Lights dim

[F-450 HOTSHOT]

I have a 99 for f-450. And recentlly i have noticed that the interior and the headlights dim a little (noticeable). Whenever i am wot peddle to the floor. And they get back to normal when i let off. My voltmeter on the truck does not move reads that evereything is fine and it does not have trouble starting in the morning. So i think that my batteries and alternator are good. My batteries r about 6 months old. Does anyone know what could be causing this. All help and comments are appreciated. Thanks a lot.

 

[JSchuricht]

Can’t help much with your voltage drop, but I can tell you that the voltmeter in the dash is useless. I was running a sound system that would almost kill all interior and exterior lights at night and the volt gauge never moved.

 

[FordMastertech]

Belt slipping or a bad alternator.
FMT