Somebody with a TH400 diagnostic chart- Can you look something up?

[FWP]

1-2 late, harsh shift. Can you list the possible causes for me please? Everything else is normal on this tranny, except the 1-2. TIA!

 

[tomseviltwin]

vacume modualtor

 

[dave_90_blazer]

vacume modualtor

first thing to check, pull the hose and look for fluid

 

[Ryan B.]

Possible causes:

Fluid level
Vacuum modulator
Missing check balls (not possible if it worked normal and just started acting up)
1-2 (rear) accumulator
burned intermediate clutches