Do you have a junction box in engine compartment on the passenger side (my SD does)? If so, jump the connections & see if it cranks. I don't remember if the ignition switch needs to be on. I do that when I want to bump the engine using my 35 yr old switch like we used to do when setting point gap. That will let you know starter, solenoid & wiring to solenoid are good. FSM should have details on which terminals.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.