I gots a 96 lumina that shudders a bit when the automatic is shifting from 3rd? to 4th (od?)- the last two gears anyway...
It will shudder/chug for about 1 second then switch to the correct 'gear'  It seems that if i lift off the gas a bit during this time it will get through the trouble quicker with less shudder.  Otherwise while IN any gear the thing works semeingly perfect.  Any Ideas?  Changed the fluid and filter proably 30-40k ago.

Two things, first you should not change the ATF fluid but take it in for a fluid flush.  Changing the fluid only gets around half of it out of the system.  Second, many GM cars have a "chuggle" transmission problem which can be fixed with a computer program upgrade (fairly cheap).  One last problem, there is supposedly a bad shift solenoid problem on a lot of GM auto transmissions.

Most of what I know I picked up from Pat Goss' radio car show.

GunLvr

thanks- I know that a lot of fluid stays in during a 'change' but I wanted to change the filter out being i bought the car used and had no idea if it was ever done.  And I did not want to pay mr. fluid changer 98 bucks...

I suppose that solenoid is buried deep inside the tranny where a complete overhaul is in order?  Also, where do you upgrade/update a trannny computer?!?!?!

For the computer, I think a dealer has to do it but it is the engine computer which is flash-upgraded.  Might only cost $100?  I have been considering this too since my Lesabre has a very small "chuggle" around 45-50 mph.

For the fluid I thinkt the full flush is worth it unless the vehicle has gotten no maintenance then you might as well do nothing.  I will do the flush every 2 yrs.

For the solenoid I am not sure where it is but the replacement is supposed to be fairly cheap but I am not sure how cheap.

GunLvr

Well if's it's like the other generic motors products I've worked on the solenids are easily accesible. The last auto trann I worked on they were accessable on the driver side very easily.

I hear about this constantly with GM auto-tranny's. It will oftern get worse.

I can't see it being a computer problem, the program doesn't change over time so if it worked before it should work now. It could be one of the sensor's the computer is using to make adjustments. A sensor that is reading differently that before might cause the computer to react strangely. A 96 lumina should have minimal electronic control on the transmission anyway.

Changing the tranny fluid yourself is fine. The only fluid you don't get is what's left in the torque converter. By changing the fluid and filter every 30K you should be fine. Now if your lax and don't do it oftern enough then maybe you should do the flush and fill. Remember auto trann's suck and therefore are more apt to failure. They NEED a fluid change EVERY 30K.

If new fluid and filter doesn't fix it take it to a tranny shop and have them run a diagnostic. If it's a solenoid you can probably do that yourself. If it a band, TC, pump, governor or anything else well get ready to spend a big chunk of change to fix it.

As a veteran of the automatic transmission business, I will provide you with the following information for FREE [;)]
The problem you are describing [b]sounds like[/b] "tourque converter chatter" ( [i]I say [/i]"sounds like", [i]because no transmission problem can be 100% diagnosed without at least a road test[/i] ).
Your trans has a "lock-up" tourque converter, the converter has a clutch inside of it that "engages" (for lack of a better term) at a certain speed. (sort of like having another gear)
A few things can cause it to chatter,
[b]The solenoid that controls it COULD be messed up[/b].
 
It could be going into "lock up" too early (like letting out the clutch on a stick shift too soon)

The [b]whole transmission[/b] could be shifting too early (in which case it could just need an adjustment to the throttle pressure cable)

The tourque converter itself could be worn out.

IMPORTANT QUESTION: does your car have an actual "overdrive" transmission? (actual selector spot for overdrive on the gear selector)

[b]ALSO[/b] 9.9 times out of ten, a fluid change/flush will have no effect on a drivability problem.
You still should try it, but dont bet the farm on it, and [b]NEVER[/b] put anything in your transmission that says it will "stop leaks", or make your transmission "brand new" again.
There [b]are[/b] some "friction additives" that will help it shift more positively sometimes though.

My advice would be to go to a REPUTABLE trans shop with the information I have given you, that way, no matter what the problem is, if they think you know a little about it...the will be less inclined to "bullshit" ya.

it is a over drive i believe, the shifter has a "O" around the little "D" thingy

If it is an actual overdrive trans (a 440) in order to get to the valve body, you have to pull the drivers side axle to get the side cover off of the transmission (and on SOME of them you have to lower the sub-frame)

Take it to a couple different trans shops, and let them road test it. Better trans shops should also have a diagnostic scanner that will tell them if it is going into lock-up correctly.