Komatsu D85EX-15 Fault Code S-5: Complete Diagnostic Guide

What is Komatsu D85EX-15 Fault Code S-5?

Fault Code S-5 on the Komatsu D85EX-15 bulldozer indicates a Steering System Pressure Sensor malfunction or abnormal hydraulic pressure reading in the hydrostatic steering circuit. This code is generated when the Electronic Control Module (ECM) detects voltage signals outside the normal operating range (typically 0.5-4.5V) from the steering pressure transducer, or when actual hydraulic pressure readings deviate significantly from expected values during operation.

This fault is critical for the D85EX-15 because this model utilizes a hydrostatic steering system (HSS) that relies on precise pressure monitoring for safe operation. The pressure sensor provides real-time feedback to the machine controller, ensuring adequate steering response and preventing hydraulic system damage. Ignoring this code can lead to steering assist failure, creating serious safety hazards on job sites.

Common Symptoms

When Code S-5 is active on your D85EX-15, operators typically experience:

• Intermittent or complete loss of power steering assist, requiring excessive force on the steering controls
• Illuminated warning light on the instrument cluster, often accompanied by an audible alarm
• Erratic steering response or unexpected resistance during turns, especially under load
• Reduced machine performance as the ECM may limit travel speed when steering system faults are detected
• Stored fault codes visible through the monitor panel or diagnostic software, even if symptoms are intermittent

Potential Causes

The most common technical reasons for S-5 codes on used D85EX-15 machines include:

• Failed steering pressure sensor/transducer due to internal component degradation or contaminated hydraulic fluid
• Damaged wiring harness between the pressure sensor and ECM, particularly at flex points near the steering orbital or blade lift cylinders where rubbing occurs
• Corroded or loose electrical connectors at the sensor plug—a frequent issue on machines operating in wet or corrosive environments
• Low hydraulic fluid level or contaminated oil causing actual pressure fluctuations that trigger sensor alerts
• Faulty ECM ground connections creating voltage reference errors
• Hydraulic pump wear in high-hour machines reducing actual steering circuit pressure below threshold values

How to Troubleshoot and Fix Code S-5

Step 1: Visual Inspection and Fluid Check Begin by checking hydraulic oil level in the HSS reservoir and inspect for contamination. On used excavators, examine the wiring harness from the steering pressure sensor (typically located on the steering control valve or orbital motor) to the ECM for chafing, cuts, or damaged insulation at known rub points near the blade pivot.

Step 2: Electrical Testing Using a digital multimeter, backprobe the pressure sensor connector with the key on, engine off. Check for 5V reference voltage from the ECM and verify sensor ground continuity (should be less than 1 ohm to chassis ground). Measure sensor output voltage at idle—it should read approximately 0.8-1.2V at rest. Disconnect the sensor and measure resistance across sensor terminals (typical specification: 250-350 ohms at 68°F).

Step 3: Live Pressure Testing Connect Komatsu diagnostic software (KOMTRAX or compatible scan tool) and monitor live steering pressure data while operating controls. Compare actual readings with sensor voltage output. If physical pressure (measured with a hydraulic pressure gauge installed at a test port) differs significantly from sensor readings, replace the pressure transducer.

Step 4: Component Replacement and Verification For used machinery, clean all connector pins with electrical contact cleaner and apply dielectric grease before reconnecting. If sensor replacement is needed, always install a genuine Komatsu pressure sensor (Part No. 7861-93-1650 or equivalent) and recalibrate using diagnostic software. Clear codes, test operation, and verify repair success.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu D85EX-15 service manual and consider professional diagnostic assistance for complex hydraulic or electrical issues. Working with pressurized hydraulic systems requires proper safety equipment and training.