Does it make sense that my car would shift correctly only when cold?

This seems to be the case, I get longer held and smoother shifts when I first take off, then the hotter the engine gets the faster it goes through the gears and the rougher they hit. Any ideas?

Would a thermostat modification, causing the car to take longer to warm up, fix my problem or does it sound like the transmission oil cooler is toast?

Sorry if I asked too many questions in one thread, but I'm dieing to know!

Thank you for ALL replys and advice!

The vehicle has a circuit that adds vacuum when the engine is below 50C. Accordingly, the shifts might be a bit smoother in this operating range. The design is to offset the cold transmission fluid which would otherwise result in shifts that are too firm for most folks.

If you've got shifts that are too firm when warm, it's time to get a vacuum gauge on the transmission modulator line and see what you've got at various operating modes.

Otherwise, we're just grasping at straws...........

Remember this with shifting action on the 300sdl transmission:

High Vacuum equals soft shifts

Low vacuum equals Hard shifts.

My 300sdl had a very soft shift, almost a slip between 3 and 4th. I disconnected the electrical connection to the device that boosts that vaccum when the engine is cold. Now my shifts are a little firm when the engine is cold, but I like it that way.

At normal operating temp, my shifts are just perfect.

John
1986 300sdl