2001 Montana trans issue?

Hey all, my parents just picked up a '01 'Tana with the 3400 v6 in it with 100k miles.

Now, cutting right to the chase, here's the issue.

When you first start driving, the trans mission shifts as smooth as warm butter. However, after maybe ten miles of driving (doesn't seem to care between highway or stop-and-go) and the trans is warm, it shifts hard. Not neck snapping, but a thump that makes you notice it, almost like popping a clutch fast on a manual. The 1-2 shift is like that, a little noticeable thump and surge. The 2-3 shift is "softer", but the thump turns into almost a "clang", in that it's sharper sounding and feeling, but not with the same surge of motion after the shift. 3-4 seems to be fine.

Also, when this symptom is showing itself, at a stop with foot on the brake, shifting from drive to reverse gives a very hard thump, sitting still. However, at a "cold" start, no such thump.

The mechanics have looked at it, and observed this behavior, and wrote it off as a "characteristic of that vehicle". What do you guys think it is? If I was not clear at any point, just say so and I'll do my best to explain.

... mechanics..wrote it off as a "characteristic of that vehicle". .
WOW! However, GM has listed the conditions you described as "problems" , and here is a cut and paste of what is relevant of your description for the 4T60:
http://www.avigex.ca/xport/12shift.jpg
... accumulators are serviced from under.
....pressure regulator though is on driver side of transmission..more complicated.
Before all, I suggest you have a "transmission capable" scan done. I know AUtozone does scans for free, but some said they scanned only for engine codes....verify that.
Some problems of that nature are often generated by vacuum modulator failure..so check all vacuum lines are OK and connected. Better invest some time for investigation rather than to go for a full surgery at which time they "also correct" vacuum line problems.....

Very good information, I had no idea it would be this serious/documented. Will driving the car in this condition cause damage or make things worse?

Also, can you break down your fix by steps? I had a little trouble catching what you meant.

FIrst step would be to have a code scan, hoping it procures some relevant hint, and going from there.
I would not think that a lot of components have gone awry at same time. I wouldthink there is a common cause to all or most of the symptoms.
What is the condition of the transmisison fluid on the dipstick? (clear red or other)
Trannyman52 gives good advise about transmission problems. I'll ring him....

EDIT: ...checked for some former advise from trannyman and got this: http://www.automotiveforums.com/vbulletin/showthread.php?t=385506

I'll scan pictures about the accumulators and post them later.

Now check this out also.
http://www.automotivehelper.com/topic213553.htm
and read galileo's answers .
As stated previously, I lean on the modulator explanation, and from the galileo's explanation, it is external to the transmission. Modulator shows in all of the possible causes for your symptoms, either directly, or as linked to oil pressure.
http://www.autozone.com/az/cds/en_us/0900823d/80/1b/ed/32/0900823d801bed32/repairInfoPages.htm
I do not have all the details for variations in years..as some "hardware" components may have gone "computer derived" in later years.

Anyway, get that scan, specilaly if you have the Cehck Engine Light ON

That's very promising, a big thanks to you for looking all of that up. I love this website so much!

What would be a good source for the parts? The one thread had 120 for a dealer part, another guy said he paid 43. Is this a part you can get from a salvage yard maybe?

I live in Canada but to get an idea about parts# and prices I use to pay a visit to www.rockauto.com in the USA.
IN fact I just searched for a modulator and could not find it for 2001, though it is available for 99....so as said before, this might have gone software instead of hardware....
ANyway, if you can have a computer scan and come back with the codes, we will keep looking for the best probability.

Alright, I'll show my father this evening and advise him of what he needs to do. Do you think driving the van this way will cause damage, or make worse existing damage?

As you describe the situation, I would drive with a light foot so shifts occur at low rpm...then accelerate afterwards. I do not think this can have immediate destructive consequences, but would not like to keep it like that.

Thanks for all your help, will post later about this.

I had a similar problem where 1-2 was harsh with 2-3 being significantly better and 3-4 having no problem. I didn't have the thumping involved with shifting into drive from reverse. One other symptom I had was the transmission not shifting until you let up on the gas like you would on a manual. It ended up being a cracked planetary gear that was fixed with a rebuilt transmission. Did you take the van to a general purpose repair shop, or one that specifically deals with transmission? If it was the former I'd get a second opinion with the latter. Heck, for that matter, I'd still get a second opinion.

Still have not gotten to an Autozone to have it scanned. I'm going on a road trip in this vehicle in two weeks, 1400 miles, all highway. Starting to get a little antsy about it. Will post when I have more info.

Autozone scanning it likely won't reveal anything. The scanners they have are quite basic and won't query the transmission specifically. You will need to go to a transmission shop to get a real scan done.

And the 2001 had the 4T65e transmission, not the 4T60e. The 4T60e was last used with the 98 Transport/Montana. The 4T65e is completely electronic and does not have a vacuum modulator on the front of the case.