Myron's

I would highly suggest it is the belt. Serpentine belts wear their grooves too wide and the belt rides on the top of the pulley instead of in the groove. Parts stores have belt gauges to measure the groove width in the belt.
Buy a new belt before you buy another alternator.

LD1993F150

i will take at look at it. See if what your suggesting could be the problem. I have been wondering why it discharges when I apply the breaks.This only happens when the light and AC are both at the same time. Could it be due to the extra tension placed on the belt from the engine. What are your thoughts.

Chris_1

iTrader: (1)

I think you want a 95 amp alternator minimum with a/c. If you have a 70 amp that could be an issue.
You could look into doing the 3G upgrade - there are writeups on here for that.
You might also want to hook up a digital voltmeter at the battery posts to get a true reading of the voltage - the dash gauge isn't real accurate - maybe it's just a little too sensitive and indicating a problem when there isn't one ?

LD1993F150

OK, I've changed the belt. Kept the old one just in case. No change in problem. I have considered getting another alternator, but I went from a 70 amp to 75 amp. This should be sufficient if you consider the truck came originally with a 60 amp, and there have been no changes to the system.

digitaltrucker

I believe I have read posts here where folks have had similar issues going larger. When they returned to correct alt, problems went away. More isnt better, and getting a good rebuilt alt is harder.