Alternator is fine battery isn’t charging.

[RyRobPer86]

Okay so I have a 1995 Saturn SC2 that I bought used for a work car and had no problems with until here within the last month. It started off idling poorly, the RPM’s would flare clear up over three thousand than drop back down below one thousand spitter sputter and sometimes die. Also it acts like it has a bad alternator because sometimes I will be driving and the radio will cut out than turn back on or turn off completely, the speedometer will sometimes drop down to zero or bounce around erratically, and the dash and head lights will fluctuate between regular brightness and extremely dim. The battery keeps running dead, but the car runs as long as it is charged and will usually start as soon as it is jumped. I put a new battery in the car, replaced the alternator, and the serpentine belt… but the problem persists. To test the alternator at home I disconnected the positive battery cable and the car died immediately. But I’ve taken the alternator back off and had it tested thinking it might have been a bad one, but it tested out just fine. The kid at the parts store… who also has a SC2 said he had the same problem and it was something with the computer, before I start buying expensive parts does anybody have a suggestion on what could be causing this problem.

[Cat Fuzz]

Bad grounds or other wiring, maybe. What you need to do is do a voltage test on the alternator while in the car. Check to voltage reading at the lug terminal on the back of the alternator with the engine running (good gawd be careful) and then check the voltage across the battery. They should be the same voltage within a tenth of a volt. If the battery voltage is significantly lower than the alternator output voltage you have wiring issues.

Also, the parts store alternator tester and your alternator voltage reading may or may not be adequate. If it's simply reading voltage output, that is only half the story. The alternator needs to be tested under load to see if it will put out the required current. You can do this while on the car by simply turning on a few accessories. If the voltage drops with accessories turned on, the alternator isn't keeping up. It could be putting out 14 volts but only a couple amps on the bench tester. A voltage reading under load will tell a better story.

[denisond3]

Yes, alternators can show good voltage, but be 2/3rds dead. They are really a 3 phase a.c. voltage generator. There are 3 sets of diodes in them to turn the a.c. they create into direct current. With 2 of the 6 diodes being bad (or having a solder joint in the alt. come loose) the alternator can lose so much of its output capacity that it will just about keep up with the needs of the engines electronics.
Its also possible there are some broken or corroded gound wires in addition to a weak alternator. I think my 92 SL2 has about 8 wires that just are just grounded to bolts on the transaxle bell housing or the back side of the block. And all of them are vital for something.