OT-What Makes An Automatic Tranny Shift With A Thud?

Yesterday I drove my Buick LeSabre 250 miles one way. After
driving 200 miles the tranny started shifting with a thud thru all
the gears.At the next town I checked the fluid and it was OK.I
let it sit for awhile and it shifted fine the last 25 miles.Its not
slipping and shifts when it should.

I started for home early this morning and it did great for the
first 100 miles.Then it started shifting with a thud again.At the
next town I let it sit for 20 minutes.It then shifted normally and
made it home OK the last 150 miles


The car has 95,000 miles,had the fluid changed at 60,000,has
never pulled anything,and has never been abused. Any ideas
whats going on?

Think you may have trouble coming, best get it to a trusted mechanic and have it checked out.
Hope I'm wrong!

Take it the dealer or someone you know is good on Automatic trannies. Don't take it to a chain Transmission shop. Most likely its in the kickdown system but could be lots of other things.
Walt

Could be the left front motor mount. Or any mount, including transmission. A separated motor mount will let the engine rock excessively when the transmission shifts and then thump when the engine settles back down.

I had the same thing happen on an Olds 88 and it was the left front mount. (Same transmission as the Buick).

What year is it? When GM went to full electronic control of the trans, they started using a line pressure control solenoid to vary line pressure which controls shift firmness. There is no kick down linkage or vacuum modulator any more. If the solenoid stick or fails, usually the trans will shift harshly.

I think Brian might be right. I had the same thing happen to me, real erratic shifting and sometimes it wanted to start out in high. It even would kick the 'check engine' light on and off. Took it to a mechanic friend and he replaced the solenoid---and put new sparkplugs and wires also. It took care of the problem.

What year and trans model?Brian is correct.I had a 4L60e in my 97 Tahoe do the exact same thing.Bought a rebuilt valve body,installed it,never did it again.Gets hot,solenoid sticks,increases line pressures,gives you the hard shifts.Let it cool down,all is fine till you get it hot again.Electronic GM trans are well known for this.

You didn't mention what year but if it is 95 or newer, that has the 4T65E trans in it. As Brian mentioned, it uses a pressure control solenoid. I have seen problems where the pc solenoid sticks at times (it is a pulse width modulated solenoid rather than an 'on/off' style). When it sticks, the pressures are not what the computer has calculated them to be and shift times will be longer than the computer expects. To compensate for this, the computer changes it's adapt table to increase pressure to try to bring the shift time down. The problem happens when the solenoid does not stick, these increased pressures cause hard shifts. Eventually, the computer will store a P1811 code, max adapts/long shift times and will no longer try to modulate pressure and let the trans run full pressure, making all shifts harsh. A p1811 code generally will not turn on the 'service engine soon' light since the government requirement is that only things that effect emissions turn the light on... harsh shifts do not increase emissions, so no light.

They redesigned the PC solenoid around 2006 but I have seen the same problems with the new ones but it is much less common. Also, the valve behind the solenoid has been known to stick so whoever changes the solenoid for you, make sure the try moving this valve when the solenoid is out and that is moves smoothly against the spring pressure.

Thanks for all the replies.I forgot to mention that its a 2000 model Buick. This is the first automatic that I've had problems with in many-many years.

I think yours and the two replies above you have pretty much nailed the problem.Thanks. Its a 2000 model.

cbop has the best description of the system and the likely culprit.The code 1811(max adapt and long shift) will almost always set.Get the codes scanned.

There is also a bulletin out on that car for the tranny harness pass-thru connector being stretched too tight and damaging terminals at the connector causing hard shifts and/or multiple tranny codes.