Ok ive got an 89 f250 custom 2wd. Everything on the truck is BRAND new. Its got 11000 kms on the new motor, cooling system etc. I believe it origonally had a 60 amp alternator so when it died i got the largest factory alternator which i believe was a 75 amp... i think. The truck has no power windows, a/c, etc. no luxurys and i love it! I'm running a 250 watt amp and a 10 inch subwoofer behind the seat. I've never had a problem with the sub since the new alternator and now all of a sudden when drivng at night and come to idle at a red light with my lights and heater on my amp gauge drops down almost to the red and the sub starts cutting out. If i turn off my heater it gets a little better and with the lights off better still. And last night i turned the key to accessory and had the stereo going and the sub was cutting out even with the truck off.. this is a problem i have never had before. The only time that it ever cut out was when the original alternator packed it in but my amp gauge is normal when the revs are above idle and the battery light never comes on so there appears to be nothing wrong with the alternator. Any advice for me??????

 

if you can still return the new alternator, i suggest you do so and install a 3g alternator from a 94-95 5.0 mustang or taurus etc etc, they are 130 amp and only require a new charge cable and some grinding on the alt mount bracket.

or you can just check the charge cable on your 60 amp alternator, they typically burn out

 

3G_Alternator
has really good wiring schematics and 3g swap details
3G Alternator TECH - FSB Forums
3g swap thread

 

here's a link to a thread of mine concerning charging problems seeing as it never happened before with that alternator i would check these things that i replaced
https://www.ford-trucks.com/forums/7...-problems.html