The alternator warning light circuit does much more than simply make the
light turn on. The circuit has two important functions. When the ignition is
switch on the 12v goes through the resistors to allow enough current to
"activate" the alternator". Some rotor current is needed to get the
alternator going. Now when the alternator goes it will provide voltage back
which will cancel out the current and causes the light to go off. Now here
is the important part. When you turn the ignition off the alternator is
still providing current. Without resistance or a blocking diode the back
feed will keep the ignition energized and engine thus running. Things get a
little more complicated with the campers as this circuit is also used to
operate the fridge relay. A unique part of this is the use of the starter
solenoid for the ground of the relay coil.  This disables the relay during
cranking and if the alternator fails.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
pickle vanagon
Sent: Thursday, June 07, 2012 4:50 PM
To: vanagon@GERRY.VANAGON.COM
Subject: more mysterious circuitry

Mysterious to me at least... I'm sure to the Beierl's of the word it makes
perfect sense!

The alternator led circuit has me confused.  (This is shown, for example, on
97.74).

It goes from 12 volt power supply, splits into two tracks.  The first track
goes through a 60 ohm resistor, the second track goes through a 470 ohm
resistor and then through the alternator led, and then they meet up again
and go through a diode on their way to the alternator.

(The 60ohm resistor is actually two 1watt 120ohm resistors in parallel).

Why does this need to be like this?

My curiosity stems from the fact that I have two 1 watt 100ohm resistors
available, and I'm wondering if I can replace those 25yr old 120ohm
resistors with them, or if I need to leave them be.  I'm also wondering what
parameters I need to worry about from the standpoint of upgrading that LEDto
something brighter.