My alternator doesn't start charging until I rev the engine over 2500rpm. Then it's fine most of the time. If I shut it off and it sits for awhile the same thing happens. I've checked all the connections to make sure they are clean. I'm thinking it's a diode going bad. Does the charging circuit go thru the gage in the dash?

 

Do you have a 1-wire alternator?

 

Ther is one wire connected to the back of the alternator that has 12 volts. There is also a wire at the same location with a plug that plugs into the side (top) of the alternator.

 

wrong pulley size maybe? The realationship between the alternator pulley and the crank pulley should be about 3:1 for the street. The alternator is probably headed out. I would just get a new one with a lifetime warranty. They can be had for about $130.00 with exchange for a 124 amp model at motor supply. thats what I recently did. That fixed my under charging alternator.

 

I had a similar issue Mine was just my pos gauge. I put the amp clamp on the alt, and it worked fine. Read 45 amps at idle. Put a volt meter on the bat, 14.2 volts at idle. gauge said 11ish. revved the car, volt meter worked fine. I later put a new cluster in it and it read correctly. The old one made friends with the trash can.

 

I'm thinking it's the gage too.