PeachParts Mercedes-Benz Forum - View Single Post - Clunk when up-shifting but only when cold

Jeremy5848 · #2 03-31-2014, 01:31 AM

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