What is Komatsu PC120-6 Fault Code S-15?

Fault Code S-15 on the Komatsu PC120-6 excavator indicates a hydraulic pump solenoid valve circuit malfunction, specifically detecting an abnormal electrical signal or open/short circuit in the pump control system. This code is generated when the machine's electronic control unit (ECU) detects voltage irregularities in the solenoid that regulates hydraulic pump output.

The hydraulic pump solenoid valve plays a critical role in optimizing fuel efficiency and hydraulic performance by adjusting pump displacement based on operational demand. When S-15 triggers, the system typically defaults to a fixed displacement mode, reducing machine efficiency and potentially causing overheating during extended operation. For the PC120-6's aging electrical architecture, this fault often stems from deteriorated wiring harnesses rather than solenoid failure itself.

Common Symptoms

• Reduced hydraulic responsiveness across all functions, with noticeably slower boom and arm movements
• Intermittent or continuous monitor panel warning light (typically a wrench or diagnostic symbol)
• Increased engine RPM needed to achieve normal work speeds, resulting in higher fuel consumption
• Machine operates in "limp mode" with fixed pump displacement, eliminating power-saving Auto-Idle function
• Occasional stalling when switching between idle and work modes under load

Potential Causes

The S-15 code on used PC120-6 excavators typically results from age-related electrical deterioration:

• Corroded or loose connector pins at the pump solenoid valve connector (most common on machines with 8,000+ hours)
• Chafed wiring harness near the hydraulic pump mounting area where vibration causes insulation wear
• Failed solenoid coil within the pump assembly (internal short or open circuit)
• ECU pin corrosion at the main controller connector, affecting signal integrity
• Damaged ground wire connections between pump body and frame causing voltage reference errors

How to Troubleshoot and Fix Code S-15

Step 1: Visual Inspection Access the hydraulic pump area and thoroughly inspect the solenoid valve connector for corrosion, oil contamination, or bent pins. On used excavators, clean connectors with electrical contact cleaner and examine the wiring harness routing for rub-through points against pump housing or frame edges.

Step 2: Electrical Testing Using a multimeter, check solenoid coil resistance at the connector (typically 18-22 ohms for PC120-6 spec). Disconnect the harness and measure resistance to ground on both wires—infinite resistance indicates proper insulation; low resistance suggests a short circuit requiring harness repair.

Step 3: Voltage Supply Check With ignition on (engine off), verify 12V power supply reaches the solenoid connector from the ECU. Check ground circuit continuity (should be less than 1 ohm resistance). For used machines, test voltage under cranking conditions as poor battery connections create voltage drops triggering false codes.

Step 4: Component Replacement If electrical tests confirm solenoid failure, replace the pump control solenoid valve (Komatsu part #702-21-57500). On older excavators, simultaneously replace the mating connector and 12-18 inches of adjacent harness to prevent recurring failures.

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This guide provides general troubleshooting information. Always consult Komatsu service manuals and qualified diesel technicians for definitive diagnostics. Improper electrical testing can damage sensitive ECU components.