I have a 1973 450 SLC blowing cold air from the center vents when using the Air Con, but hot air from the ground vents, or defrost vents. Any ideas?

There is a vacuum controlled heater valve next to the brake booster.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

I have located that. Thank you. Is there a way to test to see if it is actually closing/opening. Or do I assume it has failed? Is there an acuator under the dash inside the car that this controls?
I really am new to this, I have tried to do some work on this car, but my current repair manual is not that specific.

Thank You for your help with this

If you have a hand vacuum pump, you can "pull" vacuum on the diaphragm & see if it holds vacuum.

There are several variations of that system, post a picture & I will see if I can find the vacuum schematic of you style system.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

After an hour under the hood, the vacume lines were hooked up in error. The Heater Valve holds vacume, and is now closed.
Air condioner is not working so good, but I will handle that with an air con shop.
Thank you for your professional guidance.