1985 300D Turbo Cruise control
 

Hi all,
Cruise works sort of. If I flip it at 60 MPH the car accelerates to about 68~70 and levels off and maintains the speed. Disengage works properly and does tapping the brake. Re-engage it'll go back to 68~70. Once in a blue moon, the cruise will engage correctly at the selected speed and maintain the speed. Would this be a cruise amp problem or the column switch? My tach doesn't work either. Cruise always will engage but almost always does the accelerate thing.
Thanks

Ignore the earlier post as now the CC does not work at all! In the driver's footwell I found a black plastic part, obviously broken. Small straight-cut gear, broken in half, on a shaft that looks like the shaft was somehow maybe a part of some kind of flap. I'm going to try to get a picture in a while. Thinking about it, I am kind of thinking this is a part of a broken vent. The vent on the extreme left rotates but doesn't actually open or close anything.
Before typing that last sentence, I was thinking of a gear that read the speedometer maybe, but now I am pretty much thinking it is in fact the vent gear used to open & close the vent. So, I'm back where I was, where do I begin troubleshootng the CC?

Welcome back!

As far as I know that black plastic part has nothing to do with cruise control.

On your 300D the cruise control problem is most likely the amplifier. I've written up a set of diagnostic that can help you narrow it down:

https://sites.google.com/site/mbcruisecontrolrepair/home/diagnostics

Let me know if you have any questions!

Thanks Much! I'll have a poke around later today. The last repair I made to the car was to fix the tach. That involved replacing the gizmo in back of the glovebox and went very well. I think the plastic part is the vent breaking apart. I have a small collection of them in a toolbox someplace and will have a gander.