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Discussion Starter · #1 ·
Been trying to fix this on my own but now I must turn to those who are much smarter than I (which means most of you!)

Symptoms - Engine starts right up but runs rough. Fuel pressure reads 0.0 psi.

Fuse ok. Power on both sides of inertia switch. No power at fuel pump. Ground at fuel pump ok. Direct power to fuel pump and truck runs perfect with 65 psi displayed.

What do you guys think?
 

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There is a relay... its behind the radio in the dash. Service manual pages attached. I believe its the auxiliary relay box highlighted in the attached image file.
 

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Fuel it up again Dummy
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My truck did the same thing when I hit the fuel pump shutoff after soundproofing. Looked like pump had 12v but it was no amperage of any kind.

Make absolutely sure thats reset on passenger side kick panel.
 

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Discussion Starter · #5 ·
There is a relay... its behind the radio in the dash. Service manual pages attached. I believe its the auxiliary relay box highlighted in the attached image file.
Hey thanks for that. I thought the relay was integrated with the fuse panel and was non-serviceable.
 

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Discussion Starter · #6 ·

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Discussion Starter · #7 ·
Correct me if I'm wrong, but if the relay were bad there would be no voltage at the inertial fuel shutoff, right? :dunno:
 

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that makes troubleshooting tricky

i would be tempted to jump from the inertia pump supply side to the pump

but i cannot verify it is the last in line after the relay ( the wiring schematic is a biatch to read )
 

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Correct me if I'm wrong, but if the relay were bad there would be no voltage at the inertial fuel shutoff, right? :dunno:
You are correct.


...
i would be tempted to jump from the inertia pump supply side to the pump

but i cannot verify it is the last in line after the relay ( the wiring schematic is a biatch to read )
The inertia switch is last in line. ...kinda. according to the wiring diagram, the PCM is wired to the pump on the same wire as the inertia switch. That suggests that the PCM has control in either a PWM sense or veto sense or both.

I'd do as @ZMANN suggests but with the existing connector removed first. Then again tapping into the existing wire. That will tell you if either the PCM is overriding your power or if the wire from the PCM is otherwise effed up.

You can find wiring diagrams here:

http://www.powerstroke.org/forum/ge...res-single-page-6-0l-wiring-diagram-pdfs.html
 

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Discussion Starter · #11 ·
<snip>

The inertia switch is last in line. ...kinda. according to the wiring diagram, the PCM is wired to the pump on the same wire as the inertia switch. That suggests that the PCM has control in either a PWM sense or veto sense or both.

I'd do as @ZMANN suggests but with the existing connector removed first. Then again tapping into the existing wire. That will tell you if either the PCM is overriding your power or if the wire from the PCM is otherwise effed up.

<snip>
See, that's what I was worried about and now am confused on how to proceed. Like, how will that tell me the PCM is overriding or the wire is messed up?

Solution for now is not to simply hotwire the fuel pump - that would work and would be ok if I was on the side of the road and just wanted to get home. But how do I go about identifying the exact cause and fix it the correct way? I'm leaning towards the wire being open somewhere.

I purchased a brand new fuel pump harness to replace my existing one (I had trouble removing the plug from the fuel pump (it was full of dirt and required some heavy persuasion.) My wiring was all twisted up and had zero strain relief, as if the PO did major mods. Not sure what he did or why.
 

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... Solution for now is not to simply hotwire the fuel pump ... But how do I go about identifying the exact cause and fix it the correct way? ...
I'm not suggesting that you follow Z's recommendation as a "fix" - if you could even call it that. Neither is he. We are suggesting that you do it for diagnosis.

We know three things...
  • You have power at the inertial switch and nothing at the pump.
  • The inertia switch is wired in parallel with the PCM command/hot wire.
  • You have no power at the pump.

There are three possibilities (in my mind)...
  • The wire from the PCM is damaged in some way.
  • The wire from the inertia switch is damaged in some way.
  • The wiring is all fine and the PCM itself is dragging it down.

So...
  • You disconnect the fuel pump and feed 12V directly in it and it starts and runs.
  • That means the pump is fine and it is wiring or PCM.
  • You plug the connector back in and splice into the command/hi wire. You then jumper that splice directly to the inertia switch output.
  • If it works, that means that the wire from the inertia switch is damaged.
  • If it doesn't work, move the splice from the inertia switch output to a splice into the PCM output.
  • If *that* works, then the problem is in the PCM-to-pump wiring.

That was to what I was referring before.

Keep in mind that some additional checks along the way with a meter might help you, as well.

______________________
Edited to add...

Incidentally, there is a Pinpoint Test (M) in the PC/ED for the fuel pump. It is attached below.
 

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Discussion Starter · #13 ·
Ahhhhhh I get it now! Thanks for the in depth explanation. Sometimes my brain requires well, y'know. :tmi::banghead:

I really do appreciate all of your help!!! Thanks guys............:thumb:
 
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