Smiths Aerospace
Electronic Systems – Malvern
DOC NO.0134ED
MAINTENANCE MANUAL SUPPLEMENT
BOEING B737-100/200/300/400/500
28-41-04
Fuel Quantity Indicator (Fig. 1)
A. Digital fuel quantity indicators show the weight of fuel in each fuel tank. Each fuel quantity indicator
has a single chip microcomputer system and a digital liquid crystal display (LCD). The microcomputer
measures capacitive and resistive (electrical leakage due to contamination or water) current in the
tank units. The microcomputer also monitors other circuit parameters. When a parameter reaches
an unacceptable level for at least 30 seconds ERR is shown on the indicator face, with a blank, zero
or normal fuel quantity reading as specified below. If an intermittent fault disappears for at least 30
seconds the ERR display is blanked, but the error code remains logged in indicator memory. If the
indicator has a normal display, when the indicator test switch is activated the display will show ERR
plus all stored error codes (between 1 and 10) which will be used to help in troubleshooting the
system. If a zero or blank display is present, the power to the indicator must be cycled to read the
error codes. The subsequent define the error codes.
ERROR FUEL
QUANTITY
PROBABLE
CODE
INDICATOR READING
CAUSE
1
Normal
An open circuit or a short circuit
in the COMP LO-Z wiring *[2]
2
Zero
A short circuit in the compensator
unit (HI-Z to COMP LO-Z)
3
Normal
Too much leakage in the
compensator unit
4
Zero
An open circuit or short to ground in
one of the LO-Z wires to the tank
units *[2], [3]
5
Zero
A short circuit in one of the
tank units (HI-Z to TANK LO-Z)
6
Normal
Too much leakage in a tank unit
7
*[1]
Digital Calibration Trim Unit does
not operate correctly
8
Blank
An error in the DCTU data
9
*[1]
A problem with the indicator
memory
10
Zero
An open circuit or a short circuit
in the HI-Z line *[2]
*[1] If a problem occurs during flight, the indicator will continue to show a normal value with ERR also
shown. If a problem occurs when power is applied, the indicator will show zero with ERR also shown.
*[2] These error codes can also be caused by these conditions occurring internal to the TSU.
*[3] Holding the test switch for longer than 30 seconds will log error code 4.
B. To clear all stored error codes, including those stored by activating the self test switches, push the
indicator test switch until the display goes to zero, release the indicator test switch, then push the
indicator test switch again while the fuel quantity indicator is in self test mode. The fuel quantity
indicator will complete self-test cycle and show the word CLr.
Page 13
Apr 19/02
Airplanes with Smiths 2307 Model Fuel Quantity
Indicators with TSU
EFFECTIVITY