Battery light comes on and batteries are not charging. I put new batteries in it. Ford has replace alternator twice now. They cant figure it out.

 

If the truck runs then you know the alternators are putting out the correct current. Most likely a cabling issue to the batteries.

 

Exactly the same problem here. Saw a you tube showing a common harness area under the engine that could be the problem. i did the best i could to check it but don't think that is my problem. Changed batteries, and took it to a shop. Near dealer cost, they replaced the Alt and fooled around but could only tell me that a bad alt could have messed up other things and for loads of money they would just keep replacing things till I ran out of money. Still searching for the problem. 48 hours and a charged battery is dead. Someone help!

 

Check the small leads going to your alternator. Just had an episode of not charging on my 91 w/ 7.3. Alternator tested good 2x. New regulator. Turned out the plug that goes on the alternator wasn't snug enough.The end that went over the spade connector of the alt was stretched. As it was buried in the plastic plug I used a knife to carefully pinch it smaller. Fit snug now. Charge light is out...dash gauge reads higher.

Hope this helps.

 

False. You can completely disable the alternator on a vehicle and it'll keep running 'till the battery voltage drops below a certain point. You can also have a defective alternator not putting out the correct voltage or current and the engine can keep running indefinitely as long as the battery voltage stays within a certain range. The alternator on one of my vehicles is defective, but it's been running fine that way for over a year.