Somehow I screwed up and cannot revert from OpenWrt back to Linksys firmware on a WRT1900ACS. I used putty to change partitions but, evidently, OpenWRT is on both partitions. (No, I do not know how this happened unless it was a 30/30/30 hard reset I did that did it.)
Use the official linksys firmware with the sysupgrade command and it should work, I tested it here and works no problem. Also when you flash firmware using sysupgrade, it will write it to the other partition and boot from that preserving the current partition, also true the other way around. The idea being you'll always have a working partition to fall back on if the flash goes bad.
Did you verify what's compatible with the WRT1900ACS via the Serial Port section of the WRT AC Series ToH?
Specifically:
USB-TTL Cable: 2.0mm pitch @ 3.3v (FTDI is recommended)
I personally recommend installing a 3.5mm jack and using a USB-TTL-AJ cable, as you only have to open the case once to install, and never again to access the serial header.