I had two '78 LeBarons that had "telltale" lights in the gauges, (one of Chrysler's really long list of innovations), to get your attention when the gauge was indicating a problem. For the charging amp meter, that light just looked at system voltage. 12.6 volts is what you'll have with a good, fully-charged battery with the engine not running, and that would turn the warning light on. With the engine running and the charging system working, expect battery voltage to be between 13.75 and 14.75 volts. That voltage would turn the warning light off.
On those models, you'd be right, the warning light is on with the ignition switch on and the engine not running.
By coincidence, I have a 2014 Grand Caravan that I just took out of winter storage a few days ago. My "Battery" warning light is just to the lower right of the left "Tach" gauge. It does turn on for about two seconds as a light test when the ignition switch is turned on, but it goes off much sooner than most of the other lights. It stays off after that, before the engine is started. That's because now, the insane engineers have seen fit to add a computer to every part of the vehicle, even when they aren't needed. Chrysler builds the voltage regulator for the charging system into the Engine Computer. By the way, another one of Chrysler's innovations was the electronic voltage regulator for 1970 models. They had temperature compensation built in because storing energy in a battery is a chemical reaction, and those speed up with increases in temperature. With temperature compensation, the voltage regulator reduced the chance of over-charging the battery in hot weather.
By putting the voltage regulator in the Engine Computer, besides temperature compensation, it can modify charging voltage for a long list of other variables that the computer knows. Related to this, all vehicles have a means of turning off the electric fuel pump in case a fuel line gets ruptured in a crash. Chrysler does that with their "automatic shutdown, (ASD) relay. That relay also turns off the voltage to the ignition coil(s), injectors, and the alternator field circuit which is fed through the voltage regulator. That ASD relay turns on for one second when you turn on the ignition switch, (that's when you might hear the hum of the fuel pump), then again when the engine is rotating, (cranking or running). When you just turn the ignition switch to "run" without starting the engine, The ASD relay gets turned off after that one second. That removes the field circuit voltage that runs the alternator, and since the Engine Computer is in control of that relay, and it knows the engine is not running, it knows the charging system can't be working. The computer DOES monitor current flow through that field circuit to tell if that part is working, and it can set very specific diagnostic fault codes to indicate when there's a problem. That can be determined during that initial one second key-on cycle. When everything is determined to be responding properly, it sends a message out on the "data buss" the computers use to talk back and forth to each other. The instrument cluster, which is another very complicated computer, interprets the "Battery light off" request within that message and responds by turning the light off. This is hugely more complicated than those telltale lights in the gauges, but the bottom line is the light operates differently. Most people aren't observant enough to pick up on that.
For your reading enjoyment, if you'd like to see more on charging system testing, check out this article:
https://www.2carpros.com/articles/how-to-check-a-car-alternator
You can do the voltage test yourself, but if that passes, you need a professional load tester to perform "full-load output current", and "ripple voltage" tests.
Wednesday, April 17th, 2024 AT 4:17 PM
