th400 trouble! Tranny Gurus Please HELP!

[akincaid]

Bear with me, this is a little long, sorry. Had a recent rebuild done by a friend of a friend. Did get the majority of my money back, but would rather have a working trans. Put it back in and didn't want to shift right. Checked all the normal stuff, vacuum lines, modulator valve, governor, all that was ok. After a lot of trial and error found the front band servo was put together wrong. I reassembled the servo and thought it was ok, drove great and shifted really well. Stopped for gas and wouldn't shift to 3rd after starting back up. got it home and let it sit til the next day, ran great, drove it about 10 miles, shifted fine, until I shut it off and restarted it, no third gear. Cannot for the life of me figure out what it is. Has to be something getting hot. Pulled the modulator spool out while it was hot to verify it is moving freely. no issues there. sometimes you can shift to 2nd or Neutral and back to drive and it will shift up but not every time. Had a spare valve body so I swapped that out to eliminate that, no change. Could my kickdown solenoid be getting hot and sticking upon start up? Really running out of options. Took it to a local shop that does free diagnosis, They were not sure either and want to do a rebuild on it to find the problem. Any suggestions would be appreciated!

 

[HotRodPC]

In this type of situation, it's obvious a bad build and hard to say for sure what someone else did wrong. If I had to take a guess, there's a torn or damaged lip seal in the direct drum or forward clutch drum.

Why I suggest this is, both of those clutch packs are needed to make 3rd gear. If it's only got the problem when hot or warm, it's likely the seal are not damaged bad enough to not work all the time, but when the seal get warm and more pliable, and the trans fluid gets hotter and thinner, the fluid is able to leak past and not compress the clutch drum piston. Orrr, it might be a broken pump ring. As the trans shop said, it would probably be required to be opened up and gone through.

 

[crazy4offroad]

I don't think a folded or cut apply piston lip seal or broken pump ring follows the narrative of "warmed-up 3rd don't work, cooled down it does, sometimes can make go in 3rd when warmed up". My first guess would be not enough clearance in the direct clutch pack, or maybe not enough overall end play. Either of those conditions might be heat-sensitive. But like HR said only way to tell is to take it apart and start over, and verify everything.

 

[HotRodPC]

Every time, not once or twice, Every time I had a trans that worked when cold, but quit warmed up, it was a seal problem. Stiff seal with thicker trans fluid CAN sometimes still compress a piston. Once the seal(s) are warm and pliable, fluid thinner after getting warmed up, fluid just leaks past the seal and no go. I've never seen a trans that did not work cold but would work warm.

Clearances could certainly be a problem too. When you pulled the pan off and fixed the servo or replaced the valve body, how did the pan and fluid look? Are you getting any shavings at all in the pan? If end play was to tight, you should see some bronze babbit or silver shavings in the pan. If end plays are to tight, something should be scrubbing, rubbing or cutting. If end play is to loose like a thrust washer or bearing forgotten, maybe a clean pan and any # of things could eventually go wrong depending on where the part was forgotten, or tolerances to loose. You can still have shavings with loose end plays too cuz something in the tunnel could be slamming into something it shouldn't be, hence Thrust Washers.

Again, no telling really without taking it apart. It's much easier to diagnose a good working transmission that has failed, than diagnose one that never worked right.

If it were me, I'd probably get a $15-$20 complete seal and paper kit and just go through it completely measuring tolerances, air checking drums, checking end plays, looking for scores in hard parts where something is hitting something it shouldn't be, making sure I've got thrust washers / torrington bearings where I'm supposed to, checking all the sealing rings, check balls are in correct places.

 

[akincaid]

Thank you all for the input. I guess ill just break down and take it in. Guess I'll get the wallet out.

 

[74 Shortbed]

Could guess at that forever and not get anywhere, could just be a piece of dirt who knows, I'd just tear it all down and clean it all up and put it back together checking everything as I go along, 400's are about the simplest trannys to do..