When eng #1 is shutdown, #1 “TRANSFER BUS OFF” light comes on and power is lost to the Bus 737-3/4/500

Discrepancy: When #1 engine is shutdown, #1 “TRANSFER BUS OFF” light comes on and power is lost to the Bus

Possible Fix:
R3 relay, # 1 GCU, R349 relay, C465 circuit breaker, P5-4 AC System generator and APU panel, R350 relay, #2 GCU

To verify system operation perform AMM 24-21-00-735-001, paragraph “R. Test of the Control Circuits for the Engine Generators”. This will allow you to identify what the system malfunction is.

If both #1 and #2 Transfer Bus’s are not working per this mm ops check. You should verify 28VDC Switched Hot Bat Bus is present by checking that Circuit Breaker C169 Switched Hot Bat Bus is closed, located on P18-5 cb panel (down low behind the captains seat). If this circuit breaker is in, pull it, it should shut off a bunch of stuff in the flight deck. ( You will hear the effect). per SSM 24-30-00 if you do not hear the effect trouble shoot the source of the power back towards the battery from C169. If you hear a diffence check for the 28 VDC making it to the Transfer bus control circuit through R41, and the C4 circuit breaker (per SSM 24-20-01) or C5 circuit breaker (per SSM 24-20-02)

If the discrepancy is only with the #1 Transfer bus not being powered when #1 Gen bus or #1 APU bus is powered off or lost. Go to R3 relay contact Y2 (located in P6-4 circuit breaker panel) verify 28VDC is present and ground is present when the #1 Gen bus/#1 APU bus is lost. per SSM 24-50-02

If power and ground are present on contact Y2 and Y1, of the R3 relay you most likely have a bad R3 relay. NOTE THE R3 RELAY IS INSTALLED WITH THE CONTACT NUMBERS UPSIDE DOWN WHEN YOU LOOK AT IT.

If no power is present at Y2 of R3 you can split the circuit by going to C465 circuit breaker and looking for 28Vdc.

If power is present at C465 follow it back accross P5-4 panel, and through R349 pins B2 and B3, then to #1 GCU pins A45 and B36. It usually faster to try replacing these components rather t hen attempt to verify power at pins.

If no power is present at C465, follow it back through R350 relay contact A1 and A2 then to #2 GCU pin A45. This circuit continues back to SSM 24-20-01.

If power from generator 1 is lost, GB1 will open and the coil of relay R349 will de-energize. The circuit to normal coil of transfer relay 1 will open and the circuit to the alternate coil will close. If generator 2 is still supplying power to gen bus 2, the alternate coil of transfer relay 1 will energize, connecting transfer bus 1 to gen bus 2. A 0.25-second time delay occurs before the alternate coil energizes. This prevents bus transfer during normal power source transfer. The sequence of transfer bus control for other power source combinations is similar

The #1 Transfer bus is picked up by the other #2 Gen bus or #2 APU bus when power is lost to the #1 Gen or APU bus. This Automatic power transfer in performed by the R3 Relay, the control voltage comes through a network of relays and switch’s to make that happen. The 28Vdc control power comes from the Switched Hot Bat Bus to make this happen.

Applicable Reference:
AMM 24-51-00-001, 24-21-00/101, SSM 24-30-00, 24-50-02, 24-20-01, WDM 24-00-01, 24-56-01,24-24-01, 91-03-24
Thanks to Tom S. for his input !


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