So I'm on the freeway the other night doing about 70 so the engine is at about 2700ish. I had my radio playing like normal(only 250 watts and bass is not up) and the lights on. It gets a bit warm so I turn the vent on. The fan just happened to be on high and I noticed that when it turned on the headlights dimmed for a split second. Maybe its nothing but to me I'm not drawing enough current with the stereo and headlights for the headlight to dim when I turn the fan on from a dead stop especially when the engine is revving so high. The battery is a Red Top Optima about 2 years old. Anyone think the alternator is on its way out? I have read that the voltage gauge is just an idiot light in disguise but it reads lower than it did when I first bought the truck 4 years ago. Ideas?

 

Mine does it too. i don't have an audio system in my truck though. normally i figure its the spike of voltage drop from the initial fan start up. the regulator system of the alternator is designed to just produce enough power to keep the battery at full charge and run any accessory systems that are currently on. the start up of the fan causes a voltage drop for a split second then the system catches back up.

 

I figured as much. Thanks for you help.

 

glad i could help!