What is Sany Excavator Fault Code P979?

Fault Code P979 indicates an abnormal voltage or communication error in the main hydraulic pump control system, specifically related to the pump solenoid proportional valve circuit. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects voltage readings outside the manufacturer's specified range (typically 0.5-4.5V) or an open/short circuit condition in the pump control wiring harness.

This code is critical for Sany excavators because the proportional solenoid valve directly regulates hydraulic flow and pressure to the main pump. When this system malfunctions, the machine cannot properly modulate hydraulic power, leading to reduced operational efficiency, erratic boom/arm movements, or complete hydraulic system failure. For used excavators, this fault often stems from age-related electrical degradation rather than mechanical pump failure.

Common Symptoms

• Erratic hydraulic response with jerky or inconsistent boom, stick, or bucket movements
• Reduced engine power or automatic engine derate mode to protect the hydraulic system
• Hydraulic pump whining or unusual noise due to improper pressure regulation
• Warning light illumination on the instrument cluster, often accompanied by a check engine indicator
• Intermittent loss of hydraulic functions, particularly during cold starts or after extended operation

Potential Causes

The most common technical reasons for P979 on used Sany excavators include:

• Damaged or corroded wiring harness at known rub points near the pump mounting bracket or along the main boom chassis rail
• Faulty proportional solenoid valve with internal coil resistance outside specification (typically 8-12 ohms)
• Corroded or loose electrical connectors at the pump solenoid or ECM interface, especially in machines exposed to wet conditions
• Failed ECM output driver circuit for the pump control channel
• Voltage supply issues from the main power relay or battery connections causing insufficient solenoid actuation voltage

How to Troubleshoot and Fix Code P979

Step 1: Visual Inspection Begin by inspecting the wiring harness from the ECM to the main hydraulic pump solenoid valve. Look for abraded insulation, pinched wires, or corroded connectors—common issues on used excavators with 3,000+ operating hours. Pay special attention to harness routing near moving components.

Step 2: Electrical Testing Using a digital multimeter, disconnect the solenoid connector and measure coil resistance (should read 8-12 ohms typically). Then check for supply voltage at the connector with ignition on (should show battery voltage). Test for short to ground on both control wires with the harness disconnected from the ECM.

Step 3: Advanced Diagnostics Connect Sany diagnostic software (SY Excavator PC Service Tool) to monitor real-time solenoid command values versus actual feedback. Compare commanded current (mA) to actual response. If electrical tests pass but the code persists, the proportional valve itself may have internal contamination or wear requiring replacement.

Step 4: Repair and Verification Clean all connectors with electrical contact cleaner, repair any damaged wiring with proper gauge wire and heat-shrink connections, and replace the solenoid valve if resistance or function tests fail. Clear codes and perform a full operational cycle test under load.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Sany service manual for your specific model and consider professional diagnostic services for complex electrical issues, especially when working with high-pressure hydraulic systems.