Hello all, have had a 515 that did the same thing, but is was not easy to figure out if your as slow as I am ha ha. If you pull the pcm connector behind the dash and have a schematic of the selenoid controls, you can isolate the dead lead. On mine it has been the selenoid harness right above the tranny housing and below the cab blower motors that was the problem, but I had to wiggle the wires while testing for continuity to get it right, ( although im probly not right yet haha)
know this is an old post, hope you already found the fix already
hello to all and a great site