Cessna 170, Руководство по обслуживанию

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Cessna 170 Service Manual
Electrical System
13
TROUBLESHOOTING THE ELECTRICAL SYSTEM There are two simple and inexpensive tools with which
your electrical system troubleshooting may be performed. An inexpensive Volt/Ohm meter or MultiMeter is
the single, most useful tool you can have to troubleshoot your electrical system. Along with the MulitMeter,
an inexpensive set of heavy-duty jumper wires or “test leads” with “alligator” style clips can make any
troubleshooting an easy if not downright pleasant task, instead of a puzzling one. These items may be
purchased at local hardware and electronic shops at a total cost of less than ½ hour technician shop rates.
See Appdx EL, Fig 05 and Fig 06.
The aircraft electrical system is predominately a one-wire system, meaning that the airframe structure
provides the ground path for most circuits. As a result, the single most common fault will lie with that ground
return path. Whichever lighting circuit is causing difficulty may be diagnosed using these tools and the
following procedure.
Anytime a lighting (or other) circuit fails to operate properly the first two items to check are 1) proper, clean,
and secure ground connection, and 2) proper, clean, and secure positive connection and circuit. Item
number one will eliminate many failures of various electrical system failures. Item number two will usually
correct any outstanding defect. The most expeditious method to test these items is with the MultiMeter and
test leads.
Since one of the most frustrating systems to diagnose can be the stall warning horn/light in later aircraft, lets
use that system as an example.
Example Problem: The horn/light does not operate when the Master switch is ON and the Lift Detector is
raised.
System Description: Electrical power (12/14 volts DC) is supplied to the warning horn/light via a fuse/circuit
breaker whenever the Master Switch is ON. After the voltage passes through the warning unit it finds it’s
ground out on the wing, by passing first through the Lift Detector switch (operated by relative wind against
the lift detector vane on the leading edge) then to a nearby ground through a short wire terminating at a
screw in the airframe, thereby completing the circuit.
Procedure: Whenever troubleshooting any circuit FIRST re-read the system description to better understand
its operation. Second, always check the current limiter (fuse or circuit breaker.) Do not accept a visual
check. (Fuses may look fine but still fail a continuity check just as circuit breakers may look fine but also
have internal failures.) With the Master switch OFF: Use the continuity mode/function of your MultiMeter
across the input side and exit side of the fuse holder or C.B. to confirm continuity across the unit. If no
continuity then repair/replace the current limiter. Caution: Do not accidentally allow voltage to flow through
the MultiMeter in the continuity mode or the meter will be damaged. This is why we checked it with the
Master switch OFF.
Third, Gain access to the Lift Detector thru the left wing nearby inspection hole. Turn the Master switch OFF
and using the MultiMeter in it’s OHMmeter function (or it’s continuity test mode) check that the lift detector
switch-to-ground-wire has continuity to ground by placing one meter test-lead against the switch terminal and
the opposite lead against the airframe at a convenient location. No continuity? Problem solved. Repair the
lift detectors short lead to ground by replacement or cleaning of the terminal ends.
Continuity OK? Turn the Master switch ON and using the MultiMeter DC voltage measurement mode (15
volt range) place the positive tester lead against the lift detectors terminal leading from the cockpit, and
the negative lead to the other terminal and measure the available voltage.
Voltage near 12 volts? Use an alligator test-lead to ground the lift detectors terminal leading from the
cockpit to a convenient location on the airframe. Stall Warning Horn/Light operate satisfactory? Problem
lies with lift detector switch. Horn/Light still does not operate? Problem lies with Warning unit.
Voltage nonexistent at lift detector cockpit terminal? Problem lies with warning horn/light unit or wiring circuit
from main electrical buss. Use Multimeter at the warning unit and test for voltage at both terminals on rear
of warning unit between each terminal and airframe ground.
Voltage OK (approx 12 volts) at BOTH terminals? Connect alligator lead to warning unit terminal leading to
wing and a nearby airframe ground. Warning unit operate satisfactory? Problem lies with wiring from
warning unit to lift detector. Warning unit still not operate? Problem lies with warning unit. Remove it for
bench test and repair or replacement.
Voltage OK at one warning unit terminal but not both? Problem lies with warning unit.
Voltage nonexistent at either terminal? Problem lies with circuit from main buss. Check for power at fuse or
circuit breaker. Repair/replace faulty fuse/circuit breaker or defective wiring from the main buss.
This example demonstrates how a MultiMeter and set of test leads/alligator leads can solve most electrical
system troubleshooting problems. This procedure will work similarly for most systems.
1. Re-read the system description.
2. Test the current limiter (fuse/circuit breaker) for power and continuity.