Mark

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!!)
  
 

Gasket P/N 3876244-1

The normal takeoff procedure is to manually take the engines to 70% at the beginning of the roll. At 70% the "N1" button is pushed which causes the TMC to takeover the rest of the throttle control.

We had a aircraft abort takeoff because "N1" didn't engage (I don't know why they just didn't take it, but I don't fly these things. I'm assuming safety is the issue).

We tried the test above when we could find no other issues with the aircraft. It worked, got signed off, and hasn't had a problem sense.

www.avionix-tech.com/blog/simple-explanation-of-ads-b/

That cannot always be stated as true, but usually you’ll find most modern transponders that support TCAS will be ADS-B compliant.

’Is there a GPS input to the transponders?

A search of the part number of the transponder should answer your question.

Do your aircraft have TCAS?

Does the ATC control panel have a TA or RA selection?

If the answer to either of these questions is yes…. you have ADSB installed.

The actual ATC control panel in the pedestal. A picture of the Transponder (and part number) in the electronics rack.

Good troubleshooting and fix!

Can I get a picture of control panel, please.
Maybe one of the transponder also.

A ground test of the A/T's can only be done with the engines running. On the FMC, go to the takeoff page. Put 60 in the scratch pad and then move that into the thrust setting with the (left) side button.  That will give you a max de-rate for the day. Flaps at 5, then push the N1 button on the MCP. The autopilot does not have to be engaged and the EEC's can be on or off. The N1 bug select on the EICAS panel has to be pushed down (not in manual mode). Only the takeoff mode will work in the TMC.
  
  

The APU compressor inlet temp sensor is another example of a thermocouple.