Factory original 1990 Buick Century 3300 Auto w/OD 120K miles
Any input?:

Hi my Dad'* old Buick that I inherited I've been getting into better shape and hoping it lasts a good while.
One of the things that might need some attention is the over drive automatic transmission.

Because the transmission shifts quite nice and smooth yet when I floor the gas pedal when needed, it drops down from it'* upper gear down directly into 2nd or 1st(if I was going slow enough).
But this downshift is very pronounced CLUNK like it'* not suppose to be that rough.
(I think I even remember this terrifying Mom in the passenger seat when Dad had to floor it LOL)
I've changed the fluid and filter with a pan drop but it they weren't bad and had no effect on this clunk problem.

Is this just a symptom of an aging transmission and I should just live with it(which I can) or is this possibly an easy fix with some adjustment or control valve replacement or perhaps a sign of a bad transmission mount or engine mount or it needs some Lucas transmission slip fix additive or....?

Thank you

It'* supposed to shift harder with more throttle.

Yes but my parents also had a 92 and I had a 95 and they didn't do the kind of severe Clunk when full throttle downshift.
Oh well probably just something to live with.

Thank-you

Like an audible clunk?

If so, check motor mounts. From any other gear down to first is a big jump, and when that happens the motor mounts are supposed to control that, but if one or more are rotted out, you'll get a clunk.

My Park Avenue did something similar back in the day. I would be going about 25-30mph, floor it, and would notice it was taking longer than it should to shift, and when it grabbed, it was as if the car was slammed in to gear, making me think it was going to shatter the entire driveline in the process. I later discovered it would start doing this when it would start starving for fluid from the level being low, and line pressures not being high enough to complete the shift in a timely manner.

Everyone is dead on about checking the engine and transmission mounts, those can be checked easily by holding the breaks firmly, and trying to accelerate the car forwards and backwards while watching the engine. If something is broken, the engine will move too much.

I would also scrutinize the fluid level. That generation tends to prefer a bit of an overfill, up to 1 quart in some cases, checked at full operating temperature when running.

Finally, if you have "quite nice and smooth shift" feeling going on, something may be up in the pressure modulator. It has the vacuum line going to it on the front of the transmission. If it'* factory, I would be inclined it to swap it with an adjustable aftermarket unit to firm things up a hair. I haven't ever seen a transmission of that era shift "cushy smushy", but if line pressures are low enough that it can't grab a gear in time to safe its life come crunch time, it may benefit from a bit more help.

I know the response of "make it shift harder" seems completely counter-intuitive to the "it shatters my teeth" complaint, but it could be worth a shot. I have never thought of the shift of a 440-T4 (4T60) or 4T60-e as really plush and cushy, they usually seem pretty focused and decisive when they do their shifty bits.

If you go for the adjustable modulator, don't crank it more than a 1/2 turn at a time. It'* very easy to get lost, and then you're in a world of hurt.

Yes audible Clunk but also maybe more so the feeling throughout the car I'd say...
A bit like a rear wheel drive with a loose U joint in the drive shaft I suppose.

Ok, I'll look more at the motor mounts.
I had opened the hood and placed the car in drive and then reverse and watched for excessive engine movement but can't say for sure I saw any.

Thanks
I'll keep what you said in mind.

Well I don't that it shifts super soft smooth but it just shifts like it should under normal driving I'd say.

Come to think of it, this could be related or not:
I haven't figured out why the steering is effected when at highway speeds and going over dips in the highway, the nose of the car can like pull a bit to the left and then the right. Almost in a small circular motion.
The alignment shop did find the engine cradle pushed back from bottoming out and weren't to bring the caster completely where it should be.
However this problem seems to be something loose as well as the transmission...

If there are no alignment geometry issues found (assuming this because you said "alignment shop" without saying "it needs blah blah blah"), I'd suspect struts are tired, and one of them is more tired than the others.

Sometimes you can't see a bad mount this way. Usually this is when all of the other mounts are good. I'd crawl under it and inspect all of them.