Riding Mower Engine Will Not Crank?

I have an 18.5 H.P., Craftsman lawn tractor, model #917.275760. The problem is this: It will not crank up. The battery voltage shows 12.5 volts. I have changed the: ignition switch, interlock switch, seat safety switch, module reverse ROS (switch), starter solenoid, and starter. I have 12.5 volts to the starter solenoid, but no voltage to the starter side of the solenoid.

I have the four post type solenoid, two posts for the wiring coming from the battery (+) and two smaller spade connections. I don't remember which wire connects to which spade. The wires are color coded, one is coded orange and the other coded black. I know that one wire comes from the ignition switch and the other is the ground wire. I have reversed the two spade wires at the solenoid several times with the same results, the engine will not crank. (no voltage). All safety switches are making contact. Of course, jumping the starter solenoid, using a screw driver, will crank the engine. I'm at a loss, what more do I need to do?

There is a protection wire loop in the wiring circuit, I looked at this loop which is under the seat, near the seat safety switch. This is a covered insulated wire loop, color coded gray. This wire comes out and loops back into a small connector that is connected at the end of the wiring harness. I really don't know what happens if this wire loop is burnt out. I have no way to know this, since the wire is covered. I have looked at this loop, there is no burnt wire indication. Since this is a protective wire arrangement it should have a visual means to check it for a burn out condition. Why not use a regular fuse for this?

Can anyone give me more information, what to do next? This is really a simple electrical system, but it sure has me stumped. There is something else you ought to know, when everything was operating correctly, the engine was cranking alright, I installed a new mower drive belt. The belt was much too tight, when I tried to start the engine, it whined, but it has not cranked since. I thought I changed everthing that might have burned out, making the engine not crank. What say you? Let me know if you have an answer to this riddle. Thank you.