1. GENERAL This chapter contains information on the storage, distribution, and indicating components of the fuel system. The engine is supplied with fuel drawn out of the left or right integral fuel tank (depending on the fuel selector valve position). Fuel flows through a check valve, collector tank, selector valve, electric auxiliary fuel pump, gascolator, engine-driven fuel pump, fuel flow sensor, throttle body, fuel injection manifold, and is then distributed to the fuel nozzles. fuel line fittings installed in the cabin area of the fuselage are enclosed in a vented and drained enclosure. Fuel system troubleshooting flowchart: (See Figure 1) Fuel system layout: (See Figure 2) Fuel system schematic: (See Figure 3) For maintenance practices pertinent to the engine-driven fuel pump, fuel flow sensor, and fuel nozzles refer to Chapter 73, Engine Fuel Systems. (Refer to 73-00) Page 1
2. TROUBLESHOOTING Trouble Probable Cause Remedy Filler cap leaks. Filler cap improperly installed. Install filler cap properly. Seal improperly installed. Deteriorated seal. Improperly tightened cap fuel nuts. Replace fuel cap seal. Replace fuel cap seal. Tighten or loosen fuel cap nuts as required. Fuel connector(s) leak. Loose connector(s). Inspect and tighten connector(s). Defective seals. Defective thread. Replace seals. Replace threaded component. Insufficient fuel flow. Restricted fuel filter/screen. Clean filter or screen. Normal starting procedures result in difficult starting. No fuel flow to engine driven fuel pump. Defective mechanical fuel pump. Defective fuel pressure sensor or insufficient voltage supplied. Cracked fuel line. Faulty electric fuel pump. Fuel selector valve not in the on position. Fuel tanks empty. Fuel line disconnected or broken. Fuel tank screen plugged. Defective fuel selector valve. Plugged gascolator. Fuel line plugged. Frozen fuel line. Replace pump. Replace pressure sensor. Find voltage problem. Replace line. Replace pump. Turn selector valve on. Fill tanks with approved fuel. Repair or replace line. Clean or replace screen. Replace selector valve. Clean or replace gascolator. Remove obstruction. Thaw and drain complete fuel system. Fuel starvation after start up. Insufficient fuel flow. See remedies from above. Malfunction of engine-driven fuel pump or fuel system. Fuel vents plugged. Water in fuel. Replace pump. Remove obstruction. Remove water. Page 2 01 Jan 2016
Trouble Probable Cause Remedy No fuel output indicated from fuel flow gage when auxiliary pump is operated. TE: If the auxiliary pump is turned on when the engine is off, the fuel flow gage will show a momentary fuel flow. Mixture control set in lean position and throttle lever is pulled back. Fuel line disconnected before fuel flow sensor. Defective fuel pump switch. Open or defective circuit breaker. Adjust mixture control richer and move throttle lever forward. Check all fuel lines and connect. Replace switch. Reset or replace circuit breaker. Loose connections or open circuit. Tighten connections/repair wire. Defective electric fuel pump. Defective engine-driven fuel pump bypass or defective fuel system. Replace pump. Replace pump/troubleshoot fuel system. Page 3
Trouble Probable Cause Remedy Fuel quantity indicator indicates inaccurate fuel level. Resistance to fuel level indicator from fuel level sending units are unstable and/or greater than 66 ohms (depending on fuel level). Resistance of each fuel level sender dial from inboard through outboard locations are unstable and/or greater than 33 ohms (depending on fuel level). Fuel level out of calibration. Defective fuel sensor. Defective fuel quantity indicator. Incorrectly adjusted fuel sensor. Faulty fuel system wiring. Disconnect airplane harness from console indicator and verify resistance of RH wing (pins 4 & 9) and LH wing (pins 2 & 9) is stable and measures between 0-66 ohms (depending on fuel level). Perform System Test - Fuel Sending Unit. Check continuity in airplane wire harness between indicator and sending units. Perform Adjustment/Test - Fuel Quantity Indicator. (Refer to 28-40) Verify resistance from inboard through outboard fuel level sender dials are stable and between 0-33 ohms depending on fuel level. Perform Functional Test - Fuel Quantity Sensor Dial. (Refer to 28-40) Perform System Test - Fuel Sending Unit. Perform Adjustment/Test - Fuel Quantity Sensors. Adjust calibration using fuel indicator trim adjustments. If unable to calibrate, ensure both sending units indicate E at zero calibration and maximum at high gain. Perform Adjustment/Test - Fuel Quantity Indicator. (Refer to 28-40) Note: Indicator has active dampening. ow indicator to stabilize for approximately one minute before making adjustments. Replace sensor. Replace indicator. Adjust sensor. Repair/Replace wire(s). Page 4
