What is Komatsu PC110-7 Fault Code S-8?

Komatsu PC110-7 Fault Code S-8 indicates a hydraulic system malfunction, specifically related to the main relief valve circuit or pump delivery pressure abnormality. This diagnostic trouble code (DTC) signals that the excavator's electronic control module (ECM) has detected pressure readings outside the normal operating parameters, typically during high-demand hydraulic operations.

In the PC110-7 model, this code directly affects the machine's hydraulic efficiency and can lead to reduced digging force, slow cycle times, and potential component damage if left unaddressed. The hydraulic pump, main relief valve, and associated pressure sensors work together to maintain optimal system pressure between 315-345 bar (4,570-5,000 psi). When S-8 triggers, it indicates this critical balance has been compromised, affecting the machine's ability to perform under load.

Common Symptoms

When Fault Code S-8 is active on your Komatsu PC110-7, operators typically experience:

• Reduced hydraulic power during digging, lifting, or swing operations, with noticeably slower cycle times
• Intermittent or constant warning light on the monitor panel, often accompanied by an audible alarm
• Hydraulic oil overheating due to excessive pressure bypass through the relief valve
• Erratic boom or arm movements, especially when multiple functions are operated simultaneously
• Engine power feeling normal, but hydraulic responsiveness significantly diminished

Potential Causes

The S-8 code on used PC110-7 excavators commonly stems from these technical failures:

• Main relief valve sticking, worn, or improperly adjusted (extremely common on machines with 6,000+ hours)
• Hydraulic pressure sensor failure or providing inaccurate readings to the ECM
• Worn hydraulic pump unable to maintain specified delivery pressure under load
• Contaminated hydraulic oil causing valve spool sticking or sensor malfunction
• Damaged wiring harness or corroded connectors between the pressure sensor and ECM (check near the pump mounting area where vibration causes wear)
• ECM calibration drift or internal fault in older control modules

How to Troubleshoot and Fix Code S-8

Step 1: Connect Diagnostic Equipment Use Komatsu KOMTRAX software or a compatible diagnostic laptop with the PC110-7 interface cable. Read live data while operating the machine to observe actual pressure readings versus commanded values. Check if the pressure sensor shows realistic values (should read 0 bar at idle, rising proportionally under load).

Step 2: Inspect Pressure Sensor and Wiring Locate the main hydraulic pressure sensor on the pump delivery line. On used excavators, carefully inspect the connector for corrosion, oil contamination, or pushed-back pins. Use a digital multimeter to test sensor output: it should read approximately 0.5V at zero pressure and increase linearly to 4.5V at maximum pressure. Check harness continuity from sensor to ECM connector (typical resistance should be less than 5 ohms).

Step 3: Test Main Relief Valve With the engine running at high idle, operate the machine under full load. Install a calibrated hydraulic pressure gauge (0-6,000 psi range) at the test port near the main control valve. Pressure should reach 315-345 bar during relief. If pressure is consistently low, remove and inspect the main relief valve for wear, scoring, or debris. For used machines, this valve often requires replacement after 8,000 hours.

Step 4: Verify Hydraulic Oil Condition Sample the hydraulic oil and check for contamination, proper viscosity (ISO VG 46), and metal particles. On older PC110-7s, degraded oil frequently causes valve sticking. Replace filters and flush the system if contamination is evident.

Step 5: Check Pump Delivery If all sensors and valves test correctly, perform a pump flow test using a hydraulic flow meter. The PC110-7's pump should deliver approximately 123 L/min at rated RPM. Significant deviation indicates internal pump wear requiring rebuild or replacement.

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Disclaimer: This guide provides general diagnostic information for Komatsu PC110-7 Fault Code S-8. Hydraulic system repairs involve high pressures and specialized tools. Always consult the official Komatsu service manual and consider professional technician assistance for complex repairs, especially on used equipment where multiple wear-related issues may exist simultaneously.