1994 ford f-150 4.9 6 cyl. How could I be getting low voltage to the inertia switch ( red lead)
Bad contacts in the fuel pump relay.
Ford had 2 methods of wiring the fuel pump, pump relay, and inertia switch. One of them involved running the power for the pump through the inertia. The other involved the inertia switch being used as the trigger circuit for the pump relay with power being fed directly to the pump from the relay.
I’ll check the manuals tomorrow morning and see which one your truck has. Maybe the low voltage is the trigger circuit for the relay??
I looked at your wiring diagram, two wires at inertia switch. One carries voltage to inertia switch, the other circuit carries voltage from switch. The voltage reading on both should be the same. The red wire is voltage from inertia switch, the inertia switch may be faulty. You could use jumper wire around the inertia switch, just for testing. I wouldn’t leave it that way, that switch is a safety feature. Just my opinion.