Not positive on your truck, but *usually*....

The engine fan is for cooling the motor. If you're having overheating issues, you need to check the condition of your cooling system and especially the fan clutch.

The electric fan is an assist fan for the A/C and comes on with that whenever the A/C clutch is engaged. It might also be wired to come on if it's running hot, but mainly it's for better A/C performance at low speed/idling.

--Dan