My 2003 Chevy avalanche has a problem wen I turn my AC on my engine fan turns of why is that
My car has 100000 miles.
My car has an automatic transmission.
Your engine fan is a mechanical fan driven by the engine crankshaft damper. If the damper is broken then the crankshaft may turn and not turn the serpentine belts. When you turn on the A/C it puts pressure on the belt, causing it to stop turning. Have the crankshaft damper checked to see if the rubber is broken. If you have an electrical fan instead of the mechanical fan then you may have a bad relay or A/C cycling switch that needs to be tested. If you need some assistance with this, feel free to contact a certified technician who can diagnose the issue with your A/C and fix it accordingly. Good luck.