I have an 01 f150 5.4l. This only happens when winter hits. While i'm driving to and from work at night my voltage will randomly drop, lights dim way down, blower slows way down, and voltage needle drops to below the battery pic(normally runs just above pic). Battery light is off. After a couple minutes voltage returns, needle for gauge comes up, lights brighten, etc. Once in a while the belt squeals when voltage returns. Alternator seems to be charging because idiot light never lights. Its like there is a massive load put on the battery when this happens that the alternator can't keep up with. Truck runs and drives fine while doing this. Any takers? Thank you.

 

Most likely, the alternator is failing, probably due to brushes wearing our and making intermittent contact.

It's also possible that there is an intermittent electrical fault in the voltage sensing circuit that's used to control the output voltage.

 

Since the alternator light doesn't come on, I'm going to say that you have an intermittently bad battery. I've encountered a LOT of them in the last 6 to 8 years, ever since Johnson Controls bought out most of the other battery manufacturers. Get your battery tested but that may or may not find the fault.

 

A "bad" battery won't behave like the OP described, a bad alternator will.

The battery warning light is not a conclusive indicator one way or the other.

 

I have replaced the alternator every January for the last 3 years. Every time I do, I have no problems for the rest of the winter. Here we go again this winter.

 

Where are you buying the alternators?

 

I'd ask what Roadie did, and also:
What kind of non factory electric accessories do you have or use in the truck? Is there a plow or winch on your truck, big wattage flood lights, a big stereo amplifier, etc. Something that would stress your charging system.

 

They are AutoZone units. Everything on truck is completely stock from factory, has tow package, that's it. The drop hasn't accurd since my first post, but it's not as cold.

 

Back when I owned a 95 Mark VIII Lincoln, the forum I participated in seemed to be in agreement that the only alternator that would last on that car was one purchased from the Ford dealer. The "rebuilt" alternators as I understand just have replacements parts that just get them to work. Of course Autozone sells several and some are new.

 

I have a lifetime warranty reman from O'Reilly in mine - it's been fine for a few years.

 

I had a similar issue in my Expd, not quite as severe. The Edge CTS was showing voltage dropping to 11.0 volts at time with heat and lights running. Replaced Alternator with a DC Power Engineering 180 amp version, issue has not come back. Voltage now never drops below 12.4 volts, generally 13.5 to 14.5.

 

What did AZ say when you took the old units back? Did they test them and did they confirm that they were bad?

It sounds like it's time to find someone else to buy from. A life time warranty is great but not if you have to spend hours replacing the same part over and over again! That's why I stopped buying rebuilt items from Discount Auto Parts. Too MANY of them were bad right out of the box!