Trans Bang when downshifting to 1st
 

2003 Buick Park Ave. 3.8L, 262,000 miles
when slowing down and coming to a stop the trans downshifts fine. but when I'm slowing down, and still in 2nd or 3rd, if I hit the accelerator hard enough to downshift to 1st and speed up, it bangs hard into 1st. the harder I hit the gas pedal, the harder the bang. If I accelerate slowly but it still downshifts to 1st it's hardly noticeable.
doesn't do it in any other gear.
Any ideas would be appreciated.

More throttle ----- More Bang ------ Go together.

"Bang" like a sound?

. . . or "bang" like the feeling when it engages?

Does the engine rev up a bunch first? . . . like you put your foot in it, it tries then decides to downshift, does it rev up a bunch then engage first gear?

When was the last time the transmission filter was changed?

carfixer007 you're hilarious. Wrong kinda bang. LOL
now to more serious stuff.
no the engine doesn't rev when it downshifts, as in slipping. if I put my foot in it I get a quick response down into 1st and then an immediate bang and acceleration, no delay. it bangs when it shifts into 1st.
It's a physical thing. I can feel it.
I also checked my front motor mount near the radiator and it was practically broke in half. when I changed it it helped the banging quite a bit. but it's still doing it.
fluid and filter changed around 15,000 miles ago. Delco VI full synthetic.

could this be another motor mount?
or possibly a sticky or bad solenoid valve in the tranny?

The accumulator is what I would look at.

did you mean the accumulator pistons or accumulator solenoid valve? I hope you meant the pistons and seals.

Could be the spring, piston or seals. Need to check it out. May not be the accumulator but we have to start with most common area.

ordered new accumulator parts.
I'll keep you posted.

any codes?
Your high mileage 4t65e Is on the cut off for a few production changes...

https://cimg1.ibsrv.net/gimg/www.gmf...cb09844fc2.jpg
do you have a pic by chance of your last fluid and filter change? What kind of pan you have? does your pan have a rectangle magnet or 2 circular magnets? Did you fully insert the seal for the filter and the filter into it?

Hows the thermo element?
https://cimg7.ibsrv.net/gimg/www.gmf...9c966aea94.png
https://cimg0.ibsrv.net/gimg/www.gmf...a8c2270fc4.jpg
none of these pipes were lose, were they?

The one with the box around it 126, is replaced by the first pic I posted here on this post. That original unit has a hose piece between two hardlines. They eventually slip past the barb on one side and you have a hemorrrage of fluid pressure and that can screw things up.

Also, did you disconnect your battery for 30+ minutes or have a TAP reset with a scan tool? After your service that is. What did your fluid look like?