The battery icon on the dash came on without warning on my 99 Expedition 5.4 L, so I checked the voltage at the battery with the engine off and I read 12.5 volts. I started the engine and turned on the radio and the headlights and the A/C and checked it again and it read 12.5 volts again.
Took the alternator to Autozone and had them check it in the store and it read OK-150 A and 14.5 volts all diodes ok.
Put the same one back in and started the engine and still read 12.5 volts at the battery. Took entire vehicle to Az and had them check it in the car, result - bad alt. Went to Pep boys and bought a NEW one installed it . Light went out gauge looked normal. Drove the next day 50 Miles from home and guess what gauge read almost 0 and light came on. Had it towed to my mechanic and first thing he says is inferior quality alt. from pep boys and puts a new one on. Wife drives it home and on the way to work the next morning the light is back. She says get a new battery. We drop it off that night. He proceeds to check again and guess what finds bad alt again. Wife says get a new battery.
Puts another alt on. Now he is getting the message that something else is amiss. Finds the single wire pigtail plugged into the alt is bad. Gets a new one, light is off gauge looks good. Coming home from work the light comes on again. Wife says get a new battery. We take it back again. He puts a new battery in it. I drive it a little and I find that up to 1500 rpm gauge looks good and light is off but at 1600 light comes on guage goes to full max charge light comes on and all lights on vehicle burn really bright. Drop back down to 1500 or below everything returns to normal on dash. . Back to garage. Glad it is not the battery. Disconnects cables one at a time tries separate wires to all connections he can find. Now is sure car is possessed but also feels that 2 areas still remain to be looked into : Ignition switch and computer black box. We need some suggestions. No one he has consulted with has ever seen this before. Anybody got anything?

 

Voltage regulator?

 

Regulator is built into the alt. Every time the alt gets replaced so does the reg.

 

I wonder if you ran a scan of the PCM if any codes would show up?

Otherwise, call a priest.

 

Bad ground between engine and vehicle chassis.

 

Hey mikey dont feel bad from experience in working with auto
parts stores do not want to mention names cuz i am still with one
i have worked with auto parts for 12 years now the problem you
are describing is not uncommon in high heat conditions the regulator is gone & is not preforming the self exciting cycle
correctly. Swap it out as soon as possiable or you will litteraly
fry out your battery.There is uasaly not to many problems with
ford charging systems they are usallay bullet proof . Now i have
been a witness & fire extinguisher to many GM alternator fires.
Here in west texas it has been hot for about 3.5 months now &
you see the whole GM line up fail. Now as i said i work for a major parts store & last year i had to replace the alternator on my
lincoln it was 269.00$ at work with no discount because it was a
special order item. Called a good friend at ford dealer factory new
145.00$ with 36 month warranty .Now even if our unit was same cost & lifetime warranty was the hook i still would not buy from
my employer. Now this is all based on my experience dealing with
returns & some irate customers. There are alot of parts i will only buy oem any thing to do with charging system is number one on my list .Not cracking after market parts there is some good items
as far as bang for the buck is concerned.
good luck turboted