WHen a/c is running car kicks fan into hi gear. What causes this
Your fan should run when ac is on and car is at an idle or at low speeds.
When running the A/C, there is an increased fan RPM (caused by a relay) to create more airflow through the radiator and A/C condenser. It's the airflow that leeches heat away from the A/C coolant and engine antifreeze and ensures the continued proper function of both systems.
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