I have a '96 Legacy L wagon. Lately, we've noticed that all the lights (headlights, dash lights) briefly dim and brighten. There it no pattern, just a brief dip in brightness, usually once every few seconds. I figured likely alternator or voltage regulator, less likely the diode. I took it to the shop and they said the voltage was fine, alternator and diode seemed fine, and it was going to require disconnecting things till they found the problem. I didn't like that idea much and needed the car back for the next snowstorm. All the lights dimming said to me it had to be an overall voltage drop and, sure enough, my VOM showed voltage dipped to about 12.9V from ~14.2 whenever the lights dimmed. With an OBD2 reader, I can see the voltage briefly spike downward. It never goes above about 14.5V, making me think the regulator is ok. Revving the engine makes no difference. However, I found that with enough electrical load (e.g., the headlights on AND the fan on full speed (but not half speed) or the rear defogger also on and the fan at half speed, the spikes disappear and the voltage sits steady around 13.5V. Grounding and connections between the alternator and battery are fine.
Anyone run into this? I was ready to buy an alternator, figuring maybe the brushes were going bad, but the discovery that enough load stopped the spikes made me wonder if it was the regulator, after all.
(And can someone tell me where the stupid regulator is located? Google isn't helping and it does not seem it is in the alternator like in many cars - in fact, it costs much more than a rebuilt alternator.)