Battery light comes on when engine gets below 1500 RPM. Started about 1 month ago. Then it was at 3000 RPM battery light would come on and lights would dim. Ford dealer seems puzzled. Serp belt is fine. Battery charges back up at idle. Now cannot get over 30-40 miles per hour before light comes on and starts to discharge. Anyone have any ideas!

 

They should have done a load test and regulated output test at dfferent rpm's with there vat 60 (or 40). And that should have revealed any alternator problems. Did they check the belt tension with a tensioner gauge? Is the auto tensioner working? The centrifugal force at high rpm's may loosen the belt. Hard to tell without looking in person. I know you don't want to just throw parts at it but a new alternator would be my first try.

-Jon

 

Have you checked the pully on the Alt. sometimes they will lock up or turn slower causing you to drain your charging system and killing your battery.