I want to bring back my beloved Macbook Air 2012 (MBA5,2) to its 'original firmware'. I have identified the file as MBA51_00EF_B01.scap, which I extracted using unpkg over the downloadable offered by Apple here: MacBook Air EFI Firmware Update 2.5.
That version allows one to install OSX 10.7.4, which is the (oldest) OS that shipped with this machine. That will allow me to reactive an old M-Audio sound interface that only works with Lion OSX. But also to experience and use this hardware performing the lightest, for I mainly use it as a working station with vintage software.
Presently I have Mountain Lion (10.8.4) installed, but once I had El Capitan which upgraded me to BootROM MBA51.00EF.B04 and SMC 2.5f9. I managed to downgrade the SMC firmware to version 2.5f7, which is compatible with 10.7.4–I used rEFIt and SmcFlasher.efi for it. But Bootrom remains MBA51.00EF.B04.
I followed this guide and ran the following command in the Terminal: /usr/sbin/bless -mount / -firmware "MBA51_00EF_B01.scap" --recovery --verbose
Firmware preloads successfully. But when I reboot to start the installation a black screen remains during three continuous fast reboots (I guess trying to install the new firmware). Then it boots normally and no firmware downgrade happened.
Any idea why this is happening? Are there any other methods to load the BootROM, supposing that it is just being refused by "bless" for being older that the installed one?
Hope this contributes and appeals to those willing to revive the past.
----- UPDATE 30/07/19 -----
Still trying to downgrade firmware in MBA mid2012.
I presume apple introduced the iOS policy of no-return in newer firmwares?
An alternative way would be to inject the older firmware with 'flashrom' over linux, but I don't have another MBA with original firmware from where to extract it. Someone uploaded a similar dump here. But it is for MBA2012 11" i7, mine is MBA2012 13" i5. I guess the firmware is the same, but double risk to brick it.