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LR3D Technical Manual
Rev. IR (5/13/2020)
L-40, BC410-H, and BC425-H Alternators: Use a thin probe or small gage
wire wrapped around the probe to reach through the connector body and
measure the voltage at either field connector terminal coming out of the
alternator. Do not disconnect the field connector for this measurement.
LX60, BC460-H, and BC462-H Alternators: Turn OFF the battery and
alternator master switches.
alternator. Use an 18 AWG jumper with an alligator-clip (or fashion an 18
AWG jumper wire) approximately 6" in length to temporarily link either
active field connector terminal with the corresponding male blade coming
out of the alternator. Turn ON the battery and alternator switches. Use
DVM probe to measure voltage at the alligator clip. Turn OFF battery and
alternator master switches, remove jumper, and reinstall field connector.
Observed voltage in the above should measure within 0.5 volts of the measurement on pin
4 of the regulator. A lack of voltage may indicate an open circuit between pin 4 of the
regulator and the field terminal, or a damaged/failed regulator.
If an open field circuit is suspected, the battery and alternator master switches may be
turned OFF, the alternator field connector removed, and a resistance measurement made
between the connector and pin 4 of the regulator. Look for near 0 Ohms. Field resistance
of the alternator may also be checked at this time by measuring from either male field
terminal blade to alternator case; typically, this should measure between 3 and 10 Ohms.
Values other than these may indicate a broken field wire, or heavily worn alternator
brushes and slip rings (respectively).
Field terminal voltage:_______volts
6.
With the switches ON, check the voltage between the alternator output post (or "B"-lead)
and ground. It should be battery voltage. If not, check the wiring between the alternator
"B"-lead and the battery positive (+) terminal.
connections, broken wires, or an open breaker or fuse.
Alternator "B"-lead voltage:______volts
7.
If all of the voltages in the first 6 steps are close to the value specified, the charging system
should be operative. If not, check for a broken or loose alternator belt or broken alternator
drive coupling (if so equipped). It is also possible on some installations that the engine
speed will have to be near run-up RPM or more for the system to provide useable output.
Intermittent problems are the hardest to find. In composite aircraft a common root cause is poor
system grounds; metal aircraft can exhibit this defect, also, if proper system grounding is not a
priority.
Noise problems can also be challenging. To manage system noise problems, consider the
following:
B&C Specialty Products
bandc.com
Disconnect the field connector from the
Alternator field resistance:_______Ohms
Look for loose or contaminated
Section D
Troubleshooting
Page D - 2
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