I have a 99 S80 T6. A while back the trans stopped pulling totally and "trans service required" came on dash. so I parked it figuring I would need to replace the transmission so I started driving other vehicle to save cash up for it. After sitting for a little over a month I had to move it and started it up so could steer it and push. When I pulled shifter down it went in drive and started to pull good. I drove it around the block and was even able to chirp a tire with ease. Thought it was very odd and drove it to store and back. after about 12-25 miles it started to slip and then quit pulling. now if it sits for a month again it will be drivable for about 15 miles. I changed the fluid on it about 2 months before this started. It does have a loud whine sometimes when I start it up and sounds exactly like an old 82 camaro that the front trans seal went out in but Im not leaking or losing any fluid.. Is this something related to fluid pressure or perhaps something clogged and after sitting a few weeks it drains down and works until it clogs again. Im a single dad and cant shell out cash I dont have if there is a simple fix. or even a kinda simple one...lol

 

From what I've found the t6's huge weak spot is the tranny... The closest thing I have found to help me with this issue IS the fact that it is a gm transmission. If I am not mistaken I believe the Grand Prix might have had the same tranny, perhaps that info might help, they are more common and someone may have seen your issue before.

Personally though I would think this sound electrical, try disconnecting the battery and let it sit for about 15 minutes this will reset the computer possibly helping, though I am no transmission expert. Thats just the first thing I always do if something is wierd in my s80. Knowing that helped me get home one night when she put herself in limp mode...

Just another thought, did you do your tranny flush or did a shop do it?

 

I didn't flush I just did a fluid change twice. I had the battery disconnected for about a week and also used laptop with obd software to reset ECM. Wish knew someone close by that had a dice unit. Any other ideas.

 

Buy a piggy bank and fill it with loose change? A rebuild may be in order. I wonder if it could have something to do with the fluid tempature? I know that on my 3000gt I installed a trans cooler and it helped a ton. I am going to put one on my t6 soon too.