Starting System
Generator Control Unit LED Code Definition
 

The set of four LED (light emitting diode) fault indicators located on the face of the Left and Right Generator Control Unit (BT001 and BT002) are labeled by nomenclature which indicate the following:

NOTE:  When the L GEN or R GEN Switch (SI025 or SI026) is held in the RESET position, the GCU LEDs will illuminate displaying a fault pattern.  Refer to the Generator Control Unit LED Fault ID Display And Indications page for a description of each pattern. 

LED 

Function

Generator Control Unit (GCU)

  • If the generator control unit (GCU) LED is flashing, the generator control unit has detected an internal fault.

  • If the overvoltage (OV) LED is also flashing, the field transistor
    has shorted. Check the wire on pin DD of the GCU.  This is the return for the field suppression diode.

  • If the generator control unit (GCU) LED is steady, the internal microprocessor has shut itself down and the GCU should be replaced.
Overvoltage (OV)

  • If the overvoltage (OV) LED is flashing, the GCU has detected an overvoltage condition and has shut itself down.

  • If the system (SYS) LED is also flashing, there is a generator
    over excitation condition.

  • If the GCU LED is also flashing, the field transistor has
    shorted.
Ground Fault (GF)

  • If the ground fault (GF) LED is flashing, the GCU has detected a ground fault condition and shut itself down.

  • If the overvoltage (OV) LED is also flashing, an over excitation exists.

System (SYS)

  • The GCU has detected a problem with the airplane wiring and shut itself down.

NOTE: The GCU cycles through all the LEDs at power up to test their operation. Under some conditions the SYS LED may flash after completion of test series. This is a normal condition. The flashing LED may be
extinguished by selecting the L GEN  or R GEN switch ON/OFF/RESET switch from OFF to RESET three times within three seconds. This procedure will clear any of the fault lights if the fault has been corrected.

NOTE: The ground fault (GF) LED illuminates at a low intensity under very low ambient light conditions.  This is normal operation and not a reason for rejection of the GCU. If the GF LED is glowing at
full intensity, the ground fault wiring system may need to be investigated. The L GEN  or R GEN RESET switch may be selected to test all of the LEDs at full intensity. The GF LED illuminates at a low intensity because it is activated 0.1% of the time by the microprocessor. The low intensity illumination may or may not be seen depending on ambient light levels and the viewing angle to the GCU.