Trouble Probable Cause Remedy No fuel quantity indication. Fuel tanks empty. Fill with approved fuel. Left or right fuel tank circuit breaker open or defective. Reset or replace circuit breaker. Loose connections or open circuit. Tighten connections/repair wire. Incorrect power to fuel level indicator in console. Disconnect airplane harness from indicator and verify voltage between pins 1 & 9 measures between ~24-28v. Check airplane power source and resolve. Resistance to fuel level indicator from fuel level sending units are unstable and/or greater than 66 ohms (depending on fuel level). Disconnect airplane harness from indicator and verify resistance of RH wing (pins 4 & 9) and LH wing (pins 2 & 9) is stable and measures between 0-66 ohms (depending on fuel level). Perform Functional Test - Fuel Quantity Sensor Dial. (Refer to 28-40) Perform System Test - Fuel Sending Unit. Check continuity in airplane wire harness between indicator and sending units. Perform Adjustment/Test - Fuel Quantity Indicator. (Refer to 28-40) Defective fuel sensor. Defective fuel quantity indicator. Incorrectly adjusted fuel sensor. Faulty fuel system wiring. Replace sensor. Replace indicator. Adjust sensor. Repair/Replace wire(s). Page 5
Trouble Probable Cause Remedy Fluctuating fuel level indications. Incorrect power to fuel level indicator in console. Resistance to fuel level indicator from fuel level sending units are unstable and/or greater than 66 ohms (depending on fuel level). Resistance of each fuel level sender dial from inboard through outboard locations are unstable and/or greater than 33 ohms (depending on fuel level). Disconnect airplane harness from indicator and verify voltage between pins 1 & 9 measures between ~24-28v. Check airplane power source and resolve. Disconnect airplane harness from console indicator and verify resistance of RH wing (pins 4 & 9) and LH wing (pins 2 & 9) is stable and measures between 0-66 ohms (depending on fuel level). Perform Functional Test - Fuel Quantity Sensor Dial. (Refer to 28-40) Perform System Test - Fuel Sending Unit. Check continuity in airplane wire harness between indicator and sending units. Perform Adjustment/Test - Fuel Quantity Indicator. (Refer to 28-40) Verify resistance from inboard through outboard fuel level sender dials are stable and between 0-33 ohms depending on fuel level. Perform System Test - Fuel Sending Unit. Perform Adjustment/Test - Fuel Quantity Sensors. Fluctuating fuel flow indications. Fuel flow indicator/transducer. Replace fuel flow indicator/transducer. Fuel leaks from electric fuel pump drain manifold fitting. Vapor in fuel lines. Faulty internal fuel pump seal. Turn on auxiliary fuel pump. Replace electric fuel pump. Page 6
1 Failure to display fuel level? STEP 1A Check power to fuel level indicator in console: Disconnect airplane harness from indicator. Measure voltage between pins 1 & 9. STEP 1B Check resistance to fuel level indicator from fuel level sending units: Approximately 24 ˇ 28V? Check airplane power source and resolve. Disconnect airplane harness from console indicator. Measure resistance of RH wing (pins 4 & 9) and LH wing (pins 2 & 9). Stable and between 0-66 Ohms? Troubleshoot Fuel Level Sender Assembly. 2 Fluctuating fuel level indicator? Perform STEP 1A Perform STEP 1B Troubleshoot Fuel Level Indicator. Check continuity in airplane wire harness between indicator and sending units. STEP 2C Check resistance of each fuel level sender dial: Measure resistance starting inboard and move outboard. Stable and between 0-35 Ohms? Troubleshoot Fuel Level Sender Assembly. 3 Faulty fuel level indication? Perform STEP 1B Perform STEP 2C STEP 3C Perform fuel level calibration: ow indicator to stabilize for ~1 minute before making adjustments. Troubleshoot Fuel Level Indicator. Calibration OK? Perform Adjustment/Test ˇ Fuel Quantity Indicator. Troubleshoot Fuel Level Sender Assembly. Adjust calibration using fuel indicator trim adjustments. Ensure sending units show E at zero calibration & high gain backed just off max. SR20_MM28_3325 Figure 1 Fuel System Troubleshooting Flowchart (Sheet 1 of 3) Page 7
