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2006 Ford F250 w/ alternator problem

Hi! I have a 2006 Ford F250 Superduty 4x4 6.0L Diesel with an alternator/battery problem. We have charged the batteries (had them tested at auto zone), and replaced the alternator and still cannot get the batteries to keep a charge. I’m not talking a slow drain, with the ignition switch on for less than a minute it drains about 4 amps and the truck dies. We also noticed a trouble code “TBC fault” and discovered that was to do with the trailer brakes. Disconnected the trailer brake system (have been noticing a “check trailer” warning occasionally recently). Any other suggestions as to what could be causing such a massive drain on the batteries?

I’m not clear, did the battery drain stop after you disconnected the brake controller?

no, it did not. We have since taken the new alternator back to autozone and had it tested. They found that it was not working properly and replaced it. We put the 2nd new one in, only to have the truck run for about 10 minutes and shut off again. Now the batteries won’t even charge!! My grandfather (a mechanic of 30+ years) says he has never seen a problem like this before, any suggestions are helpful at this point!
Thank you!

Aftermarket alternators can be real junk. I would try a junkyard alternator or else spring for a high quality aftermarket part. When it comes to alternators the junkyard has been kind to me.