This problem on my 2001 Legacy sedan 2.5 AT started out slowly and subtly, but recently got much worse.

 

I noticed beginning about four months ago when I was at a stopped at an intersection the headlights would change in brightness every few seconds very slightly. I also noticed that at the same time that the dash lights were doing the same thing, but again very slightly. Stepping on the brake or turning on the highbeams changed the rate of the flickering. Also it didn't seem like this was happening all the time. Now, more recently, the flickering was much more pronounced. I begin to see this happening all the time. If I was driving along at night, for example, the lights would be flickering constantly. I noticed that the idle was affected as well now – the car would buck slightly when coasting.

 

I started to deal with the problem by confirming that all the ground wires were OK. I went to each connection on the body and cleaned and sanded the contacts, applied dielectric grease, and reattached them. Couldn't hurt, but didn't solve the problem.

 

Next went to the local AutoZone to have the electrical system checked. The employee told me that my battery was not good, but that the alternator and voltage regulator were fine. I was a little surprised that the battery was bad, because I never had trouble starting the car. But, considering that it was was over seven years old, I figured it was at the end of its useful life. So I replaced it with an Interstate battery I got at Costco. Unfortunately, that didn't fix the problem either.

 

I decided to do my own test. I started the car and attached multimeter leads to the corresponding posts on the battery, and set the dial to 20 V DC. Sure enough, the voltage was fluctuating rapidly, +/- .5 volts, I think (so much for the free analysis at AutoZone!) It seemed to me that the only possible explanation left was that the voltage regulator was going bad. Since the voltage regulator is part of the alternator, I would need to replace it. The existing one was about 35,000 miles/3 1/2 years old, which is kind of early for an alternator to go bad. That explains why I didn't think at first that the alternator was the problem here. It was not the original – it was a rebuilt replacement I had installed myself, made by USA Industries.

 

Installed a brand new one (DB Electric), which actually cost less than the one it replaced, and that did the trick. No more flickering, idle is smooth again.

 

Hope this helps anyone having a similar problem.

Not the most common alternator failure mode. The old battery was definitely past official end of life, just due to age.

Similar to my step-son's '00 OB. ...he went thru two batteries b4 replacing the alternator>>>fixed.

Fwiw to anyone having a similar issue. Bad grounds or poor connections will usually behave slightly different. Like when hitting a pothole or a bumpy street etc. Still good to check them. if it's happening at idle when stopped, it's usually the charging system. Any time an additional Load is placed on the system the lights will dim, like the radiator fan kicking in and out. Might save some time in the diagnostic portion.