His '76 is a W115 which does have the separate fans.

The A/C has vacuum and electric control. Turning the knob will cause the vacuum to be diverted to the proper diaphragms and move the internal air flaps and it sounds as though you can hear them move. The turn knob also energizes the electric portion (compressor, external cooling fan,etc.) of the system.

The blower fan (small dual squirrel cage mounted inside the center console) gets its power selected through the "change-over" switch mounted on the left side of the center console. They are easy to check. Remove either of the plastic grills on the side of the console and you will see the blades. Try to spin them using your finger or a small screwdriver with the power off. That is step one.

Crap! I just thought of something. Do you have a big dial which controls the air temperature and a row of push buttons or just one knob mounted center left which you turn to change the system?

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