Gen Main Bus Fuse Blowing

[jsenft]

I have a -12is stuck at a remote airport. Every time I try to start the plane, the prop turns about 1/4 turn and then the 30amp Gen Main Bus fuse blows. I have check all wiring for a short and found nothing. Has anyone experienced this issue?

[Mike S]

Look for a ground issue.

Make sure the battery is fully charged.

Not too familiar with the Rotax power setup, but I have heard the regulators can be an issue----can you unplug the regulator and try to start it? Just a WAG with that one.

[RFSchaller]

Does the prop turn through freely when you pull it through? If not the starter might be pulling locked rotor current.

[rcarsey]

This is very odd... if its happening exactly as you describe.

The reason is that the Rotax fusebox isn't.. or shouldn't be sending ANY power to the in-cockput fuse panel (over its X3 connector, going to the back of the fuse panel [blue connector]) until after the engine runs for 5 seconds > 2500rpm. This is why you usually will continue to see the battery discharging until after the fusebox allows the Generator B ( B for the 'big boy' 30A gen.) to power the avionics. However, some special things happen during startup.. the HIC module has some MOSFET chips that activate and act as mechanical relays.. and short the Fusebox's pins 2 & 3 and short the fusebox's Ground Plate A and B together.. somehow when these chips activate.. something is sending 12v direct to ground.

I assume your EMS Emergency power switch is OFF as usual.

The places I'd check are, in order of preference are:

1. A short or burnt traces or debris under the HIC module (behind copilot screen).. or loose connectors/contacts
2. A wire or connector in the central bundle of wires that shorts out because of engine vibration, perhaps against the side panels when they are put back on.
3. Fusebox for stuck relays (I'm throwing this in here simply because they are mechanical).
4. Fusebox - check ground wire from Grounding Plate B to the firewall (brake master cylinder screw)
5. Ground wire is secure from the starter to the battery box (i think thats where it goes)
   

[rcarsey]

Originally Posted by RFSchaller

Does the prop turn through freely when you pull it through? If not the starter might be pulling locked rotor current.

I'm not sure inrush current would cause this problem. Power goes from the battery, through the master relay, through the starter relay to the starter. There are no fuses in line at all.

I suppose if the ground wire isn't secured from the starter to the side of the battery box, maybe it would find its way to ground through some other, less beefy wires. Though I'm unsure if theres a way to cause the 30A Generator fuse to pop.

[jsenft]

Thanks for all the thoughts. The Gen Main Bus Fuse blows within milliseconds from turning the key. The is free and turns normal. The starter will slightly turn in the few milliseconds it takes the fuse to blow. I did a lot of checking of wires around the starter, master relay, and starter relay. I did not see anything that would lead to suspect any short.

[Mich48041]

This is a long shot, but is easy to do. Take the cover off from the Rotax Fuse Box and pull out the 3 relays. Then try starting the engine.

[jsenft]

I assume pulling the relays will isolate the issue being one of the relays? Note: I have both lanes off while testing. I kept the lanes off as the engine should still crank but not start.

[jsenft]

Talked to Van's. Great support help. I will give an update after I get it fixed.

[RFSchaller]

Bob,

You’re right. Don’t know what I was thinking. The iS starting circuits are very different from the ULS.

RICH