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Re: Fuel gauge polarity?....Old Dog, New Tricks

To: "Wiedemeyer" <boxweed@thebest.net>, "Pete" <pete@the-mix.com>
Subject: Re: Fuel gauge polarity?....Old Dog, New Tricks
From: "Larry Miller" <millerls@ado13.com>
Date: Tue, 29 Jun 1999 08:29:54 -0700
Cc: "spridgets list" <spridgets@autox.team.net>
References: <001e01bec23c$1b8ffc80$8c1a7ccf@default>
Reply-to: "Larry Miller" <millerls@ado13.com>
Sender: owner-spridgets@autox.team.net
This is all well and good, but, based on all of this, can someone explain
why my gas gauge works backwards? I just assumed it was hooked up backwards
and have not bothered to swap the wires. Now I'm afraid to swap them.

The gauge is a new Smith's gauge and the connections are not marked "T" and
"B". They have no markings at all.

Larry Miller
http://www.ado13.com
"If you want a Sprite, start out by asking your wife for a Porsche."






----- Original Message -----
From: Wiedemeyer <boxweed@thebest.net>
To: Pete <pete@the-mix.com>
Cc: spridgets list <spridgets@autox.team.net>
Sent: Tuesday, June 29, 1999 7:31 AM
Subject: Re: Fuel gauge polarity?....Old Dog, New Tricks


> Frank and Pete,
>
> After reading about your experience with frying the sender unit after
> reversing the wires on the fuel gauge, I just had to find out why this
> happens.  I had mistakenly believed that the fuel gauge was a simple
ammeter
> parallel type arrangement where the needle draws a very tiny amount of
> current for deflection while the bulk of the current is shunted through
from
> one pole (B) to the other pole (T).  If this were the case, reversing the
> wires would cause the needle to deflect in the opposite direction, but
would
> not result in more current going through to the tank.  After looking up
how
> a fuel gauge actually works, (in Automotive Encyclopedia by Toboldt,
> Johnson, and Gauthier -- $48.50 and worth every penny) I now see why
> reversing the wires will burn out the sender unit:  The fuel gauge is a
> "thermostatic gauge", which has a very large resistance in series between
> the B and T poles.  This reduces the current that ultimately gets to the
> sending unit.  If the wires are reversed, the large resistance is bypassed
> and much more current goes directly from the battery to the sending unit,
> which fries it.  The idea behind the thermostatic gauge seems  ingenious
to
> me because it uses the heat accummulated in the resistor to warm up the
> bi-metal fuel gauge needle, which causes it to move when the ignition is
> turned on.  There's never any current "felt" by the needle, which would
> explain why the fuel gauge itself almost never fails.  I guess it's never
> too late to learn something new.
>
> Thanks,
> Bob
> -----Original Message-----
> From: Pete <pete@the-mix.com>
> To: Frank Clarici <spritenut@Exit109.com>; Wiedemeyer
<boxweed@thebest.net>
> Cc: Doug Ingram <dougi@home.com>; spridgets list
<spridgets@autox.team.net>
> Date: Tuesday, June 29, 1999 9:56 AM
> Subject: Re: Fuel gauge polarity?
>
>
> >
> >>> Yeah.  Just reverse the wires when attaching them to the gauge.
> >>>
> >>
> >>Doug
> >>
> >>Yes it will work but DO NOT REVERSE THE WIRES!!!!
> >
> >>If you flip the wires on the gauge you send 12 volts to the sender which
> >>is grounded + connected to - = smoke!
> >
> >I can vouch for that.
> >Reversing the wires would be like putting the battery in backwards (yes,
> >I did that)
> >and frying your negative earth tach instantly (yes I did that)
> >
> >I am learning a ton though.
> >
> >Pete
>
>


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