F150 89' Fuel supply

Neither fuel pump runs. Replaced the fuel pump relay which seemed bad. Checked the inertia switch which opens & closes OK. Odd thing though is with key on, and inertia switch unplugged there is 6-8V in the pink/black line coming away from the inertia switch (a short in wiring to pump?).
Tested the wire connector to the high-pressure pump and it reads 12V when key on (not just for a second or two but stays on). Finally pulled the high pressure pump and tested with 12V off the battery and it runs. I rely on truck only for plowing my long driveway, its not on the road any more - so bypassing some things is not the end of the world. Any ideas?

Your tests narrow it down a lot. The pump runs on direct battery power, and you read 12 volts at its connector, but it will not run in the truck. That combination usually means one of two things: the circuit cannot carry current, or the meter is reading a voltage that is not really there. A meter draws almost nothing, so it will read 12 volts through a corroded connection or a few strands of wire, and the voltage collapses as soon as the pump tries to pull real current.

Two checks:
1. The ground side. Follow the pump's ground wire to where it attaches to the body or frame, then clean and tighten it. A bad ground looks fine to a voltmeter with nothing connected.
2. Load test the feed. With the pump plugged in and the key on, back-probe the pump connector and measure the voltage there, or put an incandescent test light across the harness side. If the voltage drops way down under load, there is resistance upstream: the relay contacts, the inertia switch contacts, or a connector in between.

One more thing about the readings. The 6 to 8 volts you saw on the pink/black wire with the inertia switch unplugged is not a short. The computer watches the pump circuit through a monitor wire that carries a few volts and no current, and a voltmeter picks it up any time the circuit is open. The 12 volts that stays on at the pump connector is the one to question. On these EEC-IV trucks the computer runs the pump for only a second or two at key on unless the engine is cranking or running, so put an incandescent test light on that connector instead of the meter. If the light stays bright, something is holding the relay on or feeding the circuit. If the light is dim or dead, the meter was reading a phantom voltage and the pump is not being fed at all, which puts you back at the relay socket, the relay control wire from the computer, and the inertia switch.

Dana