There is something wrong with my climate control system in my car, Im sure of it! How come my brothers 85 300SD gets hot air through the main vents (the vents in the middle of the dash) but I only get hot air through my defrost vents. I only get hot air when the defrost button is depressed...and I just dont understand how the system works! Like what does the EC mean? What do the arrows represent? Why doesnt any hot air come out the rear vent? I dont know why it does what it does...how should the climate controls work in heat!?

'Lot of diagnostics ahead of you.
1.Defrost is the "fail-safe" mode for all Mercedes (It's all you really need)
2.Lots of little vacuum operated "pods" opening and closing doors in HVAC
system.
3.You may have a combination of problems.
4.Your CCU (Climate System Controller) may have gone south on you.

5. MityVac vacuum tester helps in the diagnostics. ($30.00 USD)

EC stands for Economy

The arrows (black or white,up or down) tell you system functions and where
the air is going.

Call N.A.M.B. on Monday and order an owner's manual. 1-800-222-0100

This site and the people here will help show you where to look for the
solutions.

Probably because your brothers car has an inoperative center flap vacuum actuator. Only cool air should blow from the center vents.

The SDL, like most vehicles of it's vintage, suffers from failing vacuum pods.

You'll need to open up the access to the switchover valves (remove the carpeting on the right side of the transmission tunnel) and check each circuit for holding vacuum. You'll find several that won't hold vacuum and those pods need to be replaced.

Post on the thread when you get stuck.