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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