Alright, found another problem of many problems with this truck. I'm already $2,200+ in parts, so I hate to look back now.

If I turn the headlights on with the key in the run position, the fuel pump runs for a few seconds and turns off. When turning the headlights on with the key off, the pump doesn't run.

Anyone ever experienced this before?

 

The pump cycles (as commanded by the computer) for 1-2 seconds when the key is turned to "on" to pressurize the fuel rail.

It does this by "grounding" the fuel pump relay which is "hot" all the time with key "on". The computer will not let the pump run longer unless it "sees" a signal from the distributor.

If the pump runs continuously with key on, engine not running, this indicates a problem within computer.

Sounds like from your description that you are seeing the normal operation & ascribing the behavior to switching on the headlights, but I could be wrong.

If you turned the headlights on, key on, & the pump ran continuously, that would be an issue. Possibly from a melted headlight switch.

What year?

 

Sounds like a short in the wiring or a sensor.
What is happening is the short is causing the PCM computer to reinitiate (like it is just got power for the first time).
Evey time it reinitiates the fuel pump will run for one second and quit with the engine not running.
I had this happen to my truck when I had a short in the O2 wiring by my starter.
I reran my O2 wiring away from the exhaust manifold.

Not saying that is where your short is.

Maybe one of the headlight wires is shorted to a sensor wire.