I’m thinking this isn’t going to answer your question, but I spent some time wondering the same thing. I have an ‘80 300D that I bought new in Dallas, TX. For many years it ran winters and summers at the proper engine temperature range. Then one summer it started to run hot. The next year hotter than that. Well, that was about 8 years ago. I put a new pressure switch (that runs the fan) on the filter/drier, but that didn’t fix it. Some times with the A/C on I could hear it running. Sometimes not. Sometimes in traffic it would heat up to the point where I was rushing to get off and either shut it down, or get enough speed (air flow) to cool it down. I changed the engine driven fan assembly as well. Still no fix. I then pulled the two wires going to the pressure switch and found that a 15amp blade type fuse would connect the two of them together quite nicely. So now, every year when the outside temp hit’s the 80s, I put the fuse in. I take it out around mid November and plug the wires back into the switch. In other words: During the hot summer months, my fan comes on when the ignition switch is turned on. That was 8 years ago, and I’ve had no overheating problems in any type of traffic since.