912iS EM Battery vs Start Power Switch

- The Back-up switch is in case you need to quickly re-start the engine in the air. Or it is useful for ground maintenance.

• 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 (e.g. ran out of fuel) the ECU and fuel pumps will stop receiving power from the generators. By turning on the Back-up switch you can re-start the engine faster.
• The Back-up switch also provides the redundancy for the relays between the two generators.
• The Back-up switch must have a red guard or some means to prevent inadvertent use and to signify it as “reserved for emergency use only”.
• Caution: Do not turn ON the Back-up switch during a normal engine start as this will defeat some of the ECU’s self-checks.

-The Start-power switch is a momentary DPST(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. 

-Difference between “Start-Power” and “Back-Up” power switches: Both energize the ECU allowing it, and the fuel pumps, to power up. But…

• 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-check sequences (so that nothing interferes with an emergency re-start).

Thanks Rob, that’s a great explanation.  Much of this I was aware of but the missing link for me was the fact that the Emergency Battery switch bypasses some of the ECU self-test functions for a more immediate restart.  I thought I had read all the manuals but I was never able to find this information.