Some months ago I changed the starter. This morning I was checking under the car and saw the black wire hanging. I think I tucked it away before and forgot about it. Car's been starting, however. Well, so I think this came from the starter bolt to the engine?
I have 1996 4.9l manual. I am going to bolt it to the starter bolt, but sure appreciate your confirmation.

 

Do you have a picture? There are supposed to be 3 wires to the starter, the red positive on the solenoid, the ground cable on the side of the starter bolt and the small ignition wire that engages the solenoid.

So yes that black cable is supposed to go to the side of the starter, but the only way i could imagine it falling off is if a bolt was missing.

 

You should have a large cable from the engine to the frame (clean bare metal). When you start the truck you are pulling several hundred amps. If your engine is not properly grounded (good path back to the negative battery terminal) your vehicle may not start or it might find an alternative ground path.

I had a buddy destroy his shifter cable after leaving the motor ground disconnected. The shifter cable was the path of least resistance (ground) for the starter. It ended up getting very hot and melted the sheathing ruining the cable.

It is common for these cables to get left off when an engine is pulled. It is also possible that a vehicle will run just fine without it if there is another path for the current to follow, then again you also might start melting/burning stuff if the cable is not there

 

Thank you. It was just a newbie mistake. I put the ground cable away for better access to the ignition wire and just forgot about it. It makes me wonder what the ground cable is for. The starter was working without it.

 

Yes it goes on the starter bolt. It attaches to the frame higher up then goes to the battery. There are other engine to body grounds which is why it was starting fine.

 

It starts fine using other ground paths. However the greater resistance, the heavier the load. So your battery, solenoid, and starter have to work harder. Because the starter uses so much amperage it is given its own ground wire. Otherwise power travels through the mounting bolts, painted brackets, to the engine block which is hopefully grounded before reaching the body ground then the battery.