2011 F150 XLT 5.0
46,000 miles
Bought from a local dealer in February of this year.

For the last month or so, the battery has been dying intermittently. Sometimes overnight, sometimes after sitting for a couple days. I work from home so it's not uncommon for it to sit for 2 or 3 days at a time. First time it happened, I went to get into the truck and noticed as I was walking up to it that the buttons on the fob weren't doing anything (no locks, no lights). Thinking that the fob battery just needed replaced, I got in and turned the key....nothing. No lights, nothing on the dash, no crank...just dead. Charged the battery overnight and it was fine for a few days...then it happened again. Thinking (hoping) maybe the battery had a bad cell (it was the original battery), I bought and installed a new one. Good for a few days...then it happened again.Charged the new battery back up. Hooked up the multimeter, waited 40 min at which point the reading was 10 - 20 mA which seems fine. So whatever is drawing power doesn't seem to be doing it constantly.

It happened again last week...so after reading in some other forums, I got some dielectric grease to put in the C405 connector in case water was the issue. It has been raining A LOT here this year, so that's not unreasonable. A few days after doing that, it happened again. I have no aftermarket electronics....the truck is stock. The dash lights, dome lights, and headlights all go out after 20 seconds when I turn the truck off.

What are some other things I should be checking? How can I identify this issue? I'm hesitant to take it in to a shop yet because I don't want to be throwing tons of money into labor and not see any results. I'd rather check as many potential culprits as I can myself before I take it in somewhere.

Any thoughts or recommendations are highly appreciated.

 

 

The first couple times, I did charge with the negative cable directly on the negative post of the battery. I just put the charger on last night and this time, I did connect the negative to a body ground point.

Just to be clear, what are the consequences of charging directly on the negative terminal with the BCM? Is it possible that would cause the battery to drain down to less than 8 volts? (Doesn't explain the first occurrence since I hadn't charged the battery at that point.)

 

 

Sounds VERY similar to my issue (link). Dealer tried to screw me out of $300 by "diagnosing" a bad hood sensor (which I confirmed myself as fully functional and NOT corroded or intermittent) so I took my truck out of there! Right now I just try to drive it a little every day and thats working. What is the C405 connector and where is it located? Also, have you checked your AC voltage? Every Ford of these generation I have tested shows very high AC voltage, although conventional wisdom says thats a sign of bad alternator diodes...

 

The C405 connector is on the driver's side between the rear bumper and the spare tire. The info I got for that connector came from this thread in another forum: https://www.f150forum.com/f38/c405-c...r-help-378307/
I haven't checked anything with the alternator yet. Were you able to fix the issue you were having yet?

pdqford, I get that part of it. I'm just wondering how the computer operates the truck differently when the battery is charged via the terminal rather than a body ground. If I charge via the battery terminal, the battery still gets charged, the truck still starts and runs....what is being operated differently since the computer wasn't able to detect the battery charging?

UPDATE: So it's been 4 whole days here without rain (haven't driven the truck) and the battery is still good. I'm suspecting more and more that this is a water issue. I'm going to check it every day for a while and keep notes on the weather. I think the forecast has more rain on Thursday. So if it dies again right after, that may be an indicator that I'm getting a short somewhere when it rains. That'll be fun to track down.

 

 

I'm going to put that one in the "mental" file for future reference. Thanks

 

UPDATE: It's been 2 weeks now...we've had more heavy rain. But I have not experienced the issue again since the last time I posted here about it. There were only 2 things I did differently the last time it happened:

1) I unplugged the charging cable for my phone from the SYNC usb port. I usually left this plugged in all the time. I would be very surprised if this caused an issue, but I pulled it just in case.
2) This last time that I charged the battery, I hooked up the negative cable to a body ground point rather than the batteries negative terminal so that the BMS would detect it.

Other than that, I haven't made any changes. I did check the trailer connector but didn't see any issues there. I am clueless at this point. There is no consistency in the behavior so I don't know how to diagnose it. I'm sure a garage would tell me a similar story...only more expensively. I guess I will just wait for it to happen again (and hope that it never does). Will report back here if I ever get any more useful information about it or if I figure it out.

 

 

Common, known causes of battery drain ...
.

• LEAVING THE KEY IN THE IGNITION. *TAKE IT OUT BEFORE LEAVING THE VEHICLE.
• leaving devices plugged-into the electrial recepticles. *they're ALWAYS hot.
• faulty ignition switch. *symptom - key is loose.
• faulty shift lever micro-switch. *does not register as being in P(ark).
• faulty door -closed- switch.
• the mirror cover on the passenger visor does not close causing the light to stay on.

Don't forget to follow the instructions in the owner's manual regarding changing and/or the battery and the need to rest the BMS (Battery Management System).*NOTE: FoMoCo dealership service can reset the BMS using their IDS (Intergrated Diagnostic System) negating the eight hours of vehicle sleep time requirement.

(owner's manual) Don't have one -OR- would like a digital, searchable desktop document?.

 

Is this truck still running the stock battery?

 

UPDATE

The truck is still having this issue off and on. New battery was installed in October. Trailer plug/wiring looks ok. Finally took it in to a dealer in January for diagnosis. They were unable to find any definitive issues and said their best guess would be the alternator. Their head tech said he's seen them develop a bad spot in the core that can cause current draw if the alternator stops in just the right orientation when the vehicle is turned off. I was skeptical, but it sounded plausible enough to consider. It would also explain the seemingly randomness of the issue.

So I replaced the alternator with a NAPA brand reman unit and the very next day, the truck was dead again. Then I remembered that my truck wasn't totally stock. I had replaced the reverse light bulbs with LED replacement bulbs shortly after getting the truck. After some more research, I found a forum where a guy described a similar problem in his car. He had replaced his reverse lights with LED replacement bulbs and had used them without issue for 6 months. But then he started having battery issues and discovered that the chips in those lights were somehow providing enough feedback that the computer was not properly shutting down certain modules after the car was turned off.

So with nothing else to go off of, I put the stock bulbs back in the reverse lights. The truck was fine for over 2 months (the longest it had gone without issue since this problem started). So I was feeling more and more confident that I had found the issue. Then this past Monday, I went out and it was dead once again. Battery was drained down to 5v. I hooked up the charger but the battery did not seem to want to take a charge until I disconnected the ground terminal and hooked up the charger directly. Then I reached down and touched the alternator which was quite warm even though the truck had not been run for 2 days at that point. So apparently NAPA sold me an alternator with bad or failing diodes. I'll pull it and try doing a continuity test on it, but I'm thinking that has to be the issue this time. (Side note, it did rain lightly Sunday night...which again made me suspicious of a water issue. But I don't see how that would have had any effect on the alternator.)

Now the question is, what was the original issue? The original alternator? The LEDs? Or something else I haven't found yet?