(4)  
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.

 E. 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, 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 O-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.  Prepare for Trouble Shooting____________________________
 A. Procedure
 (1)  
Supply electrical power (Ref 24-22-00/201).

 (2)  
Make sure these circuit breakers are closed:

 (a)  On the main power distribution panel, P6: 1) EP GND FUELING 2) QTY 3) CAPT AND CTR PANEL AND INSTR LIGHT
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (b)  
AIRPLANES WITH EFIS; On the P18 panel (circuit breakers for the FMC computer and related equipment): 1) CDU No. 1 2) CDU No. 2 3) FMCS CMPTR 4) FMCS BITE

 (c)  
AIRPLANES WITHOUT EFIS; On the P18 panel (circuit breakers for the FMC and related equipment): 1) FMC (OR FMCS CMPTR) 2) CDU NO. 1 3) CDU NO. 2 4) MODE ANN

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

NOT OK NOT OK
NOT OK NOT OK NOT OK
OK

THE INDICATOR DOES NOT SHOW THE ERROR CODE (THE VALUE ON
NOT OK
THE INDICATOR DOES NOT GO TO ZERO) - Replace the fuel quantity test switch, S257.
OK
Troubleshooting for the Fuel Quantity Indicating System
Figure 101 (Sheet 1)

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