I have an '83 300SD (dubbed "The Phoenix" by the wife, see attached pic) which when I first got it had a problem with heat always coming out of the vents on the freeway regardless of how the controls were set. I was told when I got the car a few weeks ago that the A/C drier unit needed to be replaced, so I installed a new one. This did not fix the problem.

Yesterday I installed a rebuilt climate control switch assembly. While I was in there I saw a vacuum hose that had been disconnected from a round white plastic module with a single rearward facing nipple and then capped off. I reconnected the hose and hooked everything back up to the new control switch.

This seems to have helped a bit, but I am still getting slightly warmer than ambient temperature air out of the dash vents even with the fan off and the temperature control set to minimum. Turning up the temp in slow increments seems to deliver reasonable incremental increases in temperature. Curiously enough, the rear seat vent at the back end of the console puts out ambient temperature air no matter how warm the air is coming out of the dash vents. What can be causing this?

The A/C pump does not engage at all regardless of the temp setting. Am I correct to assume that I could test the control system by setting the controls to minimum temp with the fan engaged and the engine running, checking for voltage at the two wires going to the pump, and if there is no voltage I could test the pump itself by applying 12v to these wires? If so, which one is positive?

Last of all, I think I have seen here that a non functional tachometer can make the A/C not work? If this is the case I am in the process of hunting down a working tach amp and maybe this will cure my problem. Any helpful tips are greatly appreciated.

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