Date:         Tue, 23 Aug 2005 10:53:35 EDT
Reply-To:     THX0001@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         George Goff <THX0001@AOL.COM>
Subject:      Re: Two warning lights come on TOGETHER
Comments: To: eduntz@HOTMAIL.COM
Content-Type: text/plain; charset="US-ASCII"

In a message dated 8/23/05 9:09:37 AM, eduntz@HOTMAIL.COM writes:

<< I can troubleshoot for the alt and Oxy lights, but why would they always
flash on TOGETHER? >>

You need new brushes in your alternator.  The warning lights flicker because
the brushes have yet to totally queef out.

In order to have a bulb check function before the engine starts (and before
the alternator begins to do its thing), the idiot lights for the alternator and
the OXS are connected together by a diode on the low side.  If all things are
kosher whenever the alternator is spinning, the diode is reverse biased and
the OXS warning light is not lit unless, of course, the elapsed mileage has
been achieved.  If the alternator fails or its output voltage becomes low
(usually due to worn brushes making weak contact with their slip rings), the diode
will be forward biased because the battery voltage will at times be higher than
the alternator voltage.  Then, the OXS warning light and the alternator light
will glow to one degree or another.

George