AFAIK, both fans run in high or low speed. That may depend on the year of your 124. In my '95 E300D, the fans come on in low speed if it's at all warm outside and the a/c has been running for awhile. They come on especially when the car is stationary or moving at low speed but if it's warm enough they come on even at highway speed.
The fans run in high speed when switched on by a switch in the water jacket (early 124s) or by the climate control (later models). Since you mention a "green relay," you have one of the later models. I don't know when they changed from relays with external fuses. The engine has to get really hot for the fans to come on in high speed. If they do, it's probably best to turn off the a/c until the engine cools a little.
Jeremy
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"Buster" in the '95
Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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