Sounds like the temperature switch has failed closed. Trace the upper radiator hose back to the head, shortly after the rubber stops and the metal starts, there is a sensor with two or three wires. Pull a wire or two and the fan should shut off.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC