I just wanted to pass along a warning to fellow Bonneville owners that you really need to get your transmission fluid changed if you have not done so and are over 40,000 miles. I just had my 2001 Aurora with the 4t65E transmission done today and was real surprised on the difference that it made. My 2001 has 45,500 miles on it and the fluid was surprisingly dark. I think a large source of the problem is that there is not an external transmission cooler on the 65E besides the one inside the radiator. Because of the lack of the cooler the transmission reaches higher temperatures than it really should. I remember that the previous tranmission to the 65E was the 60 E and those had external transmission coolers from the factory.
I had the transmission flush done which replaced over 90% of the original transmission fluid. Typical pan-drops+ filter changes only change about 40% of the original fluid. The shop I took my Aurora to was called Marathon Auto service and the transmission service they performed on my car was a BG transmission flush. What they do is they disconnect the lines going to the transmission cooler located inside the radiator and connect it to a machine. the car is started and the machine allows the transmission'* own pump to pump out the old fluid. There is a piston that measures the rate in which the fluid is leaving the vehicle and a pump from within the machine pumps new fluid into the transmission until the fluid is totally clean. I opted for the cleaner and conditioner as well instead of just the fluid change. The difference is they put a cleaner into the transmission then drive the car for about 5 miles. Then they change the fluid out, then put a conditioner into the transmission. They'll then take it for another five mile drive to make sure everything is alright. It was.
I just really have to recommend you 4t65E transmission owners get your fluid flushed if you have over 40,000 miles because of what mine looked like! I think I'll get it done once a year to be safe. THe total cost for me was $113.04 for the flush, cleaner, and conditioner. The price with just the flush was $88.
On the right is my transmission fluid before the service. The fluid on the left is my fluid after the flush. This is with 45,000 miles remember on this 4t65E!
the fluid on the right is my mom'* 2000 SSEi'* fluid at 58,000 miles before a flush. the transmission is the 4t65E-HD.
The fluid on the left is after the flush.