When #2 engine is shut down the “Rt Transfer Bus” drops off line 737-3/4/500

Discrepancy: When #2 engine is shut down the “Rt Transfer Bus” drops off line.

Possible fix: R355 Bat Bus relay, R1 Bat Bus relay Auto.

Tip: Check to see if the “Transfer Bus” light comes on when only the battery power is on, attempt to put both APU bus’s on line #1 first then #2. If only #1 APU bus comes on line. This could be an indication of no power to the 28V DC BAT BUS. Also if the APU surge valve is open on the ground, that could be another indication of no power to the 28V DC BAT BUS.
Place the Standby power switch to BAT and try using the sequence above to place the #2 APU BUS online. If it works correctly that is a indication of a faulty R355 or R1 relay.

With no power on the aircraft, VERY SLOWLY move the Battery switch to on. The R1 relay closes first giving you the two amber “TRANSFER BUS” lights and the standby bus comes on. Continue moving the battery switch to on, next both “BUS OFF” and “GEN BUS OFF” lights come on, and standby power goes off, this is a indication that the R355 has closed.

Reason/Explanation: When Battery is switched on the 28V DC BAT BUS recieves power from the HOT BAT BUS through the R1 and R355 (SSM 24-30-00). Both “TRANSFER BUS OFF” lights are powered by the 28V DC BAT BUS (SSM 24-50-02). By putting the standby power switch to BAT the hot battery bus is supplied to the 28V DC BAT BUS through the R326 relay (SSM 24-30-00).

The reason #2 will not go online if the #1 is already online is the APU Generator control system thinks the aircraft is in the air and will only let one APU bus online at a time. The Air/Ground system gets its power from the 28V DC BAT BUS, without this power R281 can not go to ground mode thus preventing R38 from going to ground mode. If R38 is properly set to ground it bypasses both C890 and C889 (APU GEN breaker) trip/close logic contacts) so it will let both APU bus’s to come on line (SSM 32-30-00).
Applicable Reference: SSM 24-30-00, 24-20-03, 32-31-00

Thanks to Tom S. for his input !


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