Smiths Aerospace
Electronic Systems – Malvern
DOC NO.0134ED
MAINTENANCE MANUAL SUPPLEMENT
BOEING B737-100/200/300/400/500
28-41-04
FUEL QUANTITY INDICATING SYSTEM - TROUBLE SHOOTING
1. General
A. These trouble shooting procedures were developed from data from a standard system operational
test. The procedures are in a tree-type configuration. The steps related to fuel system operation are
connected by arrows to show the path through the tests that gives good (systems operable) results
(OK condition).
B. When a step in an operational test does not give a good result, follow the NOT OK line to the box that
contains the applicable problem. Refer to the procedure shown in that box. Continue to analyze the
results of each step in trouble shooting until you find the applicable solution to the problem. Do the
steps necessary to correct the problem. Then do the step where you first found the problem again to
make sure you corrected the problem.
C. You can use the error codes kept in the indicator to help you do the trouble shooting. Look at the
error codes on the indicators before you do maintenance tasks. Push the indicator test switch to
show the error codes on the display. Refer to this table for the causes of these error codes.
NOTE:
Incorrect error codes can be shown if a component of the fuel quantity indicating system is
disconnected.
(1) If a problem with the FQIS occurs that is sufficiently bad that the system cannot find the fuel
quantity, the indicator shows ERR in the "error" legend and a fuel quantity of “0”. This display
shows that the system has a "hard" fault. This type of fault can be a problem in one of the tank
units, the indicators, the wiring that connects them, or other hardware in the FQIS. The
"
0
"
in
the fuel quantity indication area must not be read as a fault code. It is not kept in the indicator as
a fault code. If this occurs, cycle the FQIS circuit breaker off and on to get more data about the
fault. The indicator then shows one of the codes in the subsequent table.
(2) Errors can also occur that permit the FQIS to continue to show the correct fuel quantity ("soft
faults"). When these errors occur, the ERR legend is illuminated, the fuel quantity display shows
the correct weight of fuel WITH REDUCED ACCURACY (+/- 3%). The error codes can be read
by pushing the FUEL QUANTITY TEST SWITCH. Note that if the test switch is held for longer
than 30 seconds error code 4 will be logged.
(3) If an intermittent fault disappears for at least 30 seconds the ERR legend is blanked, but the fault
code remains logged in indicator memory.
Page 101
April 19/02
Airplanes with Smiths 2307 Model Fuel Quantity
Indicators with TSU
EFFECTIVITY
