What is Komatsu PC220-8 Fault Code S-5?

Fault Code S-5 on the Komatsu PC220-8 excavator indicates a malfunction in the main relief valve solenoid circuit or abnormal hydraulic system pressure. This code is triggered when the machine's Electronic Control Module (ECM) detects irregular electrical signals from the main relief valve solenoid or pressure sensor, or when hydraulic system pressure exceeds or falls below specified parameters.

The PC220-8's hydraulic system relies on precise pressure regulation to maintain optimal digging force, swing speed, and overall machine performance. The main relief valve protects the hydraulic circuit from overpressure conditions while the solenoid controls pressure adjustments based on operating conditions. When fault code S-5 activates, it signals that hydraulic pressure control has been compromised, which can lead to reduced efficiency, component damage, or complete hydraulic failure if left unaddressed.

Common Symptoms

• Hydraulic power loss during high-demand operations like digging or lifting heavy loads
• Erratic boom or arm movement with inconsistent speed or pressure
• Warning alarm sounding with the monitor display showing fault code S-5
• Machine entering derate mode, limiting engine RPM or hydraulic output to protect components
• Overheating hydraulic oil due to pressure spikes or continuous relief valve operation

Potential Causes

The most common technical causes for S-5 on used PC220-8 excavators include:

• Failed main relief valve solenoid due to coil burnout, internal contamination, or mechanical wear from extended operating hours
• Damaged wiring harness at known friction points near the pump assembly where harness routing contacts the frame during machine operation
• Corroded or loose electrical connectors at the solenoid or ECM, particularly common in machines exposed to harsh environments
• Faulty main pressure sensor providing incorrect readings to the ECM
• Contaminated hydraulic oil causing the relief valve spool to stick in open or closed position
• ECM software glitch or internal ECM failure (less common but possible in high-hour used machines)

How to Troubleshoot and Fix Code S-5

Step 1: Visual Inspection and Initial Checks

Connect Komatsu diagnostic software (KOMTRAX Plus or compatible scan tool) to retrieve active and stored codes. Record all parameters including hydraulic pressure readings. Perform a thorough visual inspection of the main relief valve solenoid mounted on the main control valve assembly. On used excavators, check for oil contamination around the solenoid indicating internal seal failure. Inspect the wiring harness from the solenoid to the ECM, paying special attention to routing near the hydraulic pump where harness chafing against metal edges is common on PC220-8 models.

Step 2: Electrical Circuit Testing

Disconnect the main relief valve solenoid connector and measure resistance across the solenoid terminals using a digital multimeter. Specifications typically range between 18-22 ohms at 20°C (consult service manual for exact values). Check for voltage supply at the harness connector with ignition on—you should see battery voltage (approximately 24V on PC220-8). Test for shorts to ground by measuring resistance between each pin and machine ground (should read infinite resistance). Inspect connector pins for corrosion, bent terminals, or moisture intrusion, which are frequent issues in older machines.

Step 3: Hydraulic System and Component Testing

If electrical tests pass, verify hydraulic system pressure using a calibrated pressure gauge at the test port. Compare readings against manufacturer specifications (typically 34.3 MPa/4,970 psi for main relief). Test the main pressure sensor by measuring its output voltage while manually adjusting pressure if possible. For used excavators with high operating hours, remove and inspect the relief valve cartridge for wear, scoring, or contamination that prevents proper seating. Replace hydraulic oil and filters if contamination is evident, as degraded oil accelerates solenoid and valve failures.

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Disclaimer: This guide provides general troubleshooting information for Komatsu PC220-8 fault code S-5. Always consult the official Komatsu service manual for your specific machine serial number. Complex hydraulic and electrical diagnostics should be performed by certified Komatsu technicians with proper diagnostic equipment and training.