On the 1984 300SD there is no overheating switch for aux fans as they didn't overheat unless severe cooling system or engine problems.

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MERCEDES BENZ MASTER GUILD TECHNICIAN (6 TIMES)
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. Shopforeman
190E 2.3 ITS RACECAR
1986 190E 16V