I have 2005 F150 XLT 5.4 litre engine and my battery light comes on after letting the truck run on idle or sometimes driving.

Last Saturday went out drove it heater on and radio on I smelled something coming from the heater kinda like burning wire smell didn't think anything of it no smoke from heater (indicating the core was bad) no lights on the cluster nothing; went into the bank let it run came back out my battery light on. Drove it home and while driving the light goes off. Opened the hood and the smell was coming from the alternator. So I turned the truck off. Let it sit for an hour or so then I took my voltmeter to see if the battery was good. The battery was 12.4v with engine off. Checked my battery connection for corrosion or loose connections everything was tight there was some dust on the battery terminals so I brushed them off. I checked the battery cable that goes from positive to the alternator where it connects to the alternator with that nut it was good, clean and tight. So I started the engine up and I can hear a whining noise from one of the pulleys. Let it idle for 2 minutes no battery light so I put my voltmeter on the battery it read 13.8v, so I turned on the A/C, head lights, radio and the heater tried again the voltage dropped down to 11.9v then the battery light came back on. Also through all this my battery gauge in the cluster always stays a half way mark doesn't drop or go high. Next with the engine turned off I took the belt off checked the pulleys the idler pulleys seem fine and the belt doesn't show any glazing but the alternator was hot almost like touching a hot exhaust manifold.

So for the last 3 days I have been trying to come up with how to resolve this issue obviously it could be the alternator going bad but the battery has never lost a charge yet I let the truck sit for these past 3 days and today I started the truck and it started right up I don't see any dimming lights except it has a whining noise and the battery light didn't come on immediately I let it run for 5-10 minutes before it came on; I even drove it around the block and the whining noise gets louder every time I step on the gas. To me it's either I need new alternator, tension pulley is bad or the idler pulleys are bad but they turn fine. I'm at a lost here hopefully someone from here has some advice for me.

 

I would say alternator. The charge gauge is a joke, glorified 'idiot light'. Don't rely on that for diagnosis. With a load, your alternator is not putting out the amperage. Also the heat, the burnt smell, the noise...yeah, it's toast.

 

All signs point to alternator. Remove the belt, turn the pulley by hand, if you hear any noise, your alternator is going bad. At no point should your alternator ever be that hot. You've been very thorough, but I think you already know the issue

 

bad alternator... "normal" life is around 150K miles... sometimes more, some less. If your in that range, it is time............ and your symptoms say its time. When the belt is off, spin the idler pulleys by hand to confirm no additional problems.

 

I will be buying a new alternator tomorrow and hopefully it solves the battery light.