I have been using this forum for years to diagnose and resolve a myriad of issues with the diesel Mercedes I have owned throughout the years.

I have usually been able to resolve issues without posting however this one is a complete mystery to me and I would be thrilled if anyone had any insight into any of this.

I recently purchased the car and I am having it shipped to me. So as of right now all of this is armchair troubleshooting however the previous owner has been very pro-active with documenting the exact symptoms.

Car: 1979 300SD W116 Chassis with 722.120 transmission. Unknown mileage, however car is in astounding condition... someone loved the car cosmetically, not sure about mechanically.

Symptom: Vehicle runs and drives beautifully, however while cruising ONLY at partial throttle ( such as a country road or flat highway) the speedometer will drop about 15MPH and dance around a bit which is then followed by the vehicle dropping into neutral. Apparently this happens at nearly any speed, however not during acceleration. I can only surmise it is a neutral indication and not a "flare". This symptom only manifests randomly ( previous owner says maybe 4 times in 70 miles). Sounds like a pressure issue?

I am suspecting several issues here. Previous owner can not hear an audible click from the transmission servo when the ignition is on and the switch under the pedal is depressed.

Modulator valve on side of trans disconnected from vacuum source and shifting does not change ( shifts well according to him).

Pressure control rod has been adjusted with no change. ( he had a flaring issue but adjusting this rod corrected it)

When I receive the car I will do a thorough run through checking all vacuum systems and put a gauge on the test port and report back.

Meanwhile I have one question: Why on earth would the speedometer become erratic before dropping out of gear? ( Car goes back into gear if you lift throttle).

Where is the speedometer pickup inside the transmission? To me this sounds like a mechanical decoupling of some sort. Almost like the shifter linkage is bumping the car into neutral...

It has been a few years since I have messed with one of these cars and I am not familiar with the 116 Chassis.

Thank you in advance!