The FSS (Fuel Shut off Solenoid) is the way our trucks can turn on and off the engine. First off it is one of the most needlessly replaced parts on these engines. The most common way to kill them is the starter to stick on giving the "Pull" wire power for a long period of time, which produces a lot of heat inside the FSS, and it burns up, you most likely don't need to replace it unless it looks burnt. If your FSS is not working correctly then it's most likely your wiring or a dirty shaft.
When the FSS lever is down, the Injector lines will not receive ANY fuel.
Shut off solenoid location:
And this is the arm that needs to be up to let fuel in the injector lines, this is under the shut off solenoid. The rubber boot and the plunger coming out of the boot are part of the shut off solenoid. the lever the shut off solenoid pulls up is part of the Injector Pump
Here's a wiring diagram: (I would recommend clicking the bar on top the image to view it full size)
There are 3 wires going to the FSS, Ground, Hold and Pull.
wire, truck side is black with a orange stripe, FSS side is all black, it is connected to ground all the time.
wire, truck side is small light green with a black stripe, FSS side is normal sized Red wire, the Hold coil (an electromagnet) gets power when the key is in the run position. Since the plunger is spring loaded to push the lever down (turn off engine). The Hold coil Does not have enough power to pull the plunger up (compress the spring), and only has enough power to keep the plunger up, once it's all the way up (keep spring compressed). So in order for the hold wire to hold the lever up, the Pull coil must bring the plunger all the way up first.
wire, truck side is Red, FSS side is White, the Pull coil (an electromagnet) gets power only when the starter is engaged. it has plenty of power to pull up the lever (compress the spring), but if it's kept on to long it will create a lot of heat and will burn up the FSS. the Power wire gets power from the FSS relay(Part# SS70A), the relay gets its signal power from the small wire coming off the starter.
If you're ever on the road and it shuts off or won't start then you should be able to tie that lever up with a bungee cord, some wire, rope or something. that lever needs to be up for the truck to start.
You can test the FSS by trying to start the truck and then leaving the key in the "Run" position, feel under the FSS to make sure it's all the way up, if it's all the way up then it shouldn't be your problem, unless you have a lack of power problem then the lever might not be going all the way up then you can test it by going HERE
and check that the arm is really going all the way up, if it isn't it could be that your shaft is dirty or rusted or simply it might not be adjusted correctly.