purple wire to starter solenoid- whats it for??

Doesn anyone know what the PURPLE wire to the battery side of the solenoid is for????

I along with some others have relocated the starter solenoid to the inner fender and by accident hooked this dark purple wire (thinking it was blacK) to ground..... this gave us a no start (no fuel pump, no spark, no power to eec) but the car cranked over...... Once the purple wire was moved to the battery side of the starter solenoid... car fired right up but we now have a check engine light on-

any ideas?

thanks