Through the years the fans are controlled differently. Basically the fan comes on when the refrigerant pressure gets to about 290psi in the drier. This happens quite fast on summer days, thus the fan always seems to be on when the AC is on. This amount of fan (low speed) is generally enough to also keep the engine cool. The high speed fan comes on (same fan just different circuitry) when engine temp is above 100 varies on different years.

A common problem is the burning out of the ballast resistor that lowers the speed for the low speed function. When this happens only the high speed works if the engine gets that hot.

Another common problem is that a slightly low on refrigerant AC will not produce the high pressure necessary to turn on the fan thus the whole car runs hotter.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician