It is the proper function for your system. The electric/auxiliary fan will come on for two things, if functioning correctly. (1) The vehicle reaches 218-220° coolant temperature. (2) The A/C or defrost is turned on. The reason for the latter is because, as you noticed, the compressor adds strain to the motor. The fan turns on to combat a rise in temperature, due to said added strain, to help out the already insufficient cooling systems these jeeps were equipped with.

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