There's no difference in function between SGND and GND. They are both grounds, one to the frame, one to the engine block.
Both cause the TEN pin back at the ECU to drop from, floating high at, 12v to ground. This tells the ECU to lock the timing at 10° and to idle at 850RPM. If not, you need to adjust the CAS and/or Throttle speed screw to make this a reality.
I'll list all the changes and wires in each model year's Diag. port, but I don't have the pinouts.