Shorted Data Bus Wire

8 months 1 week ago #811 by Mark
Shorted Data Bus Wire was created by Mark
We had this.....
No FMS data on the F/O's navigation display. Same when trying the center symbol generator as alternate.

We did have navigation information on the Capt's display when selected to the right FMS.

From the prints there is one 429 data bus going from the right FMS computer to the center and right SGU's. I wasted hours checking intermediate plugs and terminal blocks on the back of the rack when I should of verified continuity from the FMS computer to the SGU's first..... AND THEN CHECKED FOR A SHORT TO SHIELD.

Blue wire of twisted pair shorted to the shielding. It took the whole data bus out.

Sometimes you get so narrow-minded on a troubleshooting path, that taking a step back and mentally rethinking the basics is sometimes a good idea.

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2 months 3 weeks ago - 2 months 3 weeks ago #837 by Mark
Replied by Mark on topic Shorted Data Bus Wire
Another example of a shorted 429 Data Bus output wire.
In this case we had a "B" channel failure of the right engine EEC (FADEC on this aircraft).The EEC outputs go several places. The Standby Engine Indicator, the PIMU (Propulsion Interface and Monitor Unit), and EICAS.
We showed a bus failure on the PIMU and the right engine "B" display was blank on EICAS. (The image above was taken after the repair.)The Thrust Management Computer receives engine data from the PIMU. (On some aircraft, the TMC receives data from the EEC directly without the PIMU as a go-between.) We showed a failure for that also on the PIMU.
We found a shorted blue wire to shield on the twisted pair wires coming from the engine. This wire was before the terminal block that splits the signal to all the user components.
Last edit: 2 months 3 weeks ago by Mark.

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