That “thing” on the side of the engine you mentioned is the injection pump controller. It rotates to 3 different positions. “STOP”, “START”, and “RUN”. If you have an extra person have them crank the engine and see if it moves during cranking. Sometimes you can hear the mechanical motion from inside the cab if you listen closely. If the controller pump isn’t moving to the “START” position while cranking, try disconnecting the transmission connector located near the glow plug connector and see if it cycles itself. This will tell you if you controller is working. There’s a few other things you could try but if you search the forum or type in google “SD22 DPC module” then google will direct you to this website with the specific forum where others go into greater detail about the operating system and doing a field test on it. Also if you PM me I can pass my info to you and see if I can help at all to show you how I operate mine. A lot of people have gone to a cable and knob system, which I haven’t done yet, but may need to if the problem stays intermittent. It’s not fun popping the hood to get the truck started half the time 😅