What is Komatsu PC120-6 Fault Code S-5?

Fault Code S-5 on the Komatsu PC120-6 excavator indicates a hydraulic pump controller system malfunction, specifically related to the pump flow control solenoid or its associated circuitry. This code appears when the electronic control unit (ECU) detects abnormal voltage, current flow, or response timing from the pump control solenoid valve, which regulates hydraulic oil flow based on operational demand.

The PC120-6 uses a sophisticated load-sensing hydraulic system where the pump controller continuously adjusts output to match work requirements. When S-5 triggers, the system typically reverts to a fixed displacement mode, significantly reducing machine efficiency and potentially causing overheating during prolonged operation. This is particularly critical for used machines where degraded components may accelerate secondary damage.

Common Symptoms

• Reduced hydraulic power across all functions—boom, arm, and bucket movements become noticeably sluggish
• Increased engine RPM without corresponding hydraulic output as the pump cannot modulate flow properly
• Hydraulic oil overheating indicated by the temperature warning light, especially during continuous operation
• Error code display on the monitor panel with possible intermittent beeping alarm
• Hard starting or stalling when hydraulic functions are engaged under load

Potential Causes

The most frequent causes for S-5 on used PC120-6 excavators include:

• Pump control solenoid valve failure due to internal coil degradation or mechanical contamination blocking the spool
• Wiring harness damage at the rub point near the main pump mounting bracket—a known wear location on this model
• Corroded or loose connector pins at the 6-pin pump controller connector, especially in machines operated in wet conditions
• ECU internal fault affecting the solenoid driver circuit, though less common
• Contaminated hydraulic oil causing solenoid valve sticking or erratic response
• Low battery voltage (below 22V on 24V systems) creating insufficient solenoid actuation

How to Troubleshoot and Fix Code S-5

Step 1: Visual Inspection
Inspect the pump control solenoid connector and wiring harness from the ECU to the main pump. Look specifically for abraded insulation where the harness crosses the pump mounting bracket. Check connector pins for corrosion or bent contacts—clean with electrical contact cleaner if necessary.

Step 2: Electrical Testing
Using a digital multimeter, check solenoid resistance at the pump-side connector (disconnect first). Specification is 15-25 ohms at 20°C. Check supply voltage from the ECU with ignition on—should read 24V ±2V. Test harness continuity between ECU pins and solenoid connector.

Step 3: Solenoid Function Test
Connect Komatsu diagnostic software (KOMTRAX PC or compatible scanner) to read real-time solenoid command values versus actual response. Command a flow change—the solenoid should respond within 200 milliseconds. If electrical tests pass but response is sluggish, remove and inspect the solenoid valve for contamination or mechanical binding.

Step 4: Used Equipment Considerations
For older machines, drain a hydraulic oil sample to check contamination levels—metal particles indicate pump wear. Inspect wiring grommets at bulkhead pass-throughs for hardening. Replace the entire harness section if multiple repair splices exist, as resistance buildup causes intermittent faults.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific machine serial number and seek professional diagnostic assistance for complex electrical or hydraulic system repairs.