The ELC (electronic level control) system is not designed to provide varying levels of suspension stiffness. It is designed to keep the car level when there is a load in the back seat or the trunk. The pump comes on when you start the car to re-fill whatever air leaked out while the car was off, but if the car is level the ELC is doing its job. With no extra weight in the car, if the butt end is raised to high you can adjust the level control arm. If the car is riding at normal height your issue with harsh ride is related to the shock you selected, not the ELC system.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)