Hi, I bought a 1985 300CD and the cruise and tachometer do not work. My service man tried a replacement transducer, but that didn't fix the tach problem. I put another instrument cluster in to see if it was the tach, and that didn't work. I hate to invest in a new cruise amp if the tach isn't working. I read about the taillamp bulbs affecting the cruise control, would they also affect the tach? Will the cruise work if the tach doesn't? Another problem I have is that the center heat/ac vents stay closed. The vacuum control seems to be working which I cn see when I remove the glove box and look at it while changing the a/c contrls. Someone said it could be one of the small vacuum controls which are in a bank behind the heater control?? if so, do you know whick one controls the center vents?
Thank you,
Carl

I don't know about the cruise, but regarding the tach, search the previous posts for a "cigarette butt" trick it worked for me.

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'82 300D Turbo 215k miles