Had a slow start issue with 1995 F250 7.3 turbo diesel. Batteries checked out fine, alternator failed, changed alternator. Still same issue, and starter failed test. Changed starter and starter relay, now battery acts dead. At starter relay and glow plug relay, I have 12.7 volts on one post and none on the 2nd post. If I have hood lifted, the factory hood light is lit. I can play the radio. When I try to crank engine, there is a loud click, hood light goes out and battery appears to be dead as a hammer. There is 12.7 volts on BOTH big posts of starter relay and glow plug relay now as well as 12.7 volts at starter. If I wait about 2 minutes, the hood light comes back on and there is only voltage on one of the posts again, instead of both. Also, when hood light goes out, entire fuse panel has almost no voltage (mV). Can it be a fusible link or wire going to PCM. Confused. Any help would be greatly appreciated.
Thanks so much to Bugman and Cortes977.3 for the info. Changed all battery terminals, took off grounds and wire brushed and sprayed with corrosion wash. Noticed very little corrosion. After cleaning up all grounds, truck acted exactly the same way. Noticed the grounds from both batteries went directly to engine ground and cable from engine ground to frame was 12G max, which surprised me. Went to AutoZone and connected 4G cable from engine ground directly to frame and it fixed it. Truck spins over super fast now. Wish I would've thought of this amazing site before buying a new starter, starter relay and alternator. At least I don't have to worry about them in the future.