strange charging problem on F250HD
This is my first post, having found this website purely by accident. I don't know if I'm supposed to start off with a joke or something but I'll just get down to brass tacks:
I have a very strange problem with my 1996 Ford 4x4 F250HD pickup truck:
A few weeks ago I started it and was driving and just by sheer luck I looked at the voltmeter gauge. The gauge is calibrated to read 8V at the extreme left hand side, 18V on the extreme right hand side. A curved word “NORMAL” is in the middle of the gauge. When all is well the needle is usually on the “R” or “M”, which would put the battery voltage at around 13V (a “normal” reading). Well, on this day I noticed that the needle was all the way down on the “N”! It stayed that way for a few minutes. After a few minutes it dropped even further and the little red battery symbol came on and I realized that I was essentially driving on the battery. I pulled over and opened the hood. The alternator belt looked fine, everything appeared to the naked eye to be normal. I got back in the truck and the needle went back to the “M” all by itelf! I started to drive home and it did this again a few times.
I took it to a shop. They checked it out and said the diodes in the alternator were cutting in & out, which was causing the output of the alt to drop to nothing. They replaced the alternator. I took the truck home and it was fine but I didn’t drive it for a couple of weeks. I started it up last week and the same problem happened again! I called the shop and they were no help, they swore the alt was the problem.
Has anyone ever seen this odd problem before? Could it be the battery?