What is likely the issue if a Hydro runs up without load but not with a full load?

When a hydraulic system runs without load but struggles to operate under full load, the most plausible issue is that the relief valve is set too low. The relief valve plays a critical role in regulating the pressure within the hydraulic system. If it is set to release pressure at a lower threshold than what is required for the system to operate under load, the system will not be able to generate the necessary force to support that load. This can lead to the hydraulic system stalling or failing to operate efficiently when the full load is applied, as it cannot build enough pressure to overcome the load’s resistance.

It is crucial for the relief valve to be properly calibrated to the system's specifications to ensure that the hydraulic system has adequate pressure when needed, especially under load. When the relief valve is functioning correctly, it helps maintain consistent pressure for the hydraulic operations, enabling the system to effectively handle varying loads.

Other potential issues, such as a defective motor, a wiring fault, or an excessively heavy load, may contribute to performance problems, but they do not specifically explain why the system can run without load but fails under load in the same manner as an incorrectly set relief valve would. Each of those scenarios indicates different system failures or operational limits, rather than a direct issue with