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I had the alternator changed 2 years ago on 2001 Boxster, with OEM replacement. I noticed cruise control would come one when first started but if not set, it would go off and you could not get it to come back on unless you shut off the car and restart it. I also noticed that the panel lights would flicker at times, like a candle flame. The alternator light would flash on for several seconds intermittently even while traveling at 80 MPH. A few weeks ago, the battery light came on and stayed on. I let the engine cool and restart but same issue. I started and rev'd the engine up to 6000 and snapped my foot off the gas. The light went out and it started to charge. Last week, I was coming home from the airport and the ABS light started to flash, then the air bag warning started to flash. BUT THERE WAS NO BATTERY LIGHT ON.
I pulled over and shut it off. It would not restart - the battery was down to 10.6 volts. It was not charging and there was no warning
The warning light operated normally. It came on when you turned the key and went off when the engine started, but there was no charging voltage. Local European cars specialist told me the battery had a shorted cell. He replaced the battery and assured me everything was okay. I drove the car for a day, then noticed that starting was sluggish. I measured and there was no charging and no warning lights. I took it back and they tested the alternator on a Snap On tester. They said it was perfect. They consulted with Porsche dealer who said it was impossible for the alternator not to charge and not have a warning light. I insisted that they change the alternator. Everything was solved. the flickering lights, the cruse control, the charging, the intermittent battery light flickering. All solved. Still no rational explanation as to why the battery light did not come one with no charging taking place.
Measurement - wise. when it was acting up, 13.5VDC stable, but sometimes very rapidly (Fluke digital meter could not track fast enough) varying between 13.5 and 14.5, and that was causing the dash lights to flicker.
When the battery was going dead with no battery light on, absolutely no charging voltage.