I have 2002 Subaru Impreza WRX. The alternator went, so I replaced it. The voltage gauge was jumping up and down when fans kicked on and when I turned on accessories. I figured the alternator was messed up. Sent it back and got another one. Installed it and it still did the same thing. I checked all the grounds and power cables, cleaned them all: still does it.
I have an aftermarket stereo in my car with old alternator before it went. Lights never dimmed or volts never jumped same with accessories. The volts always stayed around 14. Now my voltage gauge drops to 12 and at times jumps to 15+ volts. I thought maybe battery and put new battery in now. Lights don't dim as much but voltage gauge still jumps. I took to Autozone. Their tester said the volt regulator was bad. Took it to Advanced for a second look. They said everything was fine, but at the time it was staying at 14 on my voltage gauge. It wasn't jumping around. Finished the test and it started doing it again. dropped to 12v then jumped up to 14 again. Is there anything else that can cause this beside alt and battery in a car?