I replaced the battery this morning in my work truck. A 2017 F150 XL. Pretty straight forward. Dead as a door nail. Pulled battery and replaced it with a AGM battery from Autozone. All the normal stuff, cleaned the terminals and hooked the battery up positive first, then negative. As soon as the negative terminal hit the post the horn started going off. Solid tone and the key fob would not turn it off. Had to pull the horn fuse. Put horn fuse back in and now the horn works with the key fob, but not the horn button. That is the least of my worries. The headlights won't turn off, they are constantly on even with the switch off. The strangest thing is the flashers won't turn off while the key is off. When the key is in the on position the flashers turn off.
I have done the BMS reset a dozen or so times. The battery icon flashes, but after turning the truck of and off the battery icon always comes back.
Pretty straight forward battery replacement.
What am I missing?

 

You tripped the Theft system. Lights on , Horn blowing etc.
Disconnect the Battery for at least 10 minutes, then reconnect.
BMS has no part in the Theft system.
Only reset BMS for new Battery to clear out old Battery data so it handles the new Battery, correctly.
Battery Icon will always light up at ignition on, but go off with engine starting as long as that system is not in fault.
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If after this, there is a power drain trouble, that is a separate issue, or this will happen again if not found and corrected.
Some notes to be a where of:
1. The PCM will re-boot from any Power down and clear any codes and go into self diagnostics doing 7 or 8 Emissions tests that take some time and drive cycles to complete because they are tested dynamically.
2. After truck sets for about 45 minute its systems go into sleep mode and current drains reduce to 50 to 60 milliamps.
If there is a system problem that does not shut down. It might be a cause of Battery drain to quickly, over time.
A fault free system should go more than a week without any starting issue from a fully charged Battery.at last shutdown.
Good luck.

 

Why would the theft system be tripped? Because I opened the door with the key and not the fob?
Ok so I unhooked the battery for 15 minutes and as soon as the terminal touched the battery post, the horn went off. Another note, the electric door locks don't work.

 

If there is any hint of heavy sparking when Poat connection is attempted, it is caused by the issue that has run down your original Battery is still present.
The problem causes a loss of communications with all the Modules on the BUS system.
They remain in-operative.
Recommend you Goggle information on Ford F150 Thief system and what to do with it.
It gets a bit involved because it was simple; it's not a good system.
It's preventing you from using the truck by just replacing the Battery.
Only difference is the truck has a high Current drain issue >>fault<< that can't be cleared in a normal way, due to the Fault.
I can't help you any further.
You need to get the fault cleared first.
To get a handle on this, pull fuses on the main box until that load goes way down. Then what circuit on that fuse is the cause. S
Try to bring up one or both websites to get further grounding on the Theft system.
They are :
CAR CLASS ADVISOR FOR f150 Theft System.
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JohnOnWheels, Is your F150 in Thef Mode?

 

The system has a high drain fault that is your original fault that drained to old battery.
Until that is cleared, the Thef system will not re-set.
To get a handle on the circuit causing the high drain, pull fuses one at a time to find the one that greatly reduces the sparking when Battery is cable is touched.
Then run down the circuits fed off that fuse to see what function is causing the issue.
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To get more information on the truck's Thef System, try Googling thee following;
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JohnOnWheels, is your F150 in Theft Mode?
CarGlass Advisor, How to re-set Theft Mode F 150.
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These will give you back ground on the system and how to handle it.
The cause is likely the original fault that caused the old Battery to drain flat.
You just missed the relationship until running into ther problem over again.
I can't help any further.
Good luck.

 

I give up. It's a company truck so to the dealer it goes.

 

Turns out its the Body Control Module. What would make that go out?

 

Do you have any water intrusion in the passenger foot well?

 

I had an idea it would be found in the BCM, but My Crystal Ball and AI did not tell me.
The heavy lead off the Neg side Bat terminal feeds ground to that whole system.
They likely pulled the high amp fuse feeding it to see where it was coming from. That leads to an automatic replacement because it most often can't be serviced.
You would not have been able to do anything different about it.
As to the cause just one of those things happening while setting, like you say. If there was another factor involved to cause it, we can only guess it was.
Normally a Battery can be replaced without setting off the Thef system if >>intermittent<< connection is avoided. doing the change.
Good luck.

 

No, none.