strange T-350 shifting
hey guys, i'm back after lurking for awhile. I've tried doing some digging around and can't come up with anything that matches my problem. Maybe i'm just not wording my searches right or whatever.
rebuilt th350 with a little over 500 miles on her. had new clutches installed, pump checked out, new adjustable vac modulator. everything was supposed to be good to go. valvebody was looked at and was supposedly good. It was rebuilt by a transmission man that has been in business for over 45 years.
after i got it out of the shop, i babied it (i still am), no hard launches, no spinning the tires.around town it drove and felt great (35-45 mph range) all the gears were there no slipping. i had to go out of town a few days ago so i finally got to try it on the highway......that is when **** went south.
after i brought it up to around 55-60mph range it cruised great for a few miles barely turning 1800 rpm, then for no reason it dropped down to 2nd and started winding up and wouldn't go back to third. i had to manually put it in neutral then shift back to D for it to go back in third. i thought, ok wtf. maybe just a one time thing there. nope. about every 1/2-3/4 mile or so it would do it again, didn't matter if it was on a hill or perfectly flat ground.
so i tried unhooking the detent cable and making sure it was in the closed position, no change at all. problem still persisted. so i checked my vacuume lines (they are new) but no problems there, i adjusted the modulator both ways. no change.i checked the linkages, all good there too. i'm running out of ideas.everything on the truck is mechanically new with less than 750 miles on motor and trans.
what could make it work great crusin around town but absolutely fall to **** on the highway like that?
one more thing, SOMEtimes i could get it to catch third again if i stabbed the gas to the floor real quick, but most of the times i have had to shift to neutral then drive again.