My training manual says:
Ignition/Starting Function Sequence
This is the APU start sequence:
The start sequence starts when the APU switch is put to the START position and released to the ON position.
The ECU turns on the oil pressure light on the P5 panel when the ECU receives the start signal. Thelow oil pressure light goes out at 30-40 psi oil pressure.
The ECU opens the APU fuel shutoff valve and air inlet door. The inlet door switch sends a door open signal to the ECU when the air inlet door is open.
At 0 percent speed and before the start system is energized, the ECU energizes the ignition unit.
If the air inlet door is open and the APU speed is less than 7%, the ECU sends a start signal to the SCU. If the APU speed is more than 7%, the ECU waits until the speed is less than 7%.
If 115v ac is available on transfer bus number 1, the SCU sends a start signal to the ac to dc converter in the SPU. The ac to dc converter gives 270v dc power to the SCU.
If 115v ac is not available on transfer bus number 1, the SCU sends a start signal to the dc to dc converter in the SPU. The dc to dc converter gives 270v dc to the SCU. The SCU also has a control function to prevent the DC to DC converter from depleting the battery below limits (18v dc on ground, 20v dc in air) during a DC start attempt. The proximity switch electronics unit (PSEU) provides the air/ground input.
The dc to ac inverter in the SCU converts the 270 vdc power supplied by the SPU into three phase start power. This power goes to the starter- generator.
The starter-generator turns the APU turbine shaft. The SCU receives starter-generator rotor position from the starter—generator resolver.
The SCU uses this signal to synchronize the three phase start power to the starter-generator rotor position.
At 7% speed, the ECU energizes the fuel solenoid and ignition unit which supplies fuel and ignition to the APU.
At about 30% speed, oil pressure goes above 30-40 psi. The oil pressure switch removes the low oil pressure signal. The ECU deenergizes the low oil pressure light.
At 60%speed, the ECU deenergizes the ignition unit.
At 70 percent speed, the ECU removes the start signal from the SCU.
The SCU removes the start signal from the SPU ac to dc and dc to dc converters. This deenergizes the starter-generator.
At 95% speed, plus two seconds, the ECU gives the ready to load (RTL) signal to other airplane systems. This signals the airplanes systems that the APU is ready to accept loads.