[hammogger]

Oh good, you are testing the special Unimog "fry the alternator" option by running the engine in the "0" position of the switch! We've always wondered it that function works. Guess it does LOL

The "0" position is off. You should never turn the switch to "0" before stopping the engine, and Yes, the light should come on bright in the "0" position when the engine is running. I tried the 'fry your alternator" position once too. It didn't work on my truck.

ACUF, you really must have a working voltmeter to do any testing on an electrical system. Without that, questions and answers are pretty much useless.

Bob

So, The red light stays on when running on the 123 setting, but its dim, but when in the 0 position its bright red when running. the tach is working. the terminals are on tight, but will take them off and clean them.

-ACUF

 

[hammogger]

That's a really good question and I have not heard a definitive answer, but...

Alternators are coil-based devices, and coils, inductors, are energy storehouses using the fields surrounding them. When the key is off, the load is disconnected from the alternator, allowing voltage to build, and not providing a dampner for those spikes. And alternators are AC devices so require diodes to convert to DC. Diodes are susceptible to damage from high voltage spikes. So, the diodes get zapped. In reality, diodes have improved greatly over the years so a modern diode is probably well rated for their task. In the early seventies a good diode might handle 100v junction v. Today, it's easy to get a cheap 1000v diode. On the other hand, our trucks were from the early years of diodes so a 30 year old alternator could easily be damaged. IMHO.

Kinda like the 'knowledge' that setting a battery on concrete will ruin it. That was true decades ago when battery cases were made of hard rubber which became porous, absorbed acid, and conducted, thus shorting itself into concrete. That's no longer a problem. Maybe the same with diodes?

Bob

....chop...
So what actually happens to the voltage, when the engine is on, but the ignition is off? -you say fry.. but what really happens to the electrics?

Thank you all for the feedback. Very helpful.

-ACUF