The fuel pressure gauge instruments are mounted on the instrument panel in the cluster for their respective engines. The gauges are electrically connected to pressure senders for their respective engines: The senders are mounted through the aft side. and outboard half of each firewall to the fuel pressure line. running from each engine’s carburetor to the firewall. The oil pressure senders are those units with the two terminals on the aft side of the instrument.
FUEL PRESSURE SENDER TESTING. (Refer to Chart 7301.)
The fuel pressure sender works on the relationship of pressure to resistance. As the pressure rises the resistance rises and alters the position of the needle in the instrument.
1.  
The sender may be tested by removing it from the aircraft and applying a calibrated pressure to the pressure port.

2.  
Attach an ohmmeter to the two terminals on the reverse side of the pressure port, and check the resistances as shown in Chart 7301 .

3.  
To test the sender and gauge in conjunction with each other, disconnect the gauge from the aircraft system, connect it in series (using a 12 to 14-volt system) with the sender, and proceed as follows:

A.  Slowly increase the pressure to the sender till 9 + 0.5 psi is reached.
B.  Let the system set at the pressure for a couple of seconds, recheck the pressure, and lower the pressure to 8.0 psi.
C.  Reduce the pressure to 0.5 psi. The gauge should indicate 0.5, +1, - 0 needle widths.
— Note —
The sender and gauge are nonadjustable, therefore that which does not meet specifications should be replaced.
4. Reinstall the gauge and sender.
CHART 7301.  FUEL PRESSURE SENDER TEST SPECIFICATION
Pressure (Psi)  Resistance (Ohms)  Tolerance (Ohms) 
0  10  — 
0.5  11.5  +1, - 0 
5  42  ±1 
8  60  +1/2, - 0 
10  72  ±1 

The gauge (Piper P/N 86750-2) should indicate 8 psi +1/2, - 0 needle widths.
73-30-00 Page 73-13 Issued: April 1, 1997
TROUBLESHOOTING.
CHART 7302.  TROUBLESHOOTING FUEL PRESSURE GAUGE

Trouble  Cause  Remedy 
No fuel pressure indication.  Fuel valve stuck. No fuel in tanks. Defective fuel pump.  Check valve. Check fuel, fill. Check pump for pressure build up. Check diaphragm and relief valves in engine pump.  Check for obstruction in electric pump. Check bypass valve.  Air leak in intake lines. 
Pressure low or pressure surges.  Obstruction in inlet side of pump. Faulty bypass valve. Faulty diaphragm.  Trace lines and locate obstruction. Replace. Replace or rebuild pump. 
Needle fluctuation.  Surge dome or pump filled with fuel. Air in line.  Remove and empty. Loosen line at gauge, turn on electric pump. Purge line of air and retighten. 
High fuel pressure with engine shut off right after flight.  Fuel in line expanding due to heat build up in cowling.  Normal. 

73-30-00 Page 73-14 Issued: April 1, 1997
3H4

CHAPTER
74

IGNITION
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CHAPTER 74 - IGNITION 
TABLE OF CONTENTS / EFFECTIVITY 
CHAPTER/ SECTION  SUBJECT  GRID NO.  EFFECTIVITY 
74-00-00  GENERAL  3H9 
Description and Operation Troubleshooting  3H9 3H10 
74-10-00  ELECTRICAL POWER SUPPLY  3H11 
Removal of Magnetos Inspection of Magneto Magneto Overhaul Instructions Disassembly Cleaning and Inspection Assembly Internal Timing Magneto Installation and Timing to Engine  3H11 3H11 3H14 3H14 3H16 3H16 3H20 3H20  1R0202 1R0202 1R0202 
74-20-00  DISTRIBUTION  3I1 
Ignition Harness Inspection Removal Disassembly Assembly Installation  3I1 3I1 3I1 3I1 3I4 3I6  1R0202 
Spark Plugs Removal Inspection and Cleaning Installation  3I7 3I7 3I8 3I8 
74-30-00  SWITCHING  3I9 
Magneto and Starter Switches Removal and Installation  3I9 3I9 

74-Cont./Effec. Page 74-3 Revised: February 28, 2002
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74-Cont./Effec. Page 74-4 Issued: April 1, 1997
3H8
GENERAL.
DESCRIPTION AND OPERATION.
Magnetos are self-contained ignition devices. Two are installed on each engine for redundancy. As the engine crankshaft rotates, the magneto rotor shaft is turned by gears in the engine accessory case. The magneto rotor shaft turns a two pole permanent magnet adjacent to a coil winding, generating a high tension electric current which is distributed to the appropriate spark plug. Each magneto is driven at engine speed and produces four sparks in two complete engine revolutions. A mechanical impulse coupling is installed to retard magneto ignition timing (see lag angle on magneto dataplate) and provide spark for engine starting. As the engine is cranked, a spring in the impulse coupling is wound. When the engine crankshaft reachs the proper position for starting, the spring in the impulse coupling is released to spin the rotating magnet and produce the spark required to fire the engine. After the engine starts, the impulse coupling flyweights disengage the coupling due to centrifugal action. The coupling then acts as a straight drive and the magneto fires at the normal firing position of the engine.
　
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