What is Sumitomo SH350HD-6 Fault Code 638?

Fault Code 638 on the Sumitomo SH350HD-6 excavator indicates a malfunction in the hydraulic pump control system, specifically related to abnormal pump displacement sensor signal or pump regulator control circuit failure. This diagnostic trouble code (DTC) is generated when the Electronic Control Module (ECM) detects voltage readings outside the normal operating range from the pump displacement sensor or identifies a communication error with the pump's proportional solenoid valve.

This code is critical for the SH350HD-6 because the hydraulic pump is the heart of all excavator functions. The variable displacement pump adjusts flow based on operational demand, and when the ECM cannot properly monitor or control pump displacement, it directly impacts digging force, swing speed, and overall hydraulic performance. In used machines, this fault often stems from sensor degradation or electrical connection issues rather than catastrophic pump failure.

Common Symptoms

• Reduced hydraulic power across all functions, with noticeably slower boom, arm, and bucket movements
• Erratic or sluggish swing performance that doesn't respond proportionally to joystick input
• Warning light illumination on the instrument panel, typically the hydraulic system alert or check engine light
• Intermittent loss of hydraulic pressure during operation, especially when multiple functions are used simultaneously
• Engine may run normally but machine feels "weak" or unresponsive during heavy digging operations

Potential Causes

The most common technical reasons for Code 638 on used SH350HD-6 excavators include:

• Failed or drifting pump displacement sensor due to internal wear, heat exposure, or oil contamination affecting sensor accuracy
• Damaged wiring harness near the main pump area where cables experience vibration and rubbing against the pump housing or frame
• Corroded or loose electrical connectors at the pump control solenoid or sensor plugs, especially common in machines operating in wet or corrosive environments
• Faulty pump regulator solenoid valve with internal coil resistance outside specifications or mechanical sticking
• ECM power supply issues providing inconsistent voltage to the sensor circuit
• Contaminated hydraulic oil affecting sensor operation or creating internal pump control valve sticking

How to Troubleshoot and Fix Code 638

Step 1: Visual Inspection and Connector Check Begin by inspecting the pump displacement sensor and regulator solenoid connectors located on the main hydraulic pump. Check for oil contamination, corrosion, bent pins, or moisture inside connector bodies. On used excavators, these connectors often suffer from environmental damage. Clean connectors with electrical contact cleaner and apply dielectric grease before reconnecting.

Step 2: Electrical Testing with Multimeter Using a digital multimeter, check the pump displacement sensor supply voltage (should be approximately 5 volts DC from the ECM). Test sensor output signal voltage, which typically ranges between 0.5-4.5 volts depending on pump angle. Measure solenoid coil resistance, which should be within 10-25 ohms (consult service manual for exact specifications). Any readings outside these parameters indicate component failure.

Step 3: Harness Continuity and Ground Testing Inspect the wiring harness from the pump back to the ECM for chafing, especially where it passes near the pump mounting bracket—a known wear point on the SH350HD-6. Test for continuity on all wires and verify solid ground connections at both the sensor and ECM. Repair or replace damaged sections with proper gauge wire and protective loom.

Step 4: Advanced Diagnostics with Service Software Connect Sumitomo diagnostic software or compatible scan tool to access live sensor data. Monitor pump displacement sensor values during operation to identify intermittent failures or signal dropout. Compare commanded versus actual pump displacement to pinpoint whether the issue is sensor feedback or solenoid control.

Step 5: Component Replacement If testing confirms sensor or solenoid failure, replace the faulty component with OEM or quality aftermarket parts. For used machines, consider replacing both the sensor and solenoid simultaneously if they're original equipment with high hours, as failure of one often indicates the other is near end-of-life.

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Disclaimer: This guide provides general troubleshooting information for Code 638. Hydraulic system repairs involve high-pressure components and electrical systems requiring specialized knowledge. Always consult the official Sumitomo service manual and consider professional diagnostic assistance for complex repairs, especially on used equipment where multiple underlying issues may exist.