Auto trans 46re issue

46RE is based on a 727, the problem you have is called converter drain back...the converter will drain back into the pan....and once started it will need to refill itself before the truck will move.....and when it starts in park the pump does not circulate fluid to the converter...

after starting truck ..put it in neutral for a minute...then the converter will be filled before you move...

to fix the problem one of the valves in the valve body must be replaced by a Sonnax replacement or put it in neutral