There is a huge door which blocks the fresh air intake. It's operated by two vacuum elements. Both vacuum elements are dual stage. Applying vacuum to only one stage closes the door most of the way, resulting in ~80% recirculation. Applying vacuum to both stages closes the door all the way, completely shutting off outside air. Trust me, it works.

On an early 124 you can see the big 'ol door by pulling the glovebox and using a mityvac to operate the vacuum elements. I guess it could be done on a late 124 as well, but you'd need to pull the passenger airbag.

I would bet many 124 cooling complaints would trace to failure of the vacuum elements. The 2nd stage seems to fail much earlier and more frequently than the main stage. So many cars are running around with only 80% recirc operating. Getting to 100% makes a big difference.

- JimY