(This is a somewhat half-assed entry and I'd be glad to have anyone correct any wrong statements.)
We powered up the aircraft only to have a hideous horn screaming from the nose wheel well. (I didn't go look to see the actual horn location.)
It is not shown in this picture, but we had the upper right light (INS/Rack Cooling) on. We could stop the damn horn by pushing the "Reset" button, but that certainly didn't fix the problem.
In the cockpit we had the "Extract Flow" light on. (This image is with the light test switch on to show all the lights in this particular panel.)
After some digging, we found the extract flow sensor located in the forward oval belly access panel. When it was disconnected...... no horn. We had one in stock which corrected the problem. In this image is the flow sensor and extract fan.
Again..... without digging through the prints, this is my best assumption for setting off the horn. (1) No INS cooling (2) Extract fan fault (3) Cooling fan fault (4) Crew call
Cooling fan. (I didn't look to find the location of the flow sensor for the fan.)
This is a somewhat mislabeled system. From what I can guess, the lower "Blower Fault" annunciator is a blower flow sensor issue. The middle "Extract Flow" is a extract fan problem. If they're both related to "flow".... why aren't they labeled the same? Without looking at the prints, I'm guessing the top "OVBD Fault" is a disagreement between commanded and actual overboard valve positions.
The OVBD fault is exactly that because after both engines are operating, that OVBD Extraction Valve closes. We have deferred that valve for not closing completely. I do believe there are two fans operating on the ground: blower and extract that is why there are two seperate indications, from what I remember.