Troubleshooting - B&C LR3D-14 Technical Manual

Linear regulator

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LR3D Technical Manual

Rev. IR (5/13/2020)

Refer to the wiring diagram found on page E-1 and use a high impedance (preferably digital)

volt/ohmmeter (DVM) to make the following checks. Please note that the engine should not be

running, the mags should be OFF, and there should be no auxiliary power applied to the aircraft

electrical system.

Turn all switches OFF. Use the lowest resistance scale on the DVM. Check resistance

between the battery negative (-) terminal and both pin 7 of the regulator and the engine

case. Measurements over 0.5 Ohm to either would be cause for investigation; should this

be detected, check the engine ground strap, battery ground strap, and regulator ground

wire(s) for loose or contaminated connections, broken conductors or bad crimp joints. If

these measurements are less than 0.5 Ohm, any of these three points may be used as ground

reference (-) for the measurements that follow.

Resistance from battery to pin 7:_____Ohms; from battery to engine case:_____ Ohms

Turn ON the battery master and alternator field switches. Measure the voltage on the

battery bus and on pin 3 of the regulator. The voltages should be equal within 0.2 volts. A

difference of greater than 0.2 volts may be caused by using a circuit breaker as the source

for pin 3 that supplies another device of considerable load. Change to a breaker dedicated

to pin 3. It is recommended that pin 3 NOT be jumpered to pin 6. If pin 3 has no voltage,

the regulator will not operate and low voltage warning will be lost.

Bus voltage:_______ volts

Measure the voltage on pin 6 of the regulator. It should be within 0.5 volts of the bus

voltage. A difference of greater than 0.5 volts may be caused by poor contacts in the field

breaker or field switch, or poor crimp joints/loose screw terminals in the wiring between

the bus and pin 6. Absence of voltage on pin 6 will prevent the regulator from operating.

Pin 6 voltage:_________volts

Check the voltage on pin 4 of the regulator. If the bus voltage is 13.0V or less, the pin 4

voltage should be approximately 1.2 volts less than the voltage on pin 6. A difference of

significantly less than 1.2V between pins 4 and 6 may indicate an open field circuit from

pin 4 through the alternator windings to ground. Voltage differences significantly greater

than the above could indicate a bad regulator unless the bus voltage is greater than 13.0V.

Systems with batteries that have a high resting voltage (such as LiFePO

have a lower voltage on pin 4. An ideal pin 4 voltage would be approximately 10.9 to 11.4

volts on a 12.6 volt bus and 23.5 to 24.0 volts on a 25.2 volt bus.

Pin 4 voltage:________volts

Move to the engine compartment. Measure the field voltage at the alternator as follows –

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