Looking for fuel shut off switch on a 1990 f150 pick up cant find it does anyone know where it is?

There is no switch other than to switch between tanks on vehicles with dual tanks. Look for a 20A fuse, or fusable link, inside the fuse box that is inside of the engine compartment (the power distribution box). There are two fuse boxes; one inside the cab that most are familiar with and the one inside the engine compartment (the power distribution box) which the houses a few relays, the fuses for the ignition and fuel pump. In the 1990 model they may even be fusable links instead of fuses. There is a cutoff switch for the fuel system. It is an inertia switch, designed to cut off the fuel delivery system in case of a collision, and there is a manual trip method. To locate, look under the dash on the driver's side, on the firewall directly beside the main wiring harness pass-thru, directly beneath the brake linkage pass-thru. There is a black switch body there, topped by a red indicator. This is the fuel-system inertia switch. You can disable this switch by using a flat-blade screwdriver and prying up (gently) on the indicator in the notch on the front. Once disabled, the fuel pump will not operate. Then, to remove pressure from the fuel system, attempt to start the vehicle. It may start, but will die once all residual pressure in the fuel system is exhausted. To reset, simply press the indicator back down into the switch. Before trying to start the vehicle, turn the key switch to "RUN", not "START," as this will cycle the fuel pump and bring the system up to pressure. Then, when you hear the fuel pump stop, try starting the engine. It should start in one or two attempts. This will keep you from wasting battery power for nothing. I found this method in a Haynes repair manual, and have used it on several occasions when my own 1990 F150 required repair.