REAL HARD SHIFT after high speed kickdown

[tinkering]

93 TransSport with 3.8 L:
When finishing up a kickdown pass at high speeds (70 MPH +/-) I get a very unhealthy mule kick of an upshift. After overtaking the slower vehicle and when pulling back into the normal traffic lane while very gently backing off of the throttle, the shift back up feels like an extra stiff kick in the pants. It is alarming! It doesn't happen everytime; just during the more aggressive passes. I was driving through the mountains on a long trip so the vehicle was well warmed up; I don't know if it would do it before it was warmed up.
(This tranny was rebuilt a few years ago. I have always wondered how good of a job they did? There was an annoying whine present before the rebuild (it wasn't me ) and it was still present after the job was completed. I think that might be the hydraulic pump?)
What can I do in the way of code scans? Is a special code scanner required for this tranny or does an engine scanner (available at an auto parts store) do the job? Are there "most likely" parts I might replace without bothering with a scan? I'm pretty sure the fluid is right up. I will double check.

[LMP]

I know the whine : mine has it too...I think it is the pressure regulating valve...but without any consequence after more than a decade..
A basic code reader does not reveal transmission codes...will check which codes might be available in '93 with proper scanner..not many I think...
The thump after a kick down pass is not really a surprise...but might be accentuated by a degraded accumulator..acting as a damper. Increased throttle increases at same time the pressure applied to clutches to cope with the added torque...I'd expect the pressure to go down with throttle release but not really sure of where it can be accumulated..requiring some time to level off...or the degraded capacity of shift dampers to soften the pressure inrush....

the pressure modulator is an external part in 93....and it is modulated by the manifold pressure (vacuum if you prefer)

...not sure its action is really fast...there have been a few issues in past threads that were solved with replacement of this component.

..will review my documents to have more precise thoughts about that....

[500hpgtx]

A couple of years ago my wife was P*****d off and driving our van hard. SHe was doing around 30mph and got ticked and floored the throttle. At the same moment she was getting aggressive driving, I implored: "Don't beat on the van!" Well the aggressive downshift into 1st and the subsequent upshift into 2nd gear was definitely all the trans needed.
The next morning i had to out to the driveway because she said that the van would not go forward. I nursed the van in 2nd gear? to my Pontiac trans friend.
He later showed me the broken forward band. He said that could happen any time when trans is old. However, when my friend with his supercharged 3800 locked his rear brakes and hit the throttle to "smoke" the tires, he had the same problem and was told the same thing.
LMP knows that I should never point a finger. I am sure he remembers the time my trans chain exploded after I hit nitrous under full throttle going about 40mph. The extra 150hp nitrous shot was a little too much on the ole' duel chain setup! Cheers Kev

[tinkering]

Originally Posted by LMP

I know the whine : mine has it too...I think it is the pressure regulating valve...but without any consequence after more than a decade..
A basic code reader does not reveal transmission codes...will check which codes might be available in '93 with proper scanner..not many I think...
The thump after a kick down pass is not really a surprise...but might be accentuated by a degraded accumulator..acting as a damper. Increased throttle increases at same time the pressure applied to clutches to cope with the added torque...I'd expect the pressure to go down with throttle release but not really sure of where it can be accumulated..requiring some time to level off...or the degraded capacity of shift dampers to soften the pressure inrush....

the pressure modulator is an external part in 93....and it is modulated by the manifold pressure (vacuum if you prefer)

...not sure its action is really fast...there have been a few issues in past threads that were solved with replacement of this component.

..will review my documents to have more precise thoughts about that....

So an accumulator is a damper? I imagine any accumulators would be inside the tranny only accessible by removing the pan and maybe the valve body?

It could relate to an increase of engine vacuum as the throttle is slowly released; it could be related to a vacuum modulator problem; without knowing exactly what is going on in there and/or the full function of the vacuum modulator? If it is easily accessible, it could sure be worth trying it as a first place to start.. I wonder if the vac/mod is available at the auto parts store or if it is a dealer item or transmission supplies?
Any more thoughts on the vac/mod yet?

[LMP]

The modulator is listed in rockauto.com . As you say, this is the only external part....
if you plug/unplug the vacuum line to modulator at idle, you normally hear a change in the whine intensity. UNplugging the vacuum line is like full throttle asking for high transmission oil pressure ....

....for 3-4 hard shift, GM manual says:
- check accumulator cover and piston for damaged seals
- check no 10 checkball missing
- check control valve assembly
- 3-4 accumulator valve stuck
- check other causes for high/low pressure: modulator (leaks or damaged daiphragm); modulator valve struck; pressure regulator valve or springs; pressure relief valve

[LMP]

Originally Posted by 500hpgtx

I hit nitrous under full throttle going about 40mph.....and after...my trans chain exploded !

...please forgive that I changed your text so the events are listed chronologically....because I think the exhaliration had had the time to build up...before it had to fall back ...
..so yes, you did in a few seconds what can be achieved in multiple steps...and made me confortable driving mine smooth..and after 19 years, I think I will not give it too much a try .

[500hpgtx]

Expensive Rush LMP! About a $900. I did get a stronger single trans chain over the double noise canceling chain. I also lowered the drive ratio from 2.93 to 3.29 gearing. However, I have not hit nitrous since that day. THe most likely component to fail next would probably be the input shaft. An aftermarket input shaft is around $450. It is a vac melt 4340 steel.
By the way, the accumulator that is vacuum controlled likes to suck up trans fluid when they go bad. When I was an ignorant kid, I burned up a 2 speed powerglide when the engine got rich off of trans fluid....got real wise when I was losing fluid in our 95, 3800 Transport. Hard to bleieve that it is part electronic/manual. Cheers KEV

[tinkering]

Originally Posted by LMP

The modulator is listed in rockauto.com . As you say, this is the only external part....
if you plug/unplug the vacuum line to modulator at idle, you normally hear a change in the whine intensity. UNplugging the vacuum line is like full throttle asking for high transmission oil pressure ....

....for 3-4 hard shift, GM manual says:
- check accumulator cover and piston for damaged seals
- check no 10 checkball missing
- check control valve assembly
- 3-4 accumulator valve stuck
- check other causes for high/low pressure: modulator (leaks or damaged daiphragm); modulator valve struck; pressure regulator valve or springs; pressure relief valve

The vacmod is available at my favorite AP store $56. The vacmod seems like the easiest place to start so I will throw the $56 under there. I hope it is easily accessible?

[tinkering]

Originally Posted by LMP

...please forgive that I changed your text so the events are listed chronologically....because I think the exhaliration had had the time to build up...before it had to fall back ...
..so yes, you did in a few seconds what can be achieved in multiple steps...and made me confortable driving mine smooth..and after 19 years, I think I will not give it too much a try .

I don't enjoy doing or paying for the repairs enough to partake in the burnout competitions, so I tend to be more like LMP; strive to get the distance rather than the Gs.
That being said though, I like to hear about all things like from 500hpgtx because it contributes to my understanding. Thanks to both of you.