300e Climate control problem - help!
 

New to the forum and I apologize for "another" climate control post. Have searched the threads for hours but I'm on info overload and confused (and old).
1989 300e. Heat and AC works fine but only comes out of defroster and side vents. Nothing from center or foot well vents, no matter what buttons I press. Can someone please identify problem and how to fix. Thank you.

The defrost vents are the default vents if there is a vacuum problem.

Here is some reading for you.

As Jeff said above, that is the default position. There are pods with rubber diaphragms that fail after 20 years and don't hold vacuum.

Checking vent vac supply
 

Engine manifold vac => check valve => reservoir => controller { -> to vents }

Either a cracked, broken, disconnected plastic line or broken pod will cause the system to lose vac and default to defrost & side vents.
You can use any std vac hose. Save parts of plastic vac line to make connectors.

Vents have "pods" that are vacuum operated. Engine vac feeds check valve mounted behind Brake Booster, vac line routes under AC / Heater box, then battery into a reservoir [mounted inside rf fender well]. An output line goes back into eng compartment, then through the firewall to vac controller [mounted on left side of glove box cavity].

Remove the rf tire and plastic fender well [top to rear] to access the dual chamber reservoir. [Be mindful of proper in / out vac lines].
Remove the glove box to access the vac controller.

If vac controller is getting vac, then check each line feeding the vent pods to find the inoperative pod. Remove radio to access vac pod for footwell vents.

Thanks everyone for your help, I truly appreciate it. And thanks Bob for the logical roadmap for sourcing out the problem - very helpful.

If you remove the glovebox, you can see and test the solenoids that control the vacuum pods.

Jim

As Jim says, and this is also a good point at which to use a Mityvac or equivalent to see if each pod holds vacuum.