. . . and yeah what rjolly87 said about the modulator and fluid level ^^^^^^^ . A little overfill should be fine with this one, but if it is low it will starve and do nonsensical things. The modulator is probably due to replace if it still has the factory unit.

Ok I assume this is like the modulator


I think I may have already replace that modulator once before trying to fix the problem. I'll check my records, I keep good ones because unfortunately we can't remember well years later.

Thank-you

Yeah that looks about like they all do. :)

No go on the modulator,these are the rockauto listings: https://www.rockauto.com/en/catalog/...modulator,8500
'If you replaced the modulator with an aftermaket unit, it may be good to check and see if it is adjustable. The adjustment is in the vacuum nipple. If it's stock, it will usually have the vacuum port on the side, after market will usually be out the top of the modulator.

ThankX right on!

Thanks Everyone!, the modulator Seems to work Great! So from what I can tell, it was clunky hard downshifting from 35mph from 3rd into 1st directly it seemed when floored.
Now it downshifts from 3rd to 2nd and then half a second later into 1st, making it a smoother process and not clunky like before.
I got the Alto 062501 modulator on Amazon for $21.
Rockauto still won't ship to my town because they don't have city tax collection set up...

Here's the Amazon review I just posted on the product:

My 123,200 miles 1990 Overdrive Buick Century 3.3 v6 transmission would downshift harshly at about 35mph when flooring the throttle downshifting from 3rd to 1st, very harsh and clunky.
Replaced large original with this black band modulator.
Now the problem if fixed, the transmission downshifts from 3rd to 2nd and then half a second later into first so it's smooth. Evidently this transmission isn't designed to go directly from 3rd to 1st skipping 2nd.
Installation was tight for the hands but used a flashlight and very doable. 1/2" socket with an extension to remove the retaining bolt that is on the left side along with the metal retaining fork. Did not have to try to remove the wire harness old fragile plug connection that was nearby, I was careful not to stress those wires.
The instructions do require adjusting of the small screw inside the vacuum nipple for some pre 1981 vehicles but otherwise like mine it's already adjusted from the factory.
Cleaned mounting surface from dirt and lightly oiled new o ring area.
Comes with an elbow hose to replace the straight worn hose. I had to use spit and push and twist hard to get the hose on the supply pipe end over the sealing hump.
No transmission fluid came out of hole but some did come out of the nose of the old modulator perhaps indicating it had failed somewhat.

Did research before ordering. Modulators often have the different color striping to indicate they are calibrated for specific vehicle uses: Black-standard, Red-racing mode, Green or blue or -for high altitude use.
For my vehicle I saw no high altitude so I went with normal, is the best balance for original design.

Take care

Glad it's working great. Thanks for the update!

PS New modulator instructions page pic attached
 

In case anyone needs this in the future for reference:

https://cimg4.ibsrv.net/gimg/www.gmf...9cf44614a.jpeg

Thanks for following up with us!

Down shifting like factory now that I added 4 oz of Seafoam Trans Tune
 

Just want to give what seems like a final update on this problem more fully resolved by adding Seafoam Trans Tune 4 oz liquid.

As stated before, since the new exterior solenoid, on full throttle around 33mph it first downshifts from 3rd to 2nd then 2nd to 1st, where before it would go from 3rd directly and roughly into 1st.
However there was still some harshness from 3rd to 2nd on full throttle but not near as bad.

Now just today, after previously seeing a Kilmer on Youtube recommend Seafoam Trans Tune for grunting power steering racks and some transmission problems.
Suppose to remove varnish build up on internals and valving...
He seems to say it evaporates so a fluid change is not needed if not due. The bottle seems to say the same thing.
I decided to buy some and try, both for the transmission and the power steering grunt.
It's at my local auto store and Amazon at almost the same price.

I read the directions and added 4 oz to the transmission fluid and drove it around a bit and tried the downshifting by flooring the throttle.
The seafoam seems to have already worked because it downshifts from 3rd to 2nd smoothly and I've done it multiple times and different speeds.

The power steering I had turkey basted the fluid out of it multiple times in the past to put new transmission fluid in yet the steering still would make a grunting sound at times like in parking lots or turning a corner.
I don't know if it's the pump or the rack...but it's not a slipping belt. Had also added some at-205 seal restore in the past just in case.
I think the steering is also getting better already with the 4 oz seafoam, but it'll take me some time to tell for sure.

Take care