I suspect that many "rebuilts" are cleaned, painted, tested, that is all..
Right now there is a possibility that the brushes are 99% worn down..

Any time you replace an alternator with a rebuilt one, make sure that the sheave on the shaft is the same size as the original. I've see rebuilts come out of the box with the wrong sheave for the engine and accessory in your particular vehicle. In one case, externally, a 45 amp and 60 amp alternator appeared the same except the 60 amp one had a smaller sheave and was spun faster.

However, in your case, you say that the alternator works sporadiaclly and a likely cause, as the poster above stated, is brushes loosing contact with the slip rings. This can be due to worn out brushes or new brushes that are hanging up in the brush holder. Many years ago, I had an alternator with brushes of square cross section in a holder with a square hole. In less than 5000 miles, the brushes wore a groove and hung up in the holder. Chrysler fixed this problem by going to round brushes in the square holes.

Remove the brushes and inspect their condition.

A poor connection from power to the one brush or from the second brush to the computer which grounds it will cause a sufficient voltage drop through that circuit that would cause the alternator to not charge. During one of the periods when it is not charging, a technician would be able to test for and locate the problem. It is quite common for the computer connector (wiring harness) to fail and make a poor contact with the computer pins.

voltage regulator issue?

Replace the factory cable connectors at the battery before proceeding any further. The factory connectors had corrosion problems on the interior of the solid block. They look fine on the outside but only carry partial amps, part of the time, If you have already replaced those factory connectors, then recheck your aftermarket ones, you most likely have a intermittent connection problem at the battery.

dnt1 Has THE BEST advice so far. The suggestion that most rebuilds are only "cleaned-painted-& tested" is the most assinine thing I've heard. If Earthworm thinks that rebuilds are done that way, he obviously is buying them at the wrong parts house.

Posted by dnt1 (My Page) on Sun, Aug 26, 07 at 11:45

Replace the factory cable connectors at the battery before proceeding any further. The factory connectors had corrosion problems on the interior of the solid block. They look fine on the outside but only carry partial amps, part of the time, If you have already replaced those factory connectors, then recheck your aftermarket ones, you most likely have a intermittent connection problem at the battery.

Sorry, but aftermarket bolt on battery cable ends are simply a Band-Aid when someone actually needs stiches.

If the cables are really bad, the ONLY correct long term repair is to replace the cables, and associated harness if required.

BTW, the "voltage regulator" is the PCM on this vehicle.

I reckon you've got some sort of an intermittent drain somewhere and that's causing the problem.

A dodgy fuel sender (in the petrol tank) or a failing globe in the boot can cause this. Has the vehicle been involved in a rear ender or any other accident ?

(If it is this, it is not an easy problem to fix).