What does the fuel pump cutoff relay do? Does it cut the fuel off in a accident or switch the pumps off and on during driving?

 

The inertia switch is what cuts off the fuel in the event of an accident. Electrical power is routed through the Fuel pump relay and then through the Inertia fuel shut-off switch to the fuel pump module.

 

My truck has 3 different relays

Inertia Switch
Fuel pump relay
Fuel pump cutoff relay.

This is a duel tank truck. I have replaced the fuel pump relay, it is located on the side of the computer mod. The inertia switch has not been tripped I found it mounted on the passenger side just under the dash. The fuel pump cutoff relay is mounted in the engine compartment next to the brake master cylinder.

I got some info off the alldatadiy site. But it doesn't tell me what the purpose of the fuel pump cutoff relay does.

 

Heh, I guess I will have to look around for some information then. I did not see you have a 1986 model. I gave you information for my 1999.

 

According to my schematics, you have a fuel pump cut off relay and an inertia switch. From '84-'86, the first years they put an electric fuel pump in the F150, they called the fuel pump relay a fuel pump cut off relay. Starting in '87, they changed the name to fuel pump relay. They are exactly the same, in the same place in the circuit, serve the same function, and even have the same color wires. In 1987, the diesels had a fuel shut off solenoid, but that was totally unrelated to your truck.

What color are the wires going to the two relays you say are for the fuel pump?

 

I show light blue/orange from the power distribution box to the fuel pump relay and dark green/yellow from the fuel pump relay to the inertia fuel shut-off and then pink/black to the fuel pump module.

 

That's not what I show, but it's close. Is that for an '86 5.0L? Actually, I'm assuming it's a 5.0L, he hasn't said what it is. Anyway, I just want to know what the wires are going to the two relays he thinks are for the fuel pump, so we can point out which one actually is, and then try and figure out what the other one is.

 

Im thinking the terms have changed and might be as follows

fuel cutoff switch - inertia switch
fuel cutoff relay - fuel pump relay
fuel pump relay - fuel pump module

The fuel pump module may have a relay in it that my diagrams don't show or the relay may have been replaced by some solid state component.

OR

the fuel cutoff switch was in line with the coil of the cutoff relay and the cutoff relay was in line with the coil of fuel pump relay which was changed to eliminate the cutoff relay.

The result is a fuel pump relay whose coil is controlled by the pcm (and this is where the cutoff relays function went). The inertia switch is now between the fuel pump module and the contacts on the fuel pump relay which means the inertia switch has to be rated at 20 amps.

 

Anything is possible, but in my schematics, everything is the same except for the name of the "fuel pump relay" ("fuel pump cut off relay" from '84-'86).

Hang on a second...

 

Here, the first image is the '86 "fuel pump cut off relay". The second pic is the '87 "fuel pump relay". The only real difference is the wire to the inertia switch went from pink/black to brown.

 

Where do the tan/light green and red wires go on the 86 to energize the relay?

Where do the tan/light green and red wires go on the 87 to energize the relay?

Did the yellow wire switch from a starter relay to a fuel pump on the 87 or do both the 86 and the 87 go to a starter relay?

 

In both cases, the red goes to a common distribution point that feeds key on power to the injectors and the smog solenoids, the tan/green wire goes to the ECM and the self test connector. The yellow wire actually goes to the ECM, by way of the ignition switch, starter relay, and EEC relay, but wired in parallel, not in series.

 

You changed your post while I was replying. lol.

In both cases, the ECM switches on the relay.

To answer the last part, in both cases, the wire going to the inertia switch is the one that eventually goes to the fuel pump.

 

All I am seeing is ONE relay that is controlled by the ECM that supplies power to the fuel pump when it is energized. Only the names and the wire colors have changed.

 

Okay, let me try and set it up for you. First, remember that the yellow wire always has battery voltage because it goes to the starter relay post that connects to the battery pos post. When you turn the key on, the ECM energizes the fuel pump relay through the red and the tan/green wires. It then closes the relay, connecting battery power (the yellow wire) to the inertia switch. If the inertia switch is closed (meaning it hasn't been tripped in an accident), power then continues on to the fuel pump. get it?