I'm new to Mercedes. Just got an 81' 300D that I'm fixing up and have fallen in love with. Neat car. My question is this: Been having transmission shifting problems and ordered a vacuum modulator from the dealer (at least that's what I told him I needed) and the part came in today. The package says, "Vacuum Box" and carrys part number A 123 270 06 79. On the back of the package it says, "Vacuum Box, Injection Pump." It sure looks like the vacuum modulator to me and is threaded on one end, the plastic part is red in color, comes with a nylon snap on cap, o-ring, a c-clip, and a little adjusting wrench. Is this IN FACT the vacuum modulator for the transmission? Seems to me it has nothing to do with the injection pump. Can one of you veteran mercedes types fill me in?

The "red plastic" part sounds right. If it's about 2 & 1/2" across that's it.
Regards,
Randy D.

What also has me confused is that the thing screws in (has threads on it). I thought the transmission vacuum modulator was bolted in (remove a couple bolts and it slides in with an o-ring seal)?

If it has threads, it's the kind that goes in the right side of the trans near the back. There are two different 123 chassis transmissions. One for non-turbo cars, one for the turbo cars. If the car is a 1981 300D (non-turbo) that is the correct part.
Regards,
Randy D.