So, my cousin has an 07 Lariat 5.4 that has a strange electrical issue that has me stumped. Unfortunately, he is in North Louisiana and I am on the Texas Gulf Coast, so any advice will have to be relayed and they’ll be mucking around with it, not me.

sometimes, apparently it is random, if the headlights are on, when he turns the key to kill the ignition, the engine will continue to run until the lights are turned off manually. Ignition switched off and key removed.

also, twice randomly they have opened the door to get in the truck, and the radio was on before they ever put the key in. He said everything shut off like normal when he got out.

never been flooded, it was bought from a family member who upgraded and had no need for it anymore. Truck was well cared for prior to them getting it. He just got his license, and I don’t want him getting stranded somewhere.

thanks in advance for any suggestions.

 

This kind of stuff can be nearly impossible to track down. But sounds like their is a short somewhere. Probably somewhere behind the dash

 

I was thinking the same thing, but
without being able to put hands on, I was hoping someone could offer a suggestion of something specific to check.

 

I don't have an idea yet but we do need to know if lights are on auto or in the on position,lariats have auto lights . Evidently there must be some kind of back feed from the light circuit to the run circuit; It could be some miss wiring or possibly a bad diode somewhere. You may have to use some blocking diodes to prevent this during testing to see which direction its coming from . You have some delay systems already that could be suspect . Most people screw wiring up on the radio circuits as they change things out.
The radio stays on until you open door so that;s one suspect path . So also does it do it with radio off . This will not be real easy to pin down .Get your voltmeter ready and some drawings . Some cheap 10/20 amp diodes would help.They only let dc flow one way .

 

It only happens with the lights in auto.

ive been trying to find electrical schematics for the truck, but haven’t had much luck. There has to be somewhere the systems cross paths that is shorting our randomly. The only place I can think they would is the interior fuse panel and the power distribution block under the hood.

 

The fpdm is a place where signals can get crossed up when corroded aluminum inside the case starts falling onto the circuit board.

 

^good point. when the FPDM went out in my 04 the truck lost its mind. i mean likes flickering, clicking sounds behind the dash, it was crazy. once i put a new one in all was good from then on out. never had any signals that it was going out either it just crapped out all at once

 

Just to clarify, the FPDM is _ _ distribution module?can y’all clarify please? Thanks

my google search brought up the fuel pump drover
module. I’m thinking that’s not right.

 

Fuel pump driver module is correct. It is up on the frame above the spare tire. It's pretty easy to inspect and/or swap out if needed. It's a common failure item due to dissimilar metal corrosion the aluminum housing will always lose out when bolted straight to the frame.

 

it’s a place to start, thanks. Interesting that it would have that kind of effect on al sorts of things, but can certainly see it keeping the engine running if it shorts out and supplies power to the pump.

do you know of any way to test it, or is it more visual check for corrosion? If in it were mine, or if I was there where he is, I’d just pull the thing and figure it
out. But this is a 16 year old kid 5 hours away, lol.

edit: found some info on google. Gonna give him a shout and see if I can explain it to him.