Screwy Fuel gauge

[TEX TERRY, II]

From: Robert Myers <robert at s-cars.org>
(snip)
could put in only about 13 gallons.  The tank capacity is several gallons
more than that amount.  Each time after fill-up the gauge started working
again but it never showed a completely full tank only reaching about 7/8
full indicated.
This afternoon I happened to catch what was happening.  The needle was
pointing to just below 1/4 tank remaining.  Suddenly the gauge needle
started moving to the left just as iff the ignition had been switched off
and then the gas pump icon flashed on.  Fill-up took 13.46 gallons.
(snip)

Sounds like the fuel tank level float sender is faulty.  As deteriation
sets in, the resistance difference, for the level arm float, changes.  The
indicator for the low fuel level is tied with set postion on the float arm
contact board.  The other item that changes over time, is the capability of
the float to not absorb any fuel, and still remain a "float" item.  If the
float itself is becoming saturated, then the float is pulling the arm down
to a lower position on the resistor board, which in turn is giving the
false readings to the actual amount of fuel in the tank.  If you remove the
float assembly from the tank, then turn use a jumper to provide a mounting
ground, then turn on the ignition key, you should be able to manually move
the float slowly, and note the gauge needle correspond to the positions of
full and low but not empty, then real low and the icon will come on.  If
all test correctly, then the float it self is faulty and would need
replaced.  If you can not get a full tank reading, and the low fuel icon
comes on to soon, then the contacts and/or the board is faulty.

Tex Terry, II
83 5kt Sedan (non-quattro)
Franklin, PA  USA