NSEU,
Apologies, I should have said EIU's not APUC. Bear with me on this.
The 13deg inlet door sw is really only an opening(not full open) / transit indicator and duly lets the APUC know that the door is moving towards the FULL OPEN position. The APUC will not engage the starter until it sees the inlet door fully open and all other parameters are satisfied. As mentioned previously the door will fully open well under the 60sec timer limit.
With the aircraft on the ground, the air/grd relay R359 is energised, providing a ground for apu door open relay R461 through the inlet door actuator internal OPEN limit sw.
Energising R461 supplies 28V DC power to the inlet door actuator motor open windings, and the 60sec timer in the EIU's.
When the door is opened to full open at 45deg the inlet door actuator open limit sw opens removing the ground from R461 and subsequently power from the timer.
Now it gets a bit hazy with my notes, but I would go so far as to say that the EIU's signal the APUC via the ARINC bus to say that the door is fully open due no timer operation. Or timer has exceeded 60secs and no start and APU DOOR status.
McHale.
PS..... We may have similar notes... APU chapter page ref 40/41.