elektro,
as far as i can remember (on the EPSII) systems
the only communication the steering controller had with the drive controller was more or less a 'go no-go' signal and there may have been a setting to set the truck to either manual steer or wire guided steering, if all was ok and no errors were detected in the steering unit the truck would run fine, if there were errors generated from the EPS then it would shut the truck down. I think (if i remember correctly)all the steering settings were done on the EPS controller itself using trim-pots.

Now as for this system on that truck you have, it MAY be like this or it may be different.
If it is zapi then I'm sure it is different.
having the service manual for the truck and the EPS steering would be a great help. from my recollection Yale was always good about having ample information in those manuals, and if there were any discrepancies the engineers at nacco were always more than willing to help.