Hi, I've got a 1990 Cadillac seville with the 440-t4/4t60 transmission and it'* got a bizarre issue that I can't quite find an exact replica of. When I start the car it starts very well and idles smoothly and has the typical small bump when you engage drive (feels the same as my other 1990 Cadillac seville). When driving it is extremely smooth and has no slip in the 1-2, 2-3, or even 3-O/D. However, after driving for a few minutes the transmission feels like it disengages and I lose all gears (forward, reverse, 1, 2, 3). When I switch from neutral to drive in this state I still feel a very subtly engagement, but it does not go. If I sit and wait for a bit with the car on or off I'll suddenly get the gears back and it drives happily. I do notice that one of the things that will cause the disengagement is slowing down. When I start slowing down I'll feel it disengage and I find that I can keep the transmission somewhat engaged by braking as I stop, but even then it'll buck and kick all the way to a stop. Obviously, that "work around" is not sustainable for a 30y/o 4t60

Information on the car is limited as I just bought it as a fix up project, but the car has 118k miles and I just did a transmission flush. The flush didn't change the behavior of the transmission much if at all. The fluid that came out during the flush was definitely burnt and was almost black, but there were no metal flakes so it seems the clutches are indeed in fine condition. My hunch right now is a transmission control solenoid, but I wanted to field some input from those who know more than me.

Thanks so much for your time!

Sounds like stuck pressure control to me, either spring and/or solenoid . . . or maybe the pump is going bad or is filled with crud.

The more I think about it, even if we guess right on what it is it will probably still need an overhaul. Let'* say we have it all figured out and we're somehow absolutely positive we know the answer. We replace the pump and/or pressure spring and/or solenoid. Not we've disturbed the innards of an old and marginal unit and have probably created new issues. Even if we didn't create new issues, there are other parts (likely clutches) that are just about gone.

Without knowing that the fluid is almost black, I'm already pretty sure it needs an overhaul. The almost black fluid tells me it hasn't been maintained well, hasn't been treated well, or both. 118,000 miles of highway driving would probably not turn the fluid black unless it had something terribly wrong, so I suspect it did a lot of city driving and/or some other terrible things have happened to it.

Summary: Overhaul.