That ignition switch might be an anomaly in our fleet. This switch has a metal body. Therefore the body itself would be immune to melting and sagging. All the other ignition switches I have seen in our fleet have a plastic body and are configured for a "4 wire connector". But this truck is a non-TWC 7F whereas the TWC trucks might have a 4th wire in their connector............I will have to pay closer attention to the connector next opportunity I get to see a TWC model switch connector. Perhaps the metal body switches were used in the earlier 7Fs without TWC. To be honest, this situation is the first where I needed to investigate the circuits and components associated with the AM1 fuse. Never before saw this problem. Previous fuse blowing events on other 7s & 8s have all been on a branch circuit and caused mostly by wire breakage due to the extremely "vibrant" 4Y engine.
No fusible links in this electrical system. The alternator charging current feeds into the same buss bar that feeds the White-blue circuits. The alternator charge output current goes through an 80 amp plug fuse to feed the buss. This buss is connected to battery + through a heavy gauge cable. Because the AM1 fuse is fed by the same buss, a short in the cable from buss to alternator output would blow the 80 amp fuse, but not the 40 amp AM1 fuse.
Sure would be nice if members could post photos to a thread because if we could, I would post some non-copyrighted, photos of the actual wiring here.