Looking at the schematic heres how I would suggest testing the grounds.
The ecm controls these relays via ground. When the key is turn on the ECM ground the main relay to give power to the fuel pump relay and to send power to the injectors. The fuel pump relay is activated via the ecm supplying ground when it sees input from the oil pressure switch/crank sensor.
Testing wires for short to ground;
I would suggest testing for continuity between the wire at the relay and its corresponding pin at the ecm connector there should be continuity. If that checks there is no short to ground in the wiring harness and you need to check for ground on that terminal with both the key off and key on.
Testing main relay;
The main relay should only have ground when the key is on. Using ohm meter connect one probe to ground and the other probe to blue wire (pin 5) at the main relay connector. It should have no continuity to ground with the key off. If it pulses between continuity/no continuity the ecm is bad.
Testing fuel pump relay;
Ground should only be present on the white/green wire (pin 5) on the fuel pump relay for 3 to 5 seconds when key is first turned on, then constant when engine is cranking. If wire has ground at any time with key off and wiring has been confirmed to not be shorted to ground as in our first test the ecm is again suspect.
If I can assist further, please, let me know.