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A
737-300/400/500MAINTENANCE MANUAL
The generator switch or the circuit is defective.
 (a)  Examine the switch and the wires.
 1) Replace the switch or repair the wires.A time-delay failure in the generator control relay (GCR) when an undervoltage warning occurs.
 (a)  Put the GCR and the circuit in their usual condition.
 (b)  Measure the time that is necessary for the low voltage
 annunciator light to come on.
 (c)  Push the applicable generator switch (S3 or S6).
 NOTE:____ The generator switches are found on the forward
 overhead panel.
 (d)  Look at the AC voltmeter.
 (e)  If the voltage does not increase, replace the applicable GCU.

The variable resistor for the frequency adjustment is defective.
 (a)
Replace the variable resistor.
The neutral power wires for the generator are not installed
 correctly at the neutral current transformers (CT).

 (a)  
Make sure that the CT is in the correct position.

 (b)  
Look at the Wiring Diagram Manual WDM 24-23-11 and WDM 24-23-21.

 (c)  
Install the wires in their correct positions.

 NOTE: The polarity of the wires is important to the
____ correct installation.
A generator control unit failure.
 (a)  
Install a GCU that has a known good control panel.

 (b)
If the circuit operates correctly, replace the defective GCU.The control for the generator transfer breaker is defective.

 (a)  
Replace the applicable GCU with a known good GCU.

 WARNING:_______USE A WHEATSTONE BRIDGE TO DO THIS TEST. IF YOU DO NOT USE THE WHEATSTONE BRIDGE, A LARGE CURRENT CAN FLOW IN THE CIRCUIT. A LARGE CURRENT CAN CAUSE DAMAGE TO THE CONTACTS IN THE BREAKER.

 (b)  
Make sure that there is no resistance across the auxilliary normally closed contacts on the generator control breaker.

 (c)  
If you find a resistance indication, the auxiliary contacts are defective.

 (d)  
Open the interlock circuit between the transfer relays No. 1 and No. 2.

 (e)  
Remove the ac power from the airplane.

 (f)  
Examine the dc supply from the GCU to the coil of the applicable transfer relay.

 (g)
Replace the transfer relay.Remove electrical power if it is not necessary.

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 24-21-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 5. Trouble Shooting an Open Phase Condition
________________________________________
 A. References
 (1)  
AMM 24-11-11/401, Generator Removal/Installation

 (2)  
AMM 24-22-00/201, Manual Control

 (3)  
WDM 24-21-11

 (4)  
WDM 24-21-21

 (5)  
WDM 24-51-31

 (6)  
WDM 24-51-41

 (7)  
SSM 24-00-01

 B. 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.
 C. Fault isolation and repair procedure.
 (1)  
Remove electrical power from the airplane (AMM 24-22-00/201).

 (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.
　
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