The 1986 420 sel had stalled while on the highway. towed it back worked on. There is fuel in the chamber, spark very week. While testing spark I had another car hooked with booster cables, while checking the distributor to coil wire all of a sudden the starter stopped turning. checked under neath there is juice to the starter but wont turn.
Please advise on possible causes.

There is something connected to the trainy that sometimes wont let the car start unless it is in neutral that has happened a lot in the past. even in neutral it doesnt turn. Is there blown fuse? I checked what i believe is the ovp relay the fuse is in tack. is ovp relay located next to the fuel pump relay and ac realay?