DISCLAIMER: The following web page content is for informational use only and to be utilized solely by FAA licensed A&P personnel.
1. Be sure before trouble
shooting your system that ALL electrical components are OFF in the
aircraft to minimize the chance of damaging avionics or other electrical
components.
2. Always hook a
volt-meter/ multi-meter to the main buss so that you can quickly and accurrately
determine if the system is charging or discharging and most importantly is the
system in an Over Voltage state . Record the buss voltage at low load and
high load and also what buss voltage is with just battery voltage on the buss.
This will help determine how your system is operating . Ammeters do not tell us
enough about what is happening on the charging system other than the indicated
charge or dis-charge condition .
3. Use the Trouble Shooting Chart
(TSC)
with Master Switch “On” , Field Switch “On”, Engine “Off” . Write the voltages
measurements down and where you took the measurement & then go back to the
chart to see if you can get a match with the TSC .
4. Make that the system
is wired as per the diagram otherwise the
TSC will not help and
your system may not charge as designed. There are several ways to mis-wire the
system so it works but it does not put out the correct output voltage.
5. If the Resistor on the
back of the Alternator is Open or
missing the TSC will not be helpful because the Resistor is providing the
voltage being measured on the TSC. Many times a system will self excite
during run-up or takeoff indicating that the Resistor is Open or missing . If
all is normal the system will be on-line at low RPM after start up .
6. If all checks are
normal on the TSC it is possible that the Over Voltage Relay (OVR) is
tripping at startup . To diagnose this do the reset of the OVR. Field
Switch Off , Master Off, Master On, Field Switch On . If this brings the system
back on-line the OVR probably tripped at startup because of a voltage spike from
the starter contactor .
7. The wiring diagram shows a
separate ground wire or ground strap on the diagram . Be sure to install one or
inspect the system to see if one is installed. The system may work, or have been
working for years, but may not have full output capability and/or cause the
output to vary as the grounding is changing.
8. Be sure the voltage
regulator is grounded to the ground terminal on the back of the Alternator as
shown on the diagram. Firewalls are
not always good grounding points and if the Voltage Regulator does not have a
good reference ground it cannot regulate properly.
9. Be sure the External
Diode Plate on the back of the Alternator is not touching a ground point . The
External Diode Plate has power to it which then flows across the diodes imbedded
in the plate to the output terminal/stud.
10. Old Master Switches
can cause problems with the charging system. As the contacts and plastic age on
the Master Switches both the field to the alternator and the ground for the
battery contactor can be opening/closing without someone moving the switch.
Remember it is important for the charging system to always have the
battery in the circuit while the alternator is charging otherwise there is a
risk of damaging the charging system components. If the battery contactor is
opening/closing because of a bad Master Switch this can cause repetitive
failures of the charging system.
11. Low Buss Voltage can
be caused by several things. Worn
Brushes on the Alternator. Poor
Grounding . Abnormal resistance in
the output wire . Resistance in the aircrafts Field Circuit . Heavy electrical
loads - battery discharged- pitot heat - landing lights- storm scope etc. can quickly add up to a 50 amp load . Alligator clip a wire onto
the output terminal of the Alternator and compare output voltage to the voltage
on the buss . They should be within .1-.2 volts difference . If you are getting
more there could be a problem on the output wire. The Voltage Regulator is
typically not the culprit in Low Buss voltage. The vast majority of the time a
Voltage Regulator would fail completely and totally and not weaken .
12. Some mechanics
install a temporary field circuit between the Voltage Regulator field terminal
and the Alternator field terminal . They do this so they can bypass the aircraft
field circuit and associated splices/switches and see if the Alternator and
Voltage Regulator will perform properly . They start the aircraft while
monitoring buss voltage and look
for normal voltages or can shut down the system off if needed. If the systems
peform normally they focus their attention on the aircrafts field circuit.
13. Some mechanics momentarily (1 - 3 seconds) flash 12 volts to
the field of the alternator while monitoring buss voltage to see if the
alternator goes to full output. If the Alternator does not do anything
they know the problem is in the Alternator.
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