Some diesels have 2 valves. Some have 1 valve. My 84 300SD has no valve.

My transmission has an interesting shifting characteristic. It usually likes to rev up before shifting. The transmission is not slipping. Before it shifts, the car starts to accelerate faster! The shifts are firmer when the transmission is cold, and smoothens out when hot. I could not claim ever claim that this transmission shifts "as smooth as butter."

However, in an advertisement for an 81 300D/SD/CD/TD, entitled "Mercedes-Benz unleashes two more high-performance Turbodiesels," they wrote: "Not even the Turbodiesel's transmission is the same. It is a compact four-speed hydraulic wizard of an automatic, a torque convertor device so quick that it can shift gears in 1/10th of a second, so smooth that many shifts are imperceptible."

As others have mentioned, a firm shift is better than a smooth shift. I also agree. But, there is nothing wrong with an imperceptible shift since that is what the car is designed for.