IBBS Blown Fuse

[40m]

270 hours without a hitch then after switching on IBBS it shuts down. Pulled 10 amp fuse on battery to find it was blown. Sent battery out to be checked came back normal. Reinstalled, screen came up for 5-10 seconds then blew fuse. Any suggestions on where to start.

Tks.

[Andy]

270 hours without a hitch then after switching on IBBS it shuts down. Pulled 10 amp fuse on battery to find it was blown. Sent battery out to be checked came back normal. Reinstalled, screen came up for 5-10 seconds then blew fuse. Any suggestions on where to start.

Tks.

IBBS powers several different LRU (line replaceable units) of the G3X system. These LRU are:

GSU 25 ADAHRS
GDU 465 Display Unit
GEA 24 Engine interface
GMU xx Magnetomer (via GSU 25)

It seems likely that either one of these LRU is faulty or, perhaps more likely. there is a wiring fault between the IBBS and one of these units. It may take only a simple test to find out if one of these LRU has failed. Try turning on MASTER power with IBBS switch OFF. Are all functions normal on the main display? Do any CB pop?

Assuming all works ok then an IBBS power wiring fault seems quite likely. Finding where the fault is may be tricky because there is no circuit breaker between each of these LRU and the IBBS. The CB of each of these LRU only controls the primary power not the backup power.

The easiest place to get at the IBBS power feed to each of these LRU is at the IBBS 15 pin connector. Perhaps start by pulling the connector and measuring (on the aircraft side) the resistance between each backup power output and ground.

If nothing obvious there then you'll need to disconnect each individual power output from IBBS and restore them one at a time. This would require a breakout harness or pulling the individual pins from the connector.

You might think it should be easier to pull the connectors of all LRU that receive IBBS power but just getting at the GEA 24 connector would take more time than all the tests at the IBBS connector.

These are just some initial ideas based on my knowledge of the system and my understanding of the schematics. Maybe someone has already experienced this problem and knows a probable cause.

Good luck and please report what you find.

[Daveembry]

270 hours without a hitch then after switching on IBBS it shuts down. Pulled 10 amp fuse on battery to find it was blown. Sent battery out to be checked came back normal. Reinstalled, screen came up for 5-10 seconds then blew fuse. Any suggestions on where to start.

Tks.

just a note that the IBBS CB, if engaged, allows the IBBS battery to charge. It does not shut down the IBBS system. If you pull it, then the IBBS battery will still provide power out but just not recharge.

I don’t know and I’m terrible with electronics but maybe pull that CB and then see if it blows the fuse. If not, then maybe it’s something from the charging side of it instead of operations? (Power going into the battery vs going out).

just a data point to consider.

[Andy]

just a note that the IBBS CB, if engaged, allows the IBBS battery to charge. It does not shut down the IBBS system. If you pull it, then the IBBS battery will still provide power out but just not recharge.

I don’t know and I’m terrible with electronics but maybe pull that CB and then see if it blows the fuse. If not, then maybe it’s something from the charging side of it instead of operations? (Power going into the battery vs going out).

just a data point to consider.

In normal system operation (IBBS ON and MASTER ON) pulling the IBBS breaker removes charge power from IBBS and this triggers IBBS to enable all emergency outputs (IBBS power outputs are enabled when charge input drops below a threshold level)

Pulling the IBBS breaker will not have any impact if the G3X system is powered by IBBS only (MASTER OFF). However, If MASTER is ON, pulling IBBS breaker will connect IBBS power to all IBBS powered LRU. They will now have power on the primary and secondary power sources. The power sources are likely to be diode "OR'd" in each LRU. IBBS fuse may blow or not blow depending of how each LRU draws current from each of the two available sources. (IBBS fuse will blow as soon as IBBS CB is pulled if there is a short on any of the IBBS output power lines.)

If IBBS fuse does not blow then pulling the CB on each IBBS powered LRU in turn will force that LRU to use only IBBS power. If pulling one of these CB causes the IBBS fuse to blow it may help to localize the problem.

The IBBS fuse can only blow if one or more of its emergency power output pins draws excessive current. (There may be internal failure modes that could cause this but that seems unlikely)

This is the sort of problem I love solving (as long as it's not on my airplane)

[Andy]

If nothing obvious there then you'll need to disconnect each individual power output from IBBS and restore them one at a time. This would require a breakout harness or pulling the individual pins from the connector.

Here is the pin to pin info for a test harness that I think would be useful to diagnose the problem. Even if you have to buy all the parts it could be cobbled up for under $10. You don't need high quality connectors and any you can find on eBay will be ok for this temporary test harness.

[Andy]

Was this resolved? If so, what was the problem?

Thanks.