Sany Excavator Fault Code P377: Complete Diagnostic Guide

What is Sany General Fault Code P377?

Fault Code P377 indicates a Hydraulic Pump Pressure Sensor Circuit Low Voltage condition in Sany excavators. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects voltage readings below the manufacturer's specified threshold (typically under 0.5V) from the main hydraulic pump pressure sensor circuit.

This code is critical for Sany excavators because the hydraulic pressure sensor provides real-time feedback to the ECM for optimal pump output regulation. When voltage drops abnormally low, the system cannot accurately monitor hydraulic pressure, potentially leading to inefficient operation, component damage, or complete system shutdown to prevent catastrophic failure.

Common Symptoms

When Code P377 is active on your Sany excavator, you may experience:

• Warning light illumination on the instrument cluster (typically amber or red hydraulic system indicator)
• Reduced hydraulic power or sluggish boom, arm, and bucket responses
• Engine derate mode where maximum RPM is limited to protect the hydraulic system
• Intermittent loss of hydraulic functions, especially during cold starts or low-temperature operation
• Error messages on the digital display panel indicating "Hydraulic System Fault" or similar warnings

Potential Causes

The most common technical causes for P377 in used Sany excavators include:

• Damaged or corroded wiring harness between the pressure sensor and ECM (common rub points near hydraulic pump mounting brackets)
• Failed hydraulic pump pressure sensor due to contamination, internal electrical failure, or physical damage
• Poor ground connections at sensor mounting points or ECM ground terminals
• Contaminated or damaged electrical connectors exposed to hydraulic fluid leaks or environmental moisture
• ECM internal fault or corrupted software (less common but possible in high-hour machines)
• Broken or chafed sensor supply voltage wire causing open circuit condition

How to Troubleshoot and Fix Code P377

Step 1: Visual Inspection Begin by locating the hydraulic pump pressure sensor (typically mounted on the main hydraulic pump body). Inspect the wiring harness from sensor to ECM for visible damage, rubbing against metal surfaces, oil contamination, or rodent damage. On used excavators, check common wear points where harnesses cross the rotating frame or contact sharp edges.

Step 2: Electrical Testing Using a digital multimeter (DMM), disconnect the sensor connector and measure supply voltage at the harness side with ignition ON (should read 4.5-5.2V). Check ground continuity (should be less than 1 ohm resistance to battery negative). Inspect connector pins for corrosion, bent terminals, or hydraulic oil intrusion—common issues in older machines.

Step 3: Sensor Testing Measure sensor internal resistance across its terminals (typically 200-3000 ohms depending on model—consult Sany service manual). Connect Sany diagnostic software (PC Suite or equivalent) to monitor live sensor voltage while manually creating hydraulic pressure. Voltage should increase proportionally with pressure (0.5-4.5V range).

Step 4: Repair and Verification Replace faulty components identified during testing. For used excavators, always clean connector terminals with electrical contact cleaner and apply dielectric grease before reconnection. Clear codes using diagnostic software and perform operational testing under load. Monitor for code recurrence over 50+ operating hours.

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Disclaimer: This guide provides general diagnostic information. Always consult your Sany service manual and consider professional assistance for complex electrical diagnostics, especially when warranty coverage or safety systems are involved.