Smiths Aerospace
Electronic Systems – Malvern
DOC NO.0134ED
MAINTENANCE MANUAL SUPPLEMENT
BOEING B737-100/200/300/400/500
28-41-04
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.
D. Try these easier procedures to find problems with the fuel quantity indicating system before you do
the longer trouble shooting procedure:
NOTE:
This will keep the time necessary to do the trouble shooting to a minimum. Use the longer
trouble shooting procedure only if these steps do not find the problem.
(1) The integrity of the Hi-Z shields is very important because they provide the ground path from the
TSU to the indicator. The connection at the indicator/DCTU is especially prone to wear and
breakage with time. A properly functioning Hi-Z shield will have a resistance to airframe ground in
the cockpit of 1 ohm or less. Check for intermittants by twisting the airplane harness.
(2) It is necessary to have good bonding of the shields at the TSU. Check that the ground studs meet
the resistance requirements.
(3) Make sure the task “Fuel System Drainage” (Sumping) was done a short time before (a minimum
of 45 minutes after the airplane movement stops, or after the refuel operation is completed). This
procedure is necessary to drain water collected in the tank. If the conditions of operation are very
bad it is recommended that 5 gallons (19 liters) be drained from each tank for each 24 hours.
(For example, this is a good procedure if the weather is very hot and humid).
Page 102
April 19/02
Airplanes with Smiths 2307 Model Fuel Quantity
Indicators with TSU
EFFECTIVITY
