Okay, Phil, perhaps your ??? was rhetorical, but part of me was thinking you might ask about a car within the thread's roundfender topic and challenge my recollection of the voltage regulators used with DC generators. I think I could write scenarios where the ammeter might have prevented a roadside service, but none would be from my actual experience.

The experience which leads me to claim the idiot light is not foolproof comes to me most memorably with Bosch EL and my 240s. I've had 17 cars and none came with anything more informative than an idiot light. I recall some did not even have a temperature gauge. With the exception of one Chrysler product, to which I added a voltmeter in pursuit of an intermittent, the non-Volvos in my past gave me no reason to doubt the usefulness of the warning lamp.

However, the Volvos, one by one, had voltmeters added. In one, while chasing a temperature sensitive stator short in an 80A Bosch, I temporarily added an ammeter. Before doing that, I was not sure the battery wasn't having one cell shorting. Anyone else would have swapped parts to figure this out.

The problem with this idiot light, and really all of them, is its value so depends on taking inventory at key on, before starting, during lamp test. Do I do this? As anal and analytical as I am known to be, I don't. Although doing so will not guarantee the lamp warns you of failure once driving, most of the real world faults would be caught. Examples are the very common vibration caused failure in the ground wire at the alternator, or simply oxidation at the D+ (61) terminal.

But, when you are driving happily down a country road and wonder why the radio is starting to sound mushy and cut out, it could be too late to shut off all the loads, use the parking brake to avoid brake lights, and find out how long the spark will continue and the fuel will pump and squirt on 10 volts and dropping. No red lights. Remembering the $320 spent on an alternator in another town. Looking for the nearest place to buy a battery.

If I did not have those experiences, the idiot lamp would have told me all I needed to know. I think the best solution given my experiences would be a battery voltage monitor with alarm, giving an audible warning if the system voltage is ever below 13 after 10 seconds of engine run. It would not need to have a dashboard presence beyond perhaps a flashing idiot light to remind me what that noise was telling me. The voltmeters are just easy, whether they are a decor-matching instrument or a $6 cigar lighter plugin.
--
Art Benstein near Baltimore

"He's the best mechanic in town." - 1992 Annemarie Powell said of General Colin L. Powell, a Volvodad