90 Cutlass Supreme 3.1 Tranny Trouble

[Spingiri]

Had engine work done in April. Ran fine after. Was out of the country for 3 months and car sat in front of the house. Trans fluid was low when I got back. Car runs well, but transmission now shifts very rough 3rd to 2nd, and especially 2nd to 1st... with a loud "THUNK." Changed fluid and filter, and fixed slow leak. Fluid level is now fine. Problem of rough shifts persists unchanged. No idea what I am dealing with.

[GTP Dad]

Welcome to AF!!

Go to the autoparts store and purchase a can of Sea Foam trans additive. Pour it into the transmission and drive it for a while. It sounds like you have some sticking solenoids that are causing the hard shifts. The fact that the car sat for three months it is possible that that caused the issue. The seafoam should eliminate the problem or at least improve shift quality.

[95supreme]

The loud thunk isn't a good sign. These trannys are notorious for having issues with them. It seems like your tranny fluid got gummy and may have thickened over the time the car was sitting. This usually can happen if the gasket seal isn't 100% and some air can leak in. This can happen too if the tranny pan gets a little bent from rocks on a gravel road. Best to have the gasket and filter replaced. When you check the fluid, is there any residue left on the paper towel after you wipe the dipstick? Look for dark grey or black. If so, it's metal from the gears. If your fluid is also pretty dark amber, then it's definately time for a flush!

[troy1]

Check the modulator and see if its leaking. If you take it off and tip it up fluid shouldn’t run out the hole where the vacuum line was hooked up. If it does time for a new mod. Take out the little one way vacuum deal in the line next to the mod and see if that helps. Get some new vacuum line from a auto parts store to eliminate the one way vacuum diaphragm. while you have the mod off make sure the valve behind it is not stuck take a small magnet and pull it out. It only goes one way so pay attention how it comes out.

[Spingiri]

Considerable improvement and thanks for the quick reply. Still a pronounced and worrisome "thunk" (but not as bad), especially shifting up or down between 2nd and 3rd. No harm in using a second can of Seafoam, right?

[Spingiri]

Thanks for your thoughts. I expect you are on to something with the seal. I will change it. (There still appears to be a slow leak... and that may well be the source). Meantime, the dipstick comes out clean and clear.

[galoosh]

I had a similar problem with my 94 3.4 convertible - the upshift from 1st to 2nd would snap your neck back! I brought it to the local tranny shop, he just changed the filter and fluid (lots of clutch material) and he cleaned the accumulators - apparently they become clogged over time. Tranny shifted like new from that point forward! Cost just over $200 and was money well spent!