1986 Ford F-250 fuelpump

  • 1986 FORD F-250
  • 7.5L
  • 150,000 MILES
Wanting to know things I can test before just going ahead and dropping the tank to replace the whole fuel pump? Is not turning on. I normally hear it loud and clear.
Do you
have the same problem?
Saturday, June 20th, 2015 AT 5:49 PM

1 Reply

Have you checked the inertia swithch?

If you're expecting the fuel pump to run with just the ignition switch turned on, that's a common misunderstanding. The pump should only run for one or two seconds after you turn on the ignition switch to insure pressure is up for starting. After that, it only runs when the engine is rotating, (cranking or running).

Pumps are diagnosed by checking for voltage on the 12 volt feed wire. Most mechanics use a test light so they can see it from inside when they turn on the ignition switch. If they see 12 volts for one second, next they check the ground circuit for continuity. If that is good, that just leaves the pump.

If you only have a digital voltmeter, you either have to place it so you can see it from inside the vehicle, or you need a helper to crank the engine. Often digital meters don't respond fast enough to give an accurate reading during that initial one-second pulse.

The test for 12 volts is much more valid if it is done with the pump plugged in and you back-probe the wires through the rubber seal in the connector. Better yet, you should hear the pump run. The problem with testing with a voltmeter and the pump is unplugged is there can be a corroded connector terminal or a break in the wire with just a tiny carbon-track left, and the digital meter will pick that up and give a false "good" indication. You won't get enough current through corrosion or carbon-tracking to run a test light or the pump, so those will give a more accurate indication if the circuit is okay.
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Saturday, June 20th, 2015 AT 7:51 PM

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