As used in the 1983 C60 HVAC system, the defrost/bi-level damper motor is dual actuated. Both sides of the diaphragm have vacuum hoses connected to them.
If the mode select is in DEFROST, vacuum is applied to the outer chamber of the diaphragm (via the red hose) and the other side is vented to atmosphere. In this configuration, the damper will move down and direct most of the available air (80%) to the defrost vents in the dash.
If the mode select switch is in any position other than DEFROST, vacuum is applied to the inner side of the diaphragm by the blue hose. The hose to the outside chamber is vented. This moves the damper upward - which directs the majority of the air flow (80%) down to the floor vents.
You can test the function of the actuator with a length of hose (windshield washer hose, emissions vacuum hose, etc - whatever fits snugly on the actuator nipple).
The system doesn't have to be on. Disconnect the factory actuating hose from the diaphragm nipple and push your test hose onto it. Then just suck on the hose like a $2 *****.
If your hose is long enough, you can lay on your side and look up into the plenum and observe whether the diaphragm/linkage/damper reacts as designed. With vacuum applied to the outer chamber, the damper should move down and block off the flow to the floor vent.
If it does react like that... then the operating motor is okay and it would be safe to assume that there is a problem elsewhere. It could be with the mode select switch, the tubing connections at either the mode select switch or motor, the interconnecting tubing or possibly there is a lack of total system vacuum from the engine's intake manifold. Do all the other mode select settings seem to direct air flow to the various vents - as designed?
Here is the function matrix, vacuum schematic and the control panel (showing a description of each mode):
You must be registered for see images attach
You must be registered for see images attach
You must be registered for see images attach
