Mercedes: CLIMATE CONTROL/ ROUGH IDLE, water control system, vacuum leaks

Question
HELLO I RECENTLY PURCHASED A 1990 500 SL THE CAR HAS 90K ON IT, THE AC COMES ON IN CENTER DUCTS, BUT THE SIDES BLOW EXTREME HEAT? IS IT THE CONTROL UNIT OR IS THERE VACUUM ELEMENTS NOT WORKING, OR IS THERE A VACUUM LEAK THATS CAUSING A BAD IDLE AND NO FLAPS WHERE DO I BEGIN?  BTW I IN STALLED NEW SPARK PLUGS SO FAR AND THE VACUUM TUBES BETWEEN THE INJECTORS THEY WERE CRACKED. THANK YOU DAVID

Answer
Hello David,
I would suspect vacuum leaks. As the hoses age they become brittle and crack without appearing to be in bad condition. There is also a mono valve involved with the water control system. This electrically controlled valve regulates the amount and direction of the water flow. It may be sticking, however because your vents are divided, I would be more likely to say it is the defrost servo, or hoses.
The interior temp control manages the fan speed, and the amount of heat and cold to adjust interior air temp. There is also AC added anytime the defrost is on to remove the humidity on the inside of the windshield. On my 430 there are side duct to the door to defrost the side windows and this is what makes me think the defrost servo is at fault.
More that likely the blending door actuator,or vacuum hose is not operating properly. Because so much of this stuff is in and under the dash, I would start looking forward of the firewall first where heat and atmosphere can affect the system.

I would start with the most obvious and least expensive points to repair first. You borrow or purchase a hand vacuum to check the system. They have these for about 40.00 at parts supply houses. They are used to bleed brakes too.
Hope this helps. Good Luck and let me know what you determine.
Robert