds like it needs to come out for inspection :-( Here's How To Get Guaranteed Solutions In Minutes
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