Date:         Wed, 19 Sep 2012 18:48:39 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Thermo switch or resistor?
Comments: To: David Milo <dellaone@GMAIL.COM>
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              mail.com>
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At 05:33 PM 9/19/2012, David Milo wrote:
>Might not hurt to see if you are getting voltage through both wires by
>carefully piercing the insulation with a tool of your choice near the ring
>terminal.

It never hurts to actually troubleshoot the problem by testing until
the fault is unequivocally identified.  When I was working as an
electronics tech long ago we used to call the other approach
"shotgunning," i.e. throw components at the board until the problem goes away.

Our circuits in general start from a +12 V source, pass to a switch
or relay, then the load, then ground.  Occasionally the switch is on
the ground side of the load instead of the hot side.

Open circuits are diagnosed with meter or test light by discovering
where the voltage changes from +12 to zero.  In a correctly operating
circuit, everything on the supply side of the load will be near the
battery voltage, very slowly declining as you get nearer the load
supply terminal.  Supply terminal to ground terminal of the load
should be near twelve volts.  From the load ground terminal to
chassis ground the voltage will be near zero and slowly declining as
it gets nearer the chassis and battery negative terminal.

This relationship only happens when there is current flowing in the
circuit.  When the circuit is broken, battery voltage will appear on
one side of the break, and ground voltage on the other side.

In the case of resistor-controlled fans, the fan and resistor
together constitute the load, so the wire between the two will show
some proportion of 12 V, lower for lower fan speeds.  But again this
only happens when the circuit is working and current flowing.

The easiest illustration of this troubleshooting technique is finding
a broken paint stripe in the rear window defogger.  If you connect
your voltmeter to one terminal and gently run the other probe along a
working stripe, the voltage on the meter will be proportional to the
distance along the stripe.  But a broken stripe will show nothing
right up to the break, and full voltage immediately on the other side.

Yours,
David