(b)
Operate generator switch (S3) or (S6) on the forward overhead panel and observe AC voltmeter. If voltage does not build up replace GCU.
(7)
Frequency adjustment variable resistor ineffective.
(8)
BTB interlock contacts not clearing circuit for GCB to close. That is, the normally open contacts remain closed.
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(a) Check BTB control contacts or replace BTB.
G. Incorrect Routing of Leads To or Through Generator Neutral Current Transformers
(1)
Check lead routing to wiring diagram. See Wiring Diagram Manual 24-23-11 and -21.
(2)
Check position of current transformer and polarity of leads.
H. Generator Control Panel
(1) Check by replacing the generator control unit with one that is known to contain a control panel known to function properly.
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I. Short on Tie Bus
(1) Remove ac power from airplane and check visually.
J. Transfer Bus Control
(1) Failure of generator transfer breaker control.
(a)
Check by substituting a generator control unit is known to function properly.
(b)
Remove wires from terminals on No. 1 bus tie breaker and transfer relay coils and check for dc supply. See Wiring Diagram Manual 24-24-01.
(c)
Check for zero resistance between terminals of normally closed auxiliary contacts on generator control breaker. Any resistance reading indicates that the auxiliary contact fails to make proper contact.
NOTE: A wheatstone bridge is recommended for this test since heavy currents should not be passed through these contacts.
(2) Open interlock circuit between transfer relays No. 1 and No. 2.
(a)
Remove ac power from airplane.
(b)
Check dc supply from generator control unit through control interlocks of various breakers and contactors to coil of defective transfer relay.
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5. Trouble Shooting and Open Phase Condition
A. General
(1) An open phase can cause the circuit breakers to trip open for three phase loads like TRU’s, hydraulic pumps and fuel boost pumps when they are brought on line. If the load checks out OK after further investigation, the problem may be an open phase. Other apparently unrelated symptoms such as cabin lights dimming and loss of instruments may also occur.
B. Fault Isolation and Repair Procedure
(1)
Remove electrical power from the airplane (AMM 24-22-0).
(2)
Open the engine firewall disconnect connector D410 and the wing disconnect connector (D5124P - left, D5126P - right).
(a)
Inspect the contacts for signs of overheating and arcing.
(b)
Repair contacts or replace connector if problems are found.
(c)
If no problems are found, reconnect previously opened connectors.
(3)
Inspect feeder terminations (terminal lugs) going into and out of the generator circuit breaker (C801 - LEFT, C802 - RIGHT) and transfer relay (R3 - XFR BUS 1, R4 - XFR BUS 2 located in P6 panel) for signs of overheating and arcing.
(a) Repair terminations and replace relay if any problems are found.
(4)
Inspect feeder terminations at the generator for sign of overheating and arcing.
(a) Repair terminations if any problems are found.
(5)
Check ground side feeders on generator to ENG GND lug for continuity.
(a) Repair wiring and terminations if any problems are found.
(6)
If no problems are found, apply electrical power (AMM 24-22-0).
(7)
Measure the voltage drop across the contacts of the generator circuit breaker (C801 -LEFT, C802 - RIGHT) and transfer relay (R3 - XFR BUS 1, R4 - XFR BUS 2) for each phase with load applied.
(8)
If there is any significant voltage drop (greater than 1 volt ac) replace the GCB or Transfer relay.
(9)
Measure the output voltage at the generator terminals with load applied. If the voltage is outside the limit of 115 ±5 volts ac on any phase, replace the generator (AMM 24-21-11).
(10)
Remove electrical power if it is no longer necessary (AMM 24-22-0).
NOTE: The load circuit breaker may trip open when trying to apply the load. If this happens, replace GCB and transfer relay.
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