1985 300D Turbo Cruise control

[rmmagow]

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

[rmmagow]

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?

[JamesDean]

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!

[rmmagow]

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.