Clunk when up-shifting but only when cold
 

Talking with my brother about his 84 300D. He has about 260 K on it and he says that ever since he has had it (about a year and a half) it makes a clunk on all upshifts when cold, but as soon as he has driven it about 10 miles at highway speeds it does not clunk when upshifting. At first I thought U-joints, but they would clunk all the time, just not when it was cold.

My next thought was a vacuum leak causing hard shifts. He told me he has hard shifts when cold, but again, they soften once the car is driven. My last thought was trans fluid and filter, but he has had them changed twice since he has had the car but no change.

I'm out of ideas, anyone else got a shot??

Thanks,

Glenn

The '84 300D has a "vacuum amplifier" (Sixto's "blue flying saucer" or BFS) located on the left inner fender. It uses a combination of vacuum and boost pressure to regulate the vacuum to the transmission. That and the Bowden cable from the accelerator linkage tell the transmission when to shift and how firmly to shift. When the engine is cold, there's a temperature switch that cuts off the boost pressure from the BFS and makes the transmission shift at lower RPMs. I think Mercedes added that to the design to keep RPMs low until the engine has warmed up. Once the coolant is up to about 50C, the switch changes state, a valve then allows boost pressure into the BFS, and the vacuum to the transmission changes so that it shifts at higher RPM.

I'm not sure if a problem with the components in this area would also cause the transmission to shift harshly but it's certainly worth a look. If I have mis-remembered (had an '85 300D years ago), sorry.

Jeremy

Its worth taking a look at. I will be able to see the car first hand in a couple weeks when he comes to visit. Just out of curiousity, would my 83 300D have the same "BFS"? If so I cold take a look at mine beforehand and gt familiar with it.

Glenn