ATA 80

Starter Sequence
The left starter sequence is the same as the right only change No. 1 and No2. and right and left.
1. Power flows from the No. 2 distribution bus and start circuit breaker to the right start switch.
2. From the right start switch to the left start relay SR1. If the left start circuit is not in use the SR1 will be de-energized and these contacts of SR1 will be closed.
3. From SR1 to GLR2. With no system malfunction GLR2 will energize as soon as battery power is applied, and these contacts will also be closed.
4. From GLR2 to pins K and X of the right generator control unit.
5. the power that comes in pins K and X goes out pin G of the GCU assuming the engines is below 50% N2 and the stop position of the start switch has not been pressed.
6. From pin G of the right generator control unit, power goes to the right DC contactor box and does the following:
(A) Arms the ignition circuit. There is no ignition until the right power lever is moved out of stop cock.
(B) Energizes the Start Relay (SR2) and the Auxiliary Start Relay (ASR2)
7. The Start Relay (SR2) does the following:
(A) Latches in the right start circuit so the start switch can be released.
(B) Disables the right feeder fault circuit during the start.
(C) Energizes and closes the right start contactor (GSC2).
(D) Disables the left start circuit
8. The Auxiliary Start Relay (ASR2) does the following:
(A) Opens the left bus tie contactor.
(B)Opens the left feeder fault circuit during the start.
(C) Closes the left start contactor. This is used for the cross start and will only happen during the second engine start, if the operating generator switch is on and there is no external power connected to the airplane bus system.
9. At corrected 50% N2 the right engine computer will supply a ground to pin N of the right GCU. This will cause a relay in the GCU to energize which will open its contacts and stop the power from coming out of pin G. This will terminate the start.
10. Pushing the right start switch to the STOP position will also supply a ground to pin N of the GCU and terminate the start, at any RPM below approximately 50% N2.

NOTE:
The only thing the engine computer has to do with the engine start , other than the obvious safety features, is to stop the start and provide enrichment. Everything else to do with starting is strictly a function of the DC electrical system.