Troubleshoot Fuel Level Indicator Obtain axial lead resistors from Newark or any source. 2.2 ohms = p/n 38K5441 18 ohms = p/n 38K5430 24 ohms = p/n 38K5446 47 ohms = p/n 38K5482 62 ohms = p/n 38K5498 Resistors must be 0.1-0.125W power rating and a accuracy tolerance +/-1. Connect 24-28v DC power to indicator: (+) = Pin 1 (gnd) = Pin 9 Connect resistor to indicator: Left = Pins 2 & 9 Right = Pins 4 & 9 Verify needle position. Needle position can vary 2-3 gallons. 2.2 ohms = 0 gal 18 ohms = 5 gal 24 ohms = 10 gal 47 ohms = 20 gal 62 ohms = full If indicator calibration is not correct, values will vary. Ohms vs. gallons correct? Perform bench level calibration on gage. Adjust low gain screw fully to counterclockwise position. Adjust high gain to half way between clockwise and counterclockwise position. Connect resistors to gage again. Fuel Level Indicator functions correctly. Other resistors match chart? Adjust low/high pots as necessary to have needle show 0 and full. Indicator is defective. Replace Fuel Level Indicator. SR20_MM28_3327 Page 8 Figure 1 Fuel System Troubleshooting Flowchart (Sheet 2 of 3)
Troubleshoot Fuel Level Sender Assy Access inboard and outboard sending units through wing access panels. TE When sweeping dial, if needle goes beyond the full dot, the dial will indicate >32 ohms resistance. This is normal. Disconnect sending unit from airplane wire harness. Measure resistance from sending unit dial as installed. Take initial readings. Shake wing, allow fuel to settle, and re-take readings. Stable? Dial is defective. Replace Fuel Level Sender Dial. Between 0-35 Ohms? Dial is defective. Replace Fuel Level Sender Dial. Dial is defective. Replace Fuel Level Sender Dial. Operative? Remove Fuel Level Sender Dial. Use metallic object (i.e. steel ruler, rod, bar) to bottom out dial needle at E empty. Connect dial to multimeter or ohmmeter to observe resistance. Resistance between 0-3.5 ohms? Remove Fuel Level Float Assembly and inspect. Values bounce slightly (~ ±2ohm)? Values bounce greatly (> 2ohm)? Values >33 ohms at full or during sweep? Rotate metallic object slowly and watch resistance on ohmmeter. Install Fuel Level Sender Dial. Dial is OK for installation. Dial is defective. Replace Fuel Level Sender Dial. Dial is defective. Replace Fuel Level Sender Dial. SR20_MM28_3329 Figure 1 Fuel System Troubleshooting Flowchart (Sheet 3 of 3) Page 9
1 2 3 10 9 8 4 5 7 6 3 1 LEGEND 1. Fuel System Vent (Underside of Wing) 2. Right Fuel Tank 3. Fuel Tank Strainers 4. Left Collector Tank 5. Left Fuel Tank 6. Selector Valve 7. Fuel Gage 8. Engine Driven Fuel Pump 9. Gascolator 10. Electric Boost Pump Serials 1005 thru 1877, 1879 thru 1885. SR20_MM28_1124A Page 10 Figure 2 Fuel System - Serials 1005 thru 1885 (Sheet 1 of 2) Serials 1005 thru 1885
5 7 9 4 3 2 8 6 1 LEGEND 1. Fuel System Vent, NACA (Underside of Wing) 2. Fuel Tank 3. Fuel Tank Strainers 4. Collector Tank 5. Selector Valve 6. Fuel Gage 7. Engine Driven Fuel Pump 8. Gascolator 9. Electric Boost Pump SR20_MM28_2562A Serials 1886 & subs Figure 2 Fuel System - Serials 1886 & subs (Sheet 2 of 2) Page 11
VENT FILLER ANNUNCIATOR QUANTITY INDICATOR FILLER VENT L. WING TANK R. WING TANK CHECK CHECK L. WING COLLECTOR R. WING COLLECTOR DRAIN (5 PLACES) SELECTOR FIREWALL SELECTOR OPERATION ELECTRIC AUXILIARY PUMP RELAY BOOST PUMP RIGHT RETURN FEED ENGINE DRIVEN PUMP MIXTURE CNTL. GASCOLATOR OIL PRIME PRESSURE SENSOR (LOW PRESSURE) RETURN FEED LEFT STARTING CIRCUIT / DAU Serials 1228 and subs, 1005 thru 1227 after SB 20-73-02. OFF TE In Prime mode, relay allows high-speed pump operation until 10 psi oil pressure is reached then drops to low-speed operation. In Prime mode, relay allows high-speed pump operation until 2-4 psi fuel pressure is reached then drops to low-speed operation. THROTTLE METERING FLOW INDICATOR INJECTOR MANIFOLD PRESSURE SWITCH Serials 1005 thru 1877, 1879 thru 1885. ELECTRIC AUXILIARY PUMP TO SELECTOR TO GASCOLATOR RELAY BOOST PRIME PUMP Serials 1005 thru 1227 before SB 20-73-02. SR20_MM28_1016E Page 12 Figure 3 Fuel System Schematic - Serials 1005 thru 1885 (Sheet 1 of 2) Serials 1005 thru 1885
ANNUNCIATOR VENT FILLER QUANTITY INDICATOR FILLER VENT L. WING TANK R. WING TANK L. WING COLLECTOR R. WING COLLECTOR FLAPPER SELECTOR FLAPPER DRAIN (5 PLACES) FIREWALL SELECTOR OPERATIO N RIGHT RETURN FEED ENGINE DRIVEN PUMP MIXTURE CNTL. ELECTRIC AUXILIARY PUMP GASCOLATOR RELAY BOOST PUMP OIL PRIME PRESSURE SENSOR (LOW PRESSURE) RETURN FEED LEFT STARTING CIRCUIT / DAU / GEA 71 OFF THROTTLE METERING FLOW INDICATOR TE In Prime mode, relay allows high-speed pump operation until 10 psi oil pressure is reached then drops to low-speed operation. PRESSURE SWITCH INJECTOR MANIFOLD SR20_MM28_2391A Serials 1886 & subs Figure 3 Fuel System Schematic - Serials 1886 & subs (Sheet 2 of 2) Page 13
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