I have a 2010 F-150 crew cab with the 4.6L 3V motor. Date of manufacture is 10/09 according to the driver door sticker. It has never had any electrical issues before this. I did replace my timing chains and sprockets this summer, but that seems unrelated.

Last Saturday July 20 I was driving home from vacation and I noticed my truck had a very odd stumble at idle. The RPMs would drop way down into the 350-400 RPM territory they go back to normal. At this point the truck is driving fine. I have an in cab volt meter and I noticed I only had 13.7 volts. This truck has always made at least 14 volts at idle. I get on the road headed home and I've still got 13.7 volts. With headlights and AC on, I'm at 13.6 volts. I figure the only thing that could cause this is a bad alternator. I check my DTC via the OBD tool and I've got a code about a O-2 sensor too lean. This code has been intermittent for years so I reset it. No other codes ever come up. I double check the control module is also reporting the low voltage, it isn't just that my meter suddenly went bad.

I found an autozone with a reman. duralast alternator in stock. I buy the alternator and borrow the tools to change it in the parking lot. I change it & while I am doing this disconnect my negative battery cable for safety. I get back on the road and I've got 14.1 volts even at idle. The stumble is gone at this point. I figure things are fixed. That night I stay in a hotel. Next morning I am driving again and I've got 14.1 volts at the start of the trip. Somewhere after 300 miles of driving, I'm back to 13.7 volts while driving. Since I don't really intend on making lots of stops, I just keep on driving. Even after getting fuel the truck turns over just fine after getting gas. It has no running issues.

I get home and i'm curious to try and figure this out. I found on the harness near the underhood fuse box a 2 pin connector that seems to run to the rotor on the alternator. If I unplug this my idle voltage drops from 13.7 volts DC to 13.2 volts DC. So apparently the rotor is getting some current from the trucks electrical system. I check around on this and other forums and apparently a 2010 may have something called "BMS". My truck has no sensor on the ground wire, so I don't see that as a problem. This truck has always made over 14 volts DC, especially when driving. So something is wrong now.

I reseat all the connectors on the computer on the firewall and then I have 14.4 volts DC while driving and over 14 volts while idling. However, I went for a trip today to the other side of town and sure enough during the trip my voltage drops back to down to 13.7 volts again. If I turn on the AC and headlamps I've got 13.5 volts. Doesn't matter if I am driving or not, the voltage never even gets up to 13.8 volts.

So my conclusion here is whatever controls the alternator rotor current is not always sending enough current to it to create charging voltage. But it is sending some current, otherwise disconnecting it would make no difference. I looked around under the hood and it has no obvious control box. The control must come directly from the computer mounted on the firewall. So I'm guessing the computer is just not sending it enough current for some reason? Is there anything else I can check on this? If I go and reset the computer by disconnecting the negative cable on the battery, it temporarily goes back up to over 14 volts DC for a while. But it always drops back down for some reason. What do I check next here? Is there any other components I can replace other than the computer?