Since this is a THM 350 tranny, I'm guessing it has a bit of age on it. I'm also guessing that (prior to this time) the fluid has not been changed often - if ever. Rule of thumb, never, never change the fluid in an older tranny unless it has been done on a regular basis. What you are now experiencing is the result.
The reason? Over time, sludge and other tiny particles (mostly steel) build up, in the filter mostly, but also throughout the transmission. When left alone these things usually never cause a problem -- that is, until you drain out the old fluid and put in brand new. The brand new fluid is just brimming with brand new detergent and it immediately starts doing its job of cleaning things up. It dislodges these sleeping metal particles and sends them sailing through all those nice rubber seals that make your transmission work properly and very soon afterward, your transmission no longer works properly. I believe that is where you are now.
The same goes for the 400 also, either change regularly or never change. Looks like you are looking at a rebuild. You have lots of company if it makes you feel any better. Every single one of these that I personally know of (that would be at least six or seven including 3 of my own, apparently I Iearn slow) failed in this manner very soon after the change.
Answers & Comments
Since this is a THM 350 tranny, I'm guessing it has a bit of age on it. I'm also guessing that (prior to this time) the fluid has not been changed often - if ever. Rule of thumb, never, never change the fluid in an older tranny unless it has been done on a regular basis. What you are now experiencing is the result.
The reason? Over time, sludge and other tiny particles (mostly steel) build up, in the filter mostly, but also throughout the transmission. When left alone these things usually never cause a problem -- that is, until you drain out the old fluid and put in brand new. The brand new fluid is just brimming with brand new detergent and it immediately starts doing its job of cleaning things up. It dislodges these sleeping metal particles and sends them sailing through all those nice rubber seals that make your transmission work properly and very soon afterward, your transmission no longer works properly. I believe that is where you are now.
The same goes for the 400 also, either change regularly or never change. Looks like you are looking at a rebuild. You have lots of company if it makes you feel any better. Every single one of these that I personally know of (that would be at least six or seven including 3 of my own, apparently I Iearn slow) failed in this manner very soon after the change.