Smiths Aerospace
Electronic Systems – Malvern
DOC NO.0134ED
MAINTENANCE MANUAL SUPPLEMENT
BOEING B737-100/200/300/400/500
28-41-04
(4) Examine the bussing plug seals on the wing spar for indications of leakage of fuel or water. This
can cause electrical leakage paths between the connector pins. If you think the bussing plug has
a problem, replace the bussing plug with a good bussing plug. This procedure will make sure that
the bussing plug was the cause of the problem.
(5) Do the press-to-test operation of the fuel quantity indicators, with the QTY TEST switch in the
flight compartment. If none of the indicators operate correctly, there is a problem with the power
supply for the indicators. If only one indicator does not operate correctly (the other indicators do
operate correctly), the problem is usually in the bad indicator.
These are some of the usual causes of bad values on the FQIS:
(1) Large quantities of water collect in fuel tanks and cause a short circuit in the tank units and the
compensator units.
(2) Blocked drain holes in the spanwise stringers in the wing tanks which can prevent the flow of
water to the sumps.
(3) Microbes, fungus, and contamination in the internal structure of the fuel tank and on the tank units
and the compensator units.
(4) One of the tank units has a short circuit to ground because it touches the structure, tubing or
bonding straps in the fuel tank.
(5) The circuit for the electrical signal from the tank unit is open or intermittent because of these
problems:
(a) A terminal screw that is not installed.
(b) A terminal screw is loose.
(c) The incorrect terminal screw was installed.
(d) There is internal damage to the tank unit because there is too much torque on the terminal
screw (too tight). Refer to 28-41-21/401 for the torque specifications.
(6) There is a short circuit between the compensator elements, and the High-Z elements, and the
Low-Z elements.
(7) There is a broken wire in the electrical harness for the tank units that gives an intermittently open-
circuit.
(8) A bad 0-ring seal installation at the spar receptacle or the bussing plug. This permits fuel to leak
from the tank to the face of the connector and (or) water to go on the face of the connector. One
of these conditions can cause a leakage path between connector pins and change the system
capacitance values.
(9) Fuel goes through the wire harness for the tank units to the face of spar receptacle which can
cause changes in the system capacitance.
(10) Loose connections in the connectors from the indicators to the tanks.
2.
Page 103
April 19/02
Airplanes with Smiths 2307 Model Fuel Quantity
Indicators with TSU
EFFECTIVITY
