Ok, Think i am figuring this forum thing out....
2006 5.4 fuel pump fuse blew the other day while driving down the road....
so i replaced it, it blew again...so after research i jumpered the FPDM module connector and got the fuel pump to start humming...ok, so it must be the fpdm...bought a new one replaced it, (old one had no corrosion on it at all came from new mexico) started up the truck just fine...fixed problem right? died in less than a min...fuse blown again..reseated the connector on the fpdm, messed with wiring harness's replaced the fuse, and poof...another blown fuse..#40 20amp....did this sorta thing over and over again, finally pushed in the inertia plunger a dozen times, it was never popped up, then moved around harness in the engine compartment, behind the fuse panel, and replaced the fuse, started up the engine and got it running and left it like that for about 45 mins or more...gravel roads, pavement, high acceleration etc..no problems...so i wanted to know what the current was at the #40 fuse, so i pulled the fuse and put in a Fluke AMp meter in line with a 10amp internal fuse and saw 2.6 to 2.7 amps draw with it running...any thoughts?

 

Likely scenario: You HAD a short circuit to ground. When you moved the harness around, you moved the wiring enough so that the bare wire is no longer making contact with the metal of the truck.

Prognosis: It will come back.... you might want to buy a self-resetting circuit breaker.

 

Either that, or maybe you had a bit of corrosion in the Inertia Switch that was causing a high current draw. You cleaned it off when you pushed the plunger down a dozen times.

 

A bit of corrosion that causes high resistance will have exactly the opposite effect - remember Ohm's Law... it will reduce current.

 

Not if the corrosion created a path to ground.

 

That would be extraordinarily unlikely. Look at the corrosion on many battery terminals. Do you see any of them drawing excessive current (> 20A)?

 

I found the likely issue...i replaced the fuel filter last night, and followed the wire harness from the back to the engine compartment..once it started with the heat shielded silver colored wrapping, there was a place where it laid across the frame of the truck and after lifting it up and inspecting it with a bright light, i could see it had chaffed in two places, the silver heat wrap was all chewed up, but initially i didn't see that because i was looking from the top view of the engine compartment...it was likely that when i was inspecting the harness, where it came out of the firewall, veered off to the relay box and also down to the frame wall on the drivers side, i had moved it enough away from the frame that the power lead stopped shorting against the frame...i didn't see any broken wires, just the exposed wire bundle laying against the frame..i wrapped it multiple times and used some zip ties to keep it away from the frame...i had actually noticed another spot where this was occurring as well, but not as chewed up as the first one that was closer to the engine relay box. I'm pretty sure that must have been it, I haven't had any issues at all now. and i don't think i need to replace the fuel pump. I do find it bizarre that it was only drawing 2.7 amps at idle..everything i have been able to find points to 5 amps...i cant get any specs for fuel pump current draw on this fuel pump...