Fuel pump circuit

[70roadrunner]

I'm working on an 87e350 cutaway chassis and am having trouble tracing the fuel pump circuit. I have power through the inertia switch, but no trigger voltage to the fuel pump relay. When I hot wire the relay it triggers and the fuel pump runs. I can't find a fuse anywhere. It's a 460 with a carb. Any help is greatly appreciated.

 

[Chris_1]

My 87 f250 351 with a carb doesn't have an electric fuel pump. It's on the motor. Sounds like yours does though. One thing I've seen is a bad connection to the wire under the relay. You have to take the relay box apart to check that. Before that, check for power to the relay. You should get power for about 2 secs (when you cycle the key off and back on) to one connection, which primes the system and shuts off. Then if the motor is running (and only if) you should get voltage to one of the other connections to run the pump (usually at a lower voltage like 6.5). These are 2 separate circuits that go through that relay.

 

[RonD]

Ford uses the Ground on the fuel pump relay for the trigger.

The computer controls this ground.

One side of the relay's coil gets 12volts from the EEC relay when key is turned on.
But the other side of the coil, the ground, connects to the computer, computer will ground the relay for only 2 seconds when powered on, then it won't ground it again until engine RPMs are above 500, engine started.

This is secondary safety feature so if engine stops running(falls below 500rpms) from a major malfunction, or accident, the fuel pump power will be cut off.
Inertia switch would be the Primary safety feature.

Fuel pumps power path
Battery------fuel pump fuse-------fuel pump relay-------inertia switch----------fuel pump-------ground

Fuel pump relay's power path
Battery----key------EEC relay-----fuel pump relay------computer-----ground


With key off you should find one 12v contact in the fuel pump relay's socket, that's from the fuel pump fuse, it is live 24/7

With key on there should be one more 12v contact, that's the relay's 12v power from the EEC relay.

EEC relay gives power to computer, fuel injectors and most sensors, also the fuel pump relay, when key is on.

 

[70roadrunner]

There is no computer on this one. It is a duraspark ignition. There are 4 wires to the relay, one constant 12v, one ground. When I apply 12v to the 3rd wire, the relay clicks and sends 12v to the pump. The inertia switch has 12v at all times, on both wires, so I know it isn't the problem

 

[RonD]

OK, a carb on there.

I found an electrical diagram that shows an oil pressure switch supplying 12v fuel pump relay power, so engine needs to be started and have oil pressure for relay to get power.

Looks like a Red/Yellow hash wire to oil pressure switch and then to relay from a 15amp fuse.
Maybe fuse #18, hard to read that

So oil pressure switch could be bad or the fuse

battery------key-------15amp fuse--------oil pressure switch----------------fuel pump relay----------------ground

 

[70roadrunner]

Ok, thanks for the input! I will check that and let you know if that was it

 

[70roadrunner]

Thanks for the help. The oil pressure switch was the problem. I didn't crank it enough to make oil pressure. All is good now!

 

[RonD]

Good work getting it figured out