So, under normal conditions with auto/man switch in AUTO, and with AEC activated, your issue is the engine won't go above idle right?
Then when you switch it to MAN, and the A appears on the screen, you can bump the idle up/down using the back up toggle switch next to it right?
If that's the case, yes a bad engine speed sensor could cause this, but so could any other fault with the electronic control really.
On backupSw1 and backupsw2 on the drawling should one of those have power coming out of the controller or are they both just inputs?
Yes they both look like inputs, they would tell the electronic control to forget about the engine speed dial and AEC altogether. I'd have a good look over the machine visually first and look for any rubbed, melted wiring, etc
Just my thoughts.
P.S. Type your replies under the quote when quoting someone.