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I have no power to the front fuel tank (F150 '90). How do you go about shorting the wires or whatever to determine if that is the problem. I have just put in a new FDM.
You can remove the plug from the back of the selector switch - It has 6 wires. The center two get switched from the upper two to the lower two. One side is switching the fuel gauge senders in the tank and the other side switches between the two fuel pumps. I used an ammeter with one lead on the center pin of the cigarette lighter (for a power source) and the other lead to the wire for the fuel pump you want to test
On my 1990, the front tank fuel pump is the red wire and the rear tank fuel pump is the brown/white wire. The fuel pump will run continuously when you hook it up and the ammeter will measure the current
What about just jumping the red wire to ground? Would that make it run? Also the gas gauge on the front tank pegs out on the full side, past the F mark. I was wondering if that means it is getting power, but the pump is not? I just changed the pump (FDM) so it should be good.