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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