I have a 1989 FORD F150 XLT lariat 4X4 my problem is hard but maybe simple i replaced my starter truck started came out the next day truck wouldnt start without a jump.thought it was the battery replaced it didnt help took the alternator and tested it altenator was good (internal voltage reg) so i started replacing relays and harnesses checked my fuses,and ended up replacing my altenator anyway also my starting solenoid.so my problem is this my truck will start with a jump and run my voltage reading on my gauge inside the truck is low and not showing any kind of good voltage,and when i shut my truck off i cant start it with out a jump.
i really need help for i use my truck for volunteer work for my church and also work i really need it bad thanx to all who give this a shot

 

It might sound like a stupid suggestion, but have you had the new starter checked? I had a similar problem, turned out the "new" starter was actually defective. I think that would be my next step. (If you haven't already done it)
Good luck

 

Check your cables from the battery to the starter, and from the battery to motor/body.