At 10:17 PM 1/10/2001, Bill Davidson wrote:
>But wouldn't the alternator light come on after starting if it wasn't
>charging? ... he said that it doesn't come on at all... I assume that means
>after starting also...

No, it wouldn't.  The alternator needs a tickler circuit to get it started
-- that's the D+ terminal which must be supplied with a small 12v current
to begin charging, and which then itself supplies 12v while the alternator
is charging.

Having the dual need of a startup circuit and also a warning light for the
alternator, the auto-makers combined the two -- when the alternator is
drawing through the D+ terminal the light is on, and when it's supplying
voltage there the light goes out.  The current through an LED isn't enough
to trigger the alternator, so VW also put a resistor in parallel with the
LED through which most of the current flows -- might be 150 ma or something
like it.

Then because they also needed a lamp test for the OXS light (and safety
belt light, I think?) they hooked those into the circuit as well, using
diodes to isolate them.  And oh yes, they use it to drive the relay to
power the fridge when the alternator *is* charging.

If the alternator light doesn't light with ignition on, it's one of three
things: bad LED (happens, but you can win lots of money betting against
it), bad 12v supply to the panel (which ought to affect other things on the
panel), or the other end of the circuit isn't being properly grounded
through the alternator -- busted wire, unplugged wire, bad connection at
the panel plug, bad alternator.  But regardless, if it doesn't light with
the motor off it won't light with the motor on either.

david

>Bill

David Beierl - Providence, RI
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