Did you ask the dealer to change the fluid and filter or is that what they suggested after you told them of the symptom? If you told them it wasn't shifting, they should have recommended a thorough diagnosis. They would have known a new filter would not solve the problem. If you didn't tell them it wasn't shifting, they would have assumed you just wanted the preventive service and wouldn't have looked for a problem.
It's actually staying in second gear. That's "Limp-in" when the transmission computer detects a problem. Starting in second gear will greatly decrease power when starting to move. It's actually a good thing if it starts in second gear right away. That suggests a problem with a sensor has been detected and is a relatively inexpensive repair. If it starts out in first gear, then upshifts and either stays in second gear, or goes back to second gear after shifting to third or overdrive, that suggests clutch pack slippage which requires a rebuild.
The problem on the early versions of this transmission wasn't exactly seal trouble; they had what's called "tolerance buildup" on the assembly line. The first 100 transmissions were hand assembled and were bulletproof. During automated assembly, every part has a specification and a tolerance, or how much it can be off a little and still be acceptable. If a sealing surface is off just a little, and the groove for the mating seal is off just a little the other way, pretty soon, with a little normal wear, that seal might not ride exactly on the surface it seals against. That will result in internal leakage and a slipping clutch pack.
There is an input speed sensor and an output speed sensor watched by the computer. When the two don't agree, the computer knows slippage is occurring and it defaults to Limp-in mode to at least allow you to move the car slowly. The same thing happens when a sensor fails. There have been quite a few updated parts developed for this transmission. It's usually less expensive to install a remanufactured transmission than to have yours rebuilt with updated parts.
There was also a list of about ten common complaints about the shifting characteristics that didn't affect the reliability. My '93 Dynasty, for example, tends to downshift to first gear a little too late so you would feel the harshness if you did a rolling stop, (not coming to a complete stop before accelerating again). It's a minor irritation that was addressed by "reflashing" the computer, meaning to install new software. In my case, it didn't change anything so I just live with that little annoyance. If your harsh shifting is something new, it is likely related to it going into Limp-in. With the low mileage you listed, there is a chance a reflash of the computer could solve the problem by making the software less susceptible to erroneously detecting slippage during shifts.
One other thing to be aware of is letting the car sit unused for long periods. The transmission fluid will drain from the clutch packs and allow the fiber plates to dry out. This is true of any automatic transmission. Dried plate material will tend to flake off. That alone leads to rapid clutch wear, and the flakes can grind against seals before getting trapped by the filter. My 16 year old Dynasty just turned 4,000 miles this summer. I try to drive it a little every summer just to keep the plates and seals lubricated, but the long storage really is not good for it.
One of the best things you can do is to pay close attention to when the transmission goes to Limp-in. Is the engine warmed up or cold? Does it always occur when upshifting to the same gear? Does the speedometer and odometer keep working? How rapidly are you slowing down when the harsh downshifts occur? If you had this car since new, you are in the best position to know what doesn't feel normal as far as shift feel is concerned, but if you recently acquired the car, you might not yet know the characteristics of this transmission.
Saturday, December 12th, 2009 AT 2:49 AM