I have a 84' 300 CD. The A/C does blow cold, but only out of the two circular side vents and the defroster simultaneously. The buttons on the control unit doesn't change where the air comes out. No matter which button is pushed. Before I tear out the dash looking for bad vacuum lines and valves, how can you tell if the climate control unit itself is bad, and that's all that needs replacing?
And help would be appreciated........

I think this sounds more like a lack of vacuum to the climate control system. That was what caused this same symptom on mine. You don't need to tear your dash apart yet - you can start by ensuring proper routing and connections under the hood, then check the vacuum pump for proper operation and the other vacuum systems (doors for example) for leaks.

With a Mitac and about 30 minutes you can eliminate many potential causes.

Does everything else work on your climate control unit? Hot to cold change? Ambient air? Blower speed control?

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The A/C will turn off using the panel and rotating the temperature ring will cause heat to come out of the same vents the A/C is stuck on. I can change blower speed too. I just can't get the unit to change to different vents ... not good in 98 degree temps.

You could rig the center vent flap open if you were so inclined. You can access it by removing the glove box.

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Doesn't the W123 series have the vacuum switchover valves on the RH side of the console?
If so, most of the troubleshooting can be done from there.

Probably the best place to start is here: http://www.peachparts.com/shopforum/general-information/248948-mercedes-service-manual-source-cd-dvd-paper.html

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84 300SD 350K+ miles ( Blue Belle )