(g)
The CU FAIL indicator will go out when the MU is in the fail mode because proper operation of the CU requires an operational MU. This identifies the unit to be replaced, which in this case is the MU.
(h)
If the 16v dc power supply in the MU fails, the software will turn on the CU FAIL indicator. The MU FAIL and CU FAIL indicators have the same lamp driver. When the MU is in the system failed mode (either CU or MU fault) data voice contact is forced to open, returning the VHF transceiver to voice mode. The VHF transceiver PTT key line is forced open to insure that in the event of any internal MU failure, the VHF transceiver will remain in the voice transmit mode.
(15) GMT Clock and Interface
(a) The GMT interface has circuits required for the .P to read and set the GMT clock. Once set, the GMT clock will maintain the correct time even when the 115v ac circuit breaker is opened. The 12v dc power required by the GMT circuit will be derived from the 28v dc input in tne absence of a 115v ac input.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
AIRCRAFT COMMUNICATIONS ADDRESSING AND REPORTING SYSTEM - TROUBLE SHOOTING
__________________________________________________________________________
1. General_______
A. When the ACARS system is inoperative, a component failure is usually the cause. Replace the component that you think is failed and make sure the system operates correctly.
B. If the ground-based supplier of the ACARS/AIRCOM data-link service (to be referred to as the "ground station") has a problem, the aircraft ACARS may not operate correctly. A NO COMM indicator will be present in this situation. Also, other aircraft which try to use the same ground station will have the same problem.
C. A display of NO COMM does not necessarily indicate an aircraft failure. It does indicate that the data-link air/ground network was not completed. This can be the result of ground station problems or aircraft failures. The ACARS transmits over the VHF-3 system. If this system is inoperative, or the VHF path between the airplane and the ground station receiving antennas is blocked, the system will go NO COMM.
D. The table that follows is a trouble-shooting chart to help you find the cause of a problem.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
2. Trouble-Shooting Chart
______________________ ~............................................................................... | PROBLEM | POSSIBLE CAUSE | ISOLATION PROCEDURE | CORRECTION | ................................................................................. | MU FAIL light | MU failure| | Open and then close the| | |
| | Replace the MU | | is on | | ACARS AC circuit breaker.| | | | | | | | CU FAIL light | IDU failure| | Open and then close the| | |
| | Replace the IDU| | is on | | ACARS AC circuit breaker.| | | | | | | | NO COMM is | VHF multipath || Transmit from a different | Change the| |
| | | shown | distortion or | location. | location of the| | | dead spot | | airplane. Tell| | | | | the ground| | | | | station about| | | | | the dead spot.| | | | | | |
| Failed trans-| | Make sure the ground |
| | Advise the |
| | mission from | station's transceivers are | ground station| | | the ground | operational. If the ground | if necessary.| | | station | station has a problem with | This is not| | | | their equipment, many other | an aircraft| | | | aircraft that use the data | problem.| | | | link will have the same| | | | | symptoms.| | | | | | | |
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