Yes, the master switch needs to be ON to start the engine. Here is the purpose of the Back-up and Start-power switches:
-The Back-up switch is in case you need to quickly re-start the engine in the air.(or for ground maintenance: If you need to power up fuel pumps or ECU)
•It is much like the start power switch as it provides a separate ground and 12 volt power that “re-energizes” the ECU and fuel pumps.
•Certification rules require the fuel pumps to stop if the engine stops. If the engine stops for any reason (ran out of fuel) the ECU and fuel pumps stop receiving power from the generators. By turning on the Back-up switch you can re-start the engine much faster.
•The Back-up switch also provides the redundancy for the relays between the two Generators.
•The Back-up switch must have red guard/cover over it to prevent inadvertent use and to signify it as emergency use only.
•Caution: Do not turn ON the Back-up switch during engine start as this will defeat some of the ECU self-checks.
-The Start-power switch is a momentary SPST(m) switch that provides 12 volts and a ground to the ECU so the engine can perform a normal start sequence. It needs to be a momentary type switch so it is OFF once the engine has started; then the ECU can perform its own self-checks.
Both the “Start-Power” and “Back-Up” power switches excite the ECU allowing it, and the fuel pumps, to power up.
-Difference between “Start-Power” and “Back-Up” power switches:
•The “Start-power” only powers up the ECU until the generator comes online.
•The “Back-up” also powers up the ECU but it is wired differently so that it bypasses the ECU self checks (so nothing interferes with a emergency re-start)
