APU Door - EICAS Message

An "APU Door" EICAS message is a indication that the APU air inlet door is not in the commanded position.

It shows when the door is open (or not fully closed) when commanded closed or closed (or not fully open) when commanded open.

The indication system uses the same two relays (K176 and K547) that are used to drive the door open/closed. Ref:  rotate.aero/forum/767-49/399-apu-inlet-door-position

If the EICAS computers see a ground on pin "E1" of plug "F", they will show the "APU Door" fault on the crew display.

In the schematic below, the APU is shown shutdown. In this condition both relays are relaxed. If the internal switches "1 and 4" or "8 and 9"  are in their correct positions, no ground will be present at the EICAS computers.
  
     
  
We recently had the "APU Door" warning on a aircraft that had the APU off with the inlet door closed. It turned out to be the internal switch contacts on 8 and 9. They were remaining connected with the actuator fully retracted (or closed) position.

After removal, I used a 28VDC power supply and a meter to verify our results. I applied power to pins 6 and 7 (actuator retract). Before power was applied, I had continuity on pins 8 and 9. After full retraction..... I still had continuity on those same pins.
  
     
     
  
The easiest way to verify bad actuator switches with the warning on and the door visually verified fully open or closed..... is to just disconnect the actuator plug.

If the EICAS warning goes away..... the actuator is at fault. (And yes, there could be some other funky shit going on if one of the relays stuck on, the warning system had a broken or shorted wire..... but we'll just keep it as simple as we can for now!!)