Spent the afternoon doing some more analysis. Good news I can make sense of everything except the steering commands. Bad news is, it’s complicated and most of it won’t work with AOG at all. The only useful thing is the steering angle sensor. That data comes over the bus all the time. I think it has to be calibrated, though. I am not sure how that’s done but I recall it was done in the diagnostic display once. Even when the brown box monitor is disconnected from the pillar, I can still see the steering angle information. So I imagine that will always be there on a GreenStar ready machine, but could be wrong. PGN is 65535, the first data payload byte is 0x76, and the steer angle is a signed byte in position 2 of those messages. positive is to the left, negative to the right. Range is about +/- 65. Not sure if this is degrees or not.
The autosteer button on the armrest is way more complex. Short version if the tractor can not interact with a monitor with autotrac, the button does nothing. There’s a complex interaction between the monitor, the SSU computer, and the armrest. When you tell the monitor to enable the system, it broadcasts a message to the tractor. If everything is good on the tractor end, the tractor sets some flags in a message that’s broadcast back to the monitor. At that point, the button on the armrest will cause a flag to be set in that same broadcast. The monitor then does its thing and eventually the tractor’s broadcast reflects that everything is good. If you move the steering wheel or put it in neutral, the tractor sets a flag that the monitor sees and then stops steering. Simple, eh.
With the possible exception of the steering angle sensor, you’re better off connecting directly to the steering wheel encode and the autosteer button in the armrest.