In addition to what Sixto has provided -- There are two relays in the relay box (behind the fuses) that turn on the fan in high or low speed, depending on which switch, as described by Sixto, is activated. The relays are usually reliable. I would check the high-pressure switch in the refrigerant line first. Unfortunately, replacing this switch requires evacuating and refilling the refrigerant.

OTOH, the high-speed switch in the water jacket can be replaced without draining the coolant, which is nice. Sixto and I have both replaced our stock switches with one that turns on the aux fan at a lower coolant temp (100C) to additionally protect the engine. The stock switch turns on at 105C.

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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