Komatsu PC850-8R Fault Code S-8: Complete Diagnostic Guide

What is Komatsu PC850-8R Fault Code S-8?

Fault Code S-8 on the Komatsu PC850-8R indicates a malfunction in the engine oil pressure sensor circuit or abnormally low engine oil pressure detected by the Engine Control Module (ECM). This code specifically monitors the oil pressure signal voltage and compares it against predetermined thresholds programmed into the Komatsu ECM.

This fault is critical for the PC850-8R's SAA6D170E-5 engine because inadequate oil pressure can lead to catastrophic bearing failure, turbocharger damage, and complete engine seizure. The ECM uses this sensor data to protect the engine by triggering warning systems and potentially limiting machine performance when oil pressure falls outside safe operating parameters.

Common Symptoms

When Code S-8 is active, operators typically experience:

• Engine oil pressure warning light illuminated on the instrument cluster (red oil can symbol)
• Audible alarm sounding continuously in the cab
• Engine derate mode activated, limiting RPM to approximately 1,200-1,500 to prevent damage
• Potential automatic engine shutdown if pressure drops critically low during operation
• Erratic oil pressure gauge readings or gauge stuck at zero despite engine running

Potential Causes

The most common technical causes for Code S-8 on used PC850-8R excavators include:

• Oil pressure sensor failure due to internal diaphragm deterioration (common after 8,000+ hours)
• Wiring harness damage at the sensor connector, particularly where the harness routes near the engine block and experiences heat cycling
• Low engine oil level or use of incorrect viscosity grade oil
• Oil pump wear or failure, especially in high-hour machines (10,000+ hours)
• Clogged oil filter or failed oil filter bypass valve
• ECM connector corrosion at pins related to oil pressure signal (typically pin 47 on the main harness)
• Worn main or rod bearings causing actual low oil pressure conditions

How to Troubleshoot and Fix Code S-8

Step 1: Verify Actual Oil Pressure Install a mechanical oil pressure gauge directly into the engine block test port. Run the engine at operating temperature and compare readings against Komatsu specifications (typically 284-441 kPa at rated RPM). This confirms whether the issue is sensor-related or actual low pressure.

Step 2: Inspect Sensor and Wiring Disconnect the oil pressure sensor connector (located on the left side of the engine block). Inspect for corrosion, oil contamination, or bent pins. Use a multimeter to check sensor resistance (typically 50-150 ohms at room temperature). Check harness continuity from sensor to ECM and verify no shorts to ground.

Step 3: Test Sensor Signal Voltage With ignition on (engine off), backprobe the sensor signal wire. You should read approximately 5V reference voltage from the ECM. During cranking with mechanical gauge installed, verify the sensor output voltage changes proportionally with actual pressure (typically 0.5-4.5V range).

Step 4: Address Root Cause For used excavators, thoroughly inspect harness routing for rub points against engine components. Check oil level, condition, and change the oil filter. If mechanical pressure is low, investigate oil pump condition and bearing clearances before replacing electronic components.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and seek qualified diesel technician assistance for complex repairs involving engine internals or ECM programming.