Greetings.

My car is a 2002 TL base model. 220k miles. I'm the original owner.

I have a voltmeter that goes into the cigar lighter so I can monitor system voltage while driving.

Most of the time it shows ~14.3V, which seems normal to me.

To be clear, it indicates ~14.3V on the highway, and also when idling at stoplights.

Sometimes, though, it drops to ~12.6 volts (battery voltage). This does NOT seem normal.

It shows the low voltage both while idling and also while driving.

Usually it will drop to the ~12.6V, stay there for a minute or two, then recover to 14.3V. It is toggling between those two states; it does not dwell at any intermediate voltages.

I do not get any failure lights on the dashboard when this happens.

Does this indicate a failing alternator? Or some other fault?

I'm the original owner and the car has been well maintained. I have not had any electrical or charging system issues.

I've made a cursory look for loose connections at/near the alternator but found nothing unusual.

Any thoughts appreciated!