GXP transmission issue. Weird one

[skregal]

I have had the strange solenoid issue that causes the super-hard shifting for a while now, like 2 years.
I just turn the car off and it seems to reset and shut up for a month or so.

This new issue is quite strange and seems to do it all the time.
My wife usually drives this car, but this weekend we took it on a road trip and I drove.

What happens is: When on the highway doing between 60 and 80mph, and you come
to a downhill section of highway and you let off the pedal and start coasting. Normally
the car will coast for a long time maintaining good highway speed even if the incline (decline)
is not very steep. However now, just after coasting for a few seconds, it feels like the
transmission down shifts (or pops out of overdrive) and the car slows down rapidly even
going down a relatively steep grade.
It does it almost every time you let off the gas at 70mph but becomes much more obvious
in situations where you know the car should not be decelerating, like steep downhills.
It is so obvious that my wife even said it's been doing that for a while now, and
she's usually clueless to any automobile functionality.

I can't think of any part on the car that would suddenly cause drag on the highway, and
it certainly feels like transmission, however the tach doesn't suddenly jump when it happens.

Is this possible for the transmission to shift itself out of overdrive? If so, what would cause
it to think the parameters are correct to downshift at that time and speed?

[havoc1482]

Sounds like typical engine braking to me. Correct me if I'm wrong, but I thought OD (4th) was only maintained when keeping your foot on the pedal (or using CC)?\

How rapidly (slowing down) are we talking about? And you say the tach doesn't show any movement when it "shifts"?

[CMNTMXR57]

No, it is a known problem in 4T65E's and 4T80E's. It is your pressure control solenoid. Some built up debris has broken loose and is floating around in the fluid causing the pintle in the solenoid to jam. The PCM sensing something isn't right with pressure, thinks all hell in a handbasket has happened, and then commands maximum shift pressure. This is the result... Hard shifting. Although it normally happens on acceleration, it can affect deceleration too. Shutting it off, depressurizes the system, and hopefully allows the solenoid to work again.

This comes from higher mileage, age, and/or dirty fluid. This is also one of the reasons power purges aren't recommended on high mileage transmissions that are still on the original fluid. You'll blast debris loose.

The obvious fix is clean fluid and solenoid R&R (if it gets to the point where it won't stop even after shutting down). But again, the only way to do that and get 100% of the old fluid out is a purge. Then on the N*'s, in order to replace the solenoid (should it get to the point where shutting it off and restarting won't reset it), you have to drop the subframe with engine/trans enough to clear the side case (it is the side case where the fill is), go in and R&R the solenoid.

[skregal]

It decelerates very fast, almost like having a foot moderately on the brake.
I have had the trans fluid changed twice on the car, once at 50K and then 100K.
The car now has 143K, so I guess I could have it done again and see
if it helps. Dealer probably wants $2000 to change that *dang* solenoid.

[CMNTMXR57]

It's not a fluid issue necessarily. It's something debris wise, from age, mileage, etc, that has broken loose and clogs that solenoid.

Changing the fluid can help, but it won't gaurantee you get it out.

[94SilverSSEi]

I could be off base here but when I asked at a cadillac shop they told me it was a waste of time to replace this solenoid while I had the drivetrain out because there is a seal that goes bad and that causes the solenoid to fail. I believe that was in regard to the TCC though? At which point he said the trans basically needs to be rebuilt.

Here's to hoping maybe a fluid change will take care of the issues though. Can you have it scanned and see if you are getting any codes? Mine had the TCC code. Cleared it and never had an issue again while I had it.

[CMNTMXR57]

Could be that too, which I would replace the seal when replacing the solenoid anyway. Still, in order to "fix" it, the case needs to be opened, which requires what I posted above.

If it acts up once in a blue moon (mine did), it isn't necessarily a case of trans needing rebuilt. If it does it every time you run the car and can't back out of your driveway without it happening, then yea, something more serious is at hand.