Date:         Sun, 8 Oct 2006 15:09:27 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Missing a high-beam... suggestions?
Comments: To: John Runberg <jrunberg@MAC.COM>
In-Reply-To:  <9467174.1160331509746.JavaMail.jrunberg@mac.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 02:18 PM 10/8/2006 -0400, John Runberg wrote:
>- grounds on both sides appear OK. Tested left high-beam on right
>ground and OK, left on right + fails.
>- removed all wired from the plugs and inspected. Look OK
>- voltmeter shows 12v on left side, and that or slightly higher on
>the right (why would it be higher??)
>- meter I have can't check amps so I'm sidelined until I can get one.

John, you have a total of about twelve volts distributed along the
entire length of the headlight circuit from battery + through the
wires and panel and switches and more wires and lamp socket and lamp
and socket again and ground wire and chassis and back to  battery
--.  Use your voltmeter from one part to another and discover where
the "missing" voltage is that should  be appearing across the lamp
filament.  You don't need/want an ammeter for this.

The voltage in the circuit is distributed in proportion to the
resistance in the various parts.  The lamp filament is *supposed* to
have the highest resistance in the whole circuit by two orders of
magnitude or so, so that almost all the voltage will appear across
its terminals and almost all the power dissipated in the circuit will
go to blinding deer instead of melting terminals and such.  But tiny
resistances are hard to measure, so you turn the vehicle into a giant
high-current ohmmeter by measuring the voltage drops instead...

david


--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"