I have been having fuel supply issues on my 1988 F150 300-6 which led me to change the in tank pump first, then the high pressure pump. After the last test drive I parked the truck and upon return, no start, the pumps wouldn't turn on. I replaced the fuel pump relay which had shown serious heat. After changing it, the truck started. I noticed the marker lights were coming on without turning on the light switch. I then found the wires under the dash to the inertia switch had melted which had shorted to the marker lights. The wires going to the fuel pump relay got hot enough in the short time to give me a minor burn on my hand. I had put the original factory high pressure pump back due to concerns about the aftermarket replacement being hooked up backwards due to no instructions after the truck died and was found to have been fine. I checked the wires at the pumps and neither were hot.

At this point, only the in tank pump is new. I am concerned it is for some reason drawing too many amps causing the mayhem. What I don't quite understand is why the inertia switch wiring melted. I believe that to be control side of the relay, not power supply.

So am I on the right thinking that the pump may be causing all this, or did the relay fail spectacularly and now not making good connection leading to the heating of the wiring? I can change the pigtail if that will resolve that issue. Thanks for any help on this !

 

According to my 1994 FSM, the inertia switch is on the power side of the fuel pump circuit. Easiest thing to do would be to remove the relay and use a multimeter on the AMP setting to see how many amps your drawing. It should be 4-6 amps. If it's more than that, remove the connector from one of the pumps to see which one it sucking up the most amps.
If you're melting the insulation off wires, then you're probably pulling 15-25 amps.

 

That is interesting considering they are considerably lighter gauge wires.

 

Did you ever figure this out? I'm having the same issue!