What is Sany Excavator Fault Code P790?

Fault Code P790 on Sany excavators indicates a malfunction in the hydraulic pump solenoid valve control circuit, specifically detecting an abnormal current or voltage signal from the main pump's proportional control system. This diagnostic trouble code (DTC) is generated when the Electronic Control Module (ECM) detects that the electrical signal to the hydraulic pump's proportional solenoid valve is outside the manufacturer's specified parameters, typically indicating an open circuit, short circuit, or excessive resistance.

This code is critical for Sany excavators because the proportional solenoid valve directly controls hydraulic flow and pressure distribution to the main pump. When this system fails, the machine cannot properly regulate hydraulic output, severely compromising digging force, swing speed, and overall operational efficiency. In used equipment, this fault often results from electrical degradation rather than mechanical pump failure.

Common Symptoms

• Reduced hydraulic power across all functions, with noticeably slower cycle times during digging, lifting, or swing operations
• Erratic hydraulic response where the machine operates normally intermittently, then suddenly loses power or becomes sluggish
• Warning light activation on the instrument cluster, often accompanied by an audible alarm
• Engine derating or automatic throttle reduction as the ECM enters a protective mode
• Complete loss of hydraulic function in severe cases, though the engine continues running normally

Potential Causes

Electrical connector corrosion at the pump solenoid valve harness—extremely common in used excavators exposed to moisture, mud, and debris infiltration at the pump mounting location.

Damaged wiring harness due to vibration-induced chafing, particularly where the harness routes near the pump housing or swing bearing, creating intermittent shorts or open circuits.

Failed proportional solenoid valve internally, with coil resistance outside the 8-15 ohm specification range typical for Sany systems.

ECM power supply issues, including corroded grounds or voltage drops in the machine's main electrical system affecting solenoid driver circuits.

Previous repair attempts using incorrect voltage specifications or non-OEM replacement solenoids incompatible with the control system.

How to Troubleshoot and Fix Code P790

Step 1: Visual Inspection and Connector Assessment Begin by locating the main hydraulic pump and tracing the electrical harness to the proportional solenoid valve connector. Disconnect the connector and carefully inspect both the male and female terminals for corrosion, bent pins, moisture intrusion, or discoloration. On used excavators, this connector frequently accumulates conductive debris. Clean contacts with electrical contact cleaner and examine the harness routing for signs of rubbing against metal surfaces—a notorious wear point on Sany models.

Step 2: Electrical Testing with Multimeter Using a quality digital multimeter, measure the coil resistance directly at the solenoid valve with the harness disconnected. Typical specification is 8-15 ohms; readings outside this range indicate solenoid failure. Next, check for supply voltage at the harness connector with the ignition on—you should see battery voltage (approximately 12-14V). Test for continuity between the ECM connector and the solenoid harness to identify breaks in the wiring.

Step 3: Advanced Diagnostics and Replacement Connect Sany-specific diagnostic software (or compatible scan tool) to monitor real-time solenoid command signals from the ECM. Compare commanded current versus actual current draw. If electrical tests pass but the code persists, suspect ECM driver circuit failure—common in high-hour used machines. Before replacing expensive components, perform a ground circuit verification at the pump mounting bolts, as poor grounding creates phantom electrical faults. Replace components only after confirming the exact failure point to avoid unnecessary parts costs.

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Disclaimer: This guide provides general troubleshooting information for educational purposes. Hydraulic and electrical systems operate under high pressure and voltage. Always consult your Sany service manual, and consider engaging a certified heavy equipment technician for complex diagnostics and repairs, especially on used machinery with unknown service history.