typically when you replace your alternator you either replace your battery or completely remove it from the vehicle and charge it so that it is fully charged. The battery loses power when the alternator is too weak to recharge it. You might not have enough voltage going through your engine. try this. answer if it is only coming on at 3,000 rpms, I would suggest you look at your serpentine belt and make suer it is not slipping on the alternator pulley. Your idler pulley spring may be weak and is not keeping the belt tight on the alternator. If you bought the alternator from Pep Boys or AutoZOne I would suggest taking it back and having them test it. I haeve already purchased parts from them, realizing they are rebuilt and not new and have found some defective. I replaced a starter 3 times and had to go to another store to find a good one when I asked Autozone to test them before I took it out of the store. They had a whole batch of bad starters..........just something to think about. If your alternator is brand new, not rebuilt, then one of the diodes inside may be going bad.