I don't think that the auxiliary fans come on at that kind of speeds. Whenever I drive that fast in summertime I have a constant temperature reading of 95 degrees.
I have driven with the auxiliary fans continuously on (no ground connection with the dash cluster = engine temp control) in summertime and at high speeds and the engine temp only reached about 90 degrees...so there is no need fro the auxiliary fans to come on at those speeds with sufficient airflow.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)