Not to dispute you, but how do you know that "...The fan ... is always on, even at highway speed." -- this assumes you're talking about the mechanical, engine driven fan, because your '82 shouldn't have an electric fan unless you installed it yourself.

So what I mean by asking is that, given how noisy an '82 240 is (lots of wind noise), how do you know the fan isn't freewheeling when driving at highway speeds -- you didn't ask your wife to crawl under the hood and watch it while you're driving, did you? (not meaning to be sarcastic, but only a bit humorous).

There is a test of the clutch (and you don't have to be driving) -- although you may not get definitive results in frigid weather.
First, after the engine is slightly warmed up, rev the engine (twist the spool for the throttle by hand as you watch the fan) and listen to the sound of the fan as it declutches -- the initial roar will quiet slightly as the fan decouples from the water pump hub.
Next, let the car fully warm up to near overheating temperature -- you may even need to block the radiator with some cardboard -- and then (after removing the cardboard) rev up the engine again and hold it. You should hear the fan's sound change as it couples and decouples, cycling "on" and "off".

Hope this helps.