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E.  The oil quantity tank units are mounted in fittings on the bottom of each engine oil tank (Fig. 1).
3.  Oil Quantity Indicator
A.  The oil quantity indicator is housed in a hermetically sealed cylindrical case and contains a motor driven self-balancing capacitance bridge circuit. The essential parts of the circuit are a reversible motor which drives the rebalance potentiometer, a dial pointer assembly, a capacitance bridge network and a four-stage transistorized amplifier. A rectifier provides dc power for the transistors. "Full" and "Empty" adjustment screws are accessible through holes in the rear cover.
B.  The oil quantity indicators, one for each engine, are located on the center instrument panel. The indicators are secured to the panel by a standard clamp mounting and have an electrical plug in back.
4.  Operation
A.  Electrical power for the oil quantity indicating system is 115 volt, 400 cycle ac, supplied through circuit breaker panel P6. The system is operative whenever electrical power is supplied to the airplane and the applicable circuit breakers are closed.
B.  The oil level in the engine oil tank determines the capacitance of the tank unit sensing probe, thus the capacitance of the probe varies proportionally to the oil level in the tank. Whenever the oil level in the tank changes, the resulting change in capacitance of the probe unbalances the bridge network. The error signal (unbalance) is detected by the amplifier in the indicator, and causes the reversible motor in the indicator to drive a balancing potentiometer and indicator pointer. The motor drives the balancing potentiometer until the bridge network is again in balance. The indicator then displays the usable quantity of oil in the tank in U.S. gallons.
C.  Operating the PUSH-TO-TEST switch simultaneously connects the active areas of the tank sensing probes to ground, thus simulating a "tank empty" signal. As a result, both indicators, if working properly, will drive toward a tank empty indication. The indicators will return to the correct quantity indication when the test switch is released.
D.  Deleted
500 
Nov 15/78  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  79-31-0 Page 3 

OIL QUANTITY INDICATING SYSTEM - TROUBLE SHOOTING
1.  General
A.  To determine and correct the cause of oil quantity indicating system malfunctions, the symptoms should be studied carefully. Each possible cause should be checked, beginning with the most probable, until the exact cause of the malfunction is determined. The following chart lists troubles likely to occur in operation of the system.
2.  Equipment and Materials
A.  Any of the following capacitance testers may be used:
(1)  
Field Calibration Unit, Part No. 387016-1 or 387991-003, Simmonds Precision Products, Inc., Tarrytown, New York

(2)  
Variable Capacitance Tester, Type 1429-A or MD-1, with current calibration chart, General Radio Co., West Concord, Massachusetts

(3)  
Variable Capacitance Tester, Type HT-109, Honeywell Inc., Annapolis Operation, Annapolis, Maryland (two required)

(4)  
Capacitance - Type Liquid Quantity System Test Set, Model TF-20, Consolidated Airborne Systems Inc., Carle Place, Long Island, New York

B.  Ohmmeter - for continuity checking
C.  Cleaning solvent - Acetone
D.  Stoddard solvent or equivalent
500 
May 01/98  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  79-31-0 Page 101 

TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Indicator reads low Indicator reads high  Indicator out of adjustment Compensator capacitance high Defective indicator Indicator out of adjustment Compensator capacitance low  Perform adjustment procedure Remove tank unit and immerse portion of tank unit below flange assembly in acetone for a maximum of twenty minutes. Agitate unit in the solvent to remove foreign material. Rinse in clean Stoddard solvent and allow to air dry. Measure compensator Capacitance between center of pin 1 and pin 3 while grounding pin 2 to flange. Capacitance should be 27.50 MMF. If not, adjust outer compensator tube assembly by loosening cap screw and adjusting assembly until capacitance is exactly 27.50 MMF. If desired capacitance can not be obtained, compensator tubes are defective Perform adjustment procedure Remove tank unit and adjust compensator capacitance to 27.50 MMF. (Tank unit must be dry.) If desired capacitance cannot be obtained, compensator tubes are defective  Replace tank unit Replace indicator (Ref. 31-00-11 R/I) Replace tank unit 
Open compensator circuit Defective indicator  Continuity check system wiring and check tank unit capacitance  Repair defective wiring or replace tank unit Replace indicator (Ref. 31-00-11 R/I) 
　
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