Last time, you remember, having burned out three LH fuel gauges in as
many flight hours, I'd received my repaired gauge cluster back from
Air Parts Lock Haven and been told to go tell my radio shop to hunt
for voltage spikes. Well, said radio shop found nothing, nada,
zilch, zip wrong with my electrical system. As my battery was three
years old, they said to replace it and eliminate that potential
source of trouble, even though I'm not convinced there was anything
wrong with the old battery.

But when we hooked up the freshly repaired fuel gauge, it read way
too low. It took 60 ohms of resistance between the sender terminal
post and ground for the gauge to read full, vs. the spec of 30 ohms.
So out came the cluster yet again for another round trip to Lock
Haven. They fixed one cold solder joint, and now the gauge is back in
spec.

The other thing the radio shop found was that the resistance through
the sender when the fuel tank was full was about 25 ohms, 5 short of
spec. It seemed to me this couldn't make much difference, but with an
in-spec fuel gauge, it turns out that it's the dofference between
correctly reading full and incorrectly reading a little more than
half a tank. I'll say it again a different way: a sender that's only
a little out of spec makes for a huge error in the fuel gauge reading.

Moreover, having had my attention drawn to the sender, I now have a
new idea about what may have happened to the gauge. Suppose that
there's an intermittent connection somewhere in the sender, say
somewhere in its travel where the wiper is not in contact with the
coil. This could happen if, for instance, the wiper is worn through
at some point. When the fuel float is at that point, the resistance
will go up to infinity, peg the fuel gauge high, and burn out its
meter movement literally in a flash.

This explanation sounds plausible to Lock Haven, and they confirm
that wipers sometimes wear right through with age. They rebuild
sender units, too, so that seems to be my next, and hopefully final,
step.

I don't know if this helps anybody, other than to serve as a warning
that you might want to rebuild your sender too when your fuel gauge
has a problem.

Craig MacCallum
Sierra N525SB
Montclair, NJ





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