John Deere 318 electrical problem. Dies when I engage transmission

[GooseRancher]

I inherited a John Deere 318 that was working fine. I had to replace a battery cable because the terminal clamp was broken. Everything seemed fine, and it started up and ran after the replacement fine. Then I saw smoke that smelled like burning plastic. I disengaged the battery cables, and looked for the problem. The end of the battery cable that I got had a lot of exposed steel, and because it was such a tight fit around the starter, it had rotated into contact with the chassis and created a short. I next found that the two wire connector to the front PTO was melted and burned. I replaced the connector and started the tractor back up. I started fine, but when I engaged the front PTO, the 20 amp fuse burned out immediately. I thought, fine, don't need the front PTO right now (just the back), so I disconnected it. Then, started up fine, but engine dies when I try to engage the transmission. I next found that the 3 wire connector to the voltage regulator was a little melted. So I replaced that connector. Problem still there. I took a long look at the wiring, and couldn't see any more problems. Chasing the problem around the engine, I found what I *think* might be the problem. My multimeter was showing that the oil pressure switch is closed when the engine is off. Is it supposed to be? The oil pressure light stays on after the engine is started. Looking at the wiring diagram, there is a direct circuit from the PTO/Transmission switches through the oil pressure switch. Could something have been messed up on that line when I created the short with the battery cable?

I've been wrestling with this issue for some weeks now, and I really need the tractor and back pto going so I can till my garden. Any help would be really appreciated!

Thanks!