>Now with the new send unit, still using only the battery, the gauge
>registers in reverse, E when it should be F and vice versa.
>Any suggestions? It's really starting to baffle me since the electrical
>theory seems so simple.
Fuel tank sending units are variable resisters. At one end of their
travel the resistance is maximum and at the other end of their travel
resistance is minimum. They can be constructed to have maximum
resistance at tank full or tank empty.
The fuel gauge is basically a volt meter wired in series with this
variable resister. They can be internally wired to read full when the
sender unit is at max resistance or at min resistance.
That said it becomes obvious that the construction of the gauge needs to
match the construction of the sender unit or the meter will read the
inverse.
Smith's is pretty good at standardization. All the positive earth gauges
and senders are wound in the same way. Along comes this switch from
positive earth to negative earth and the Smith's engineers find a
technical reason to change the polarity of the sending unit when the
polarity of the car's electrical system is changed. So for negative
earth cars they offer a sending unit wired opposite from the positive
earth sending unit along with a matching wired gauge.
Herein lies the great divide. You need a positive earth sending unit to
go with a positive earth gauge and visa versa. Crossing over the divide
with one of the two components will lead to an inverse reading no matter
how you wire it up.
Universal Lucas sending unit replacements, last I looked only came in the
negative earth flavour. Your sending unit is neg earth and your gauge is
positive earth.
But wait! all is not lost... I had a gauge failure a few years back and
sent my fuel gauge in for repair. They ended up replacing the
internals... with negative earth internals. Somewhere. packed away I may
have the only TR3 face fuel gauge wired for a negative earth sending unit.
When it stops raining I'll try to remember to go out in the shed and look
for it.
TeriAnn Wakeman If you send me direct mail, please
Santa Cruz, California start the subject line with TW -
twakeman@cruzers.com I will be sure to read the message
http://www.cruzers.com/~twakeman
"How can life grant us the boon of living..unless we dare"
Amelia Earhart 1898-1937
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