I need some help with an odd electrical issue. (I could not find any existing threads on this, but please point me toward them if they exist!) This is a 1993 Legacy L Wagon with 226k.
About a year ago, during the cold Chicago winter, I started noticing the following behavior. At random times, the entire electrical system in the car experiences what seems to be a pulsating voltage change, like a sine wave, at around 80-120 Hz. All the lights pulsate bright-dim bright-dim in unison, and I can hear the cabin fan spin faster/slower in unsion with the lights. Sometimes, the Brake warning light on the instrument cluster will flash on and off in unison as well.
I know that the Brake light coming on is a failure mode for the alternator. The alternator is quite new and I had it and the rest of the charging system checked last year - it's all fine. I replaced the battery (needed it anyway) and also the plugs and wires (was having a starting problem). No change.
Over the summer, I did not notice this issue. This week, as the cold sets in again (30-50 F), I see the same issue.
The only suggestion I've heard is to check the grounds, but since the issue seems related to cold weather, I'm not sure that is the issue. I will have them checked at next service.
Any ideas on this? Thanks!