Thanks for the input on the SCSSI. After some trials and some bricks I managed to make the eside talk while the engine is cranking.
First I tried to reenable comms at oc4i. It didn`t worked as expected. I ended oraa $20 to the SCCR2, which make the comms worked on a very rare random basis.
I was tired of experiments and skipped the scssi disable code in the irq. This made the eside talk. It is not clear what will happen on a higher rpms, so I think to put a rpm threshold and reenable the comms at oc4i after the rpms drop. I will also cut all aldl chatter on the bus. I suspect the interrupt is activated on any aldl bus activity.

I wish I have an opti simulator for testing. How hard will be to make one like yours with a knob to regulate rpms.


The plan for cracking the tpu from the backdoor is solid. You can chart the unused pins ram locations and possible found more things left on the table from factory.
Tpu and eside exchange data each run of the irq. Eside calculates cyl fire id and sends it back to the tpu. I also found some code used for loss of sync[cyl ID] situations and resync on the fly. Really good job from GM.