What is Komatsu PC100-6 Fault Code H-21?

Fault Code H-21 on the Komatsu PC100-6 indicates a hydraulic pump control solenoid malfunction in the proportional pressure reduction valve circuit. This code specifically flags an electrical abnormality in the solenoid that regulates main pump displacement based on operational demands.

The PC100-6's ECM (Engine Control Module) continuously monitors the solenoid valve's resistance and current draw. When values fall outside the 8-12 ohm specification range or the circuit shows open/short conditions, H-21 triggers. This system is critical because it directly affects the excavator's ability to modulate hydraulic flow efficiently, impacting fuel economy and operational smoothness during combined functions like swing-and-dig operations.

Common Symptoms

• Reduced hydraulic response during combined operations, particularly noticeable when simultaneously operating boom and bucket functions
• Intermittent loss of fine control in excavator movements, making precision work difficult
• Warning lamp illumination on the instrument cluster with possible audible alert
• Increased fuel consumption as the pump runs at fixed displacement instead of load-sensing mode
• Hard or jerky hydraulic movements replacing the normally smooth proportional control

Potential Causes

The most common root causes in used PC100-6 excavators include:

Solenoid coil degradation from heat cycles and contamination exposure over 5,000+ operating hours. Connector corrosion at the 6-pin harness plug near the main pump, a known vulnerability point where moisture enters through damaged rubber boots. Wiring harness chafing where the cable runs along the pump mounting bracket—vibration causes insulation wear. ECM voltage supply issues from aging capacitors in 15+ year-old control modules. Contaminated hydraulic oil causing valve spool sticking, increasing electrical load beyond parameters.

How to Troubleshoot and Fix Code H-21

Step 1: Visual Inspection Locate the proportional solenoid valve mounted on the left side of the main hydraulic pump. Inspect the connector for oil contamination, bent pins, or green corrosion. Check the wiring harness for visible abrasion where it contacts metal edges—a critical check for used excavators.

Step 2: Electrical Testing Disconnect the solenoid connector and measure resistance across the coil terminals using a digital multimeter. Specification is 8-12 ohms at 20°C. Next, measure voltage supply from the ECM with ignition on (should read 12V±0.5V). Check for continuity to ground on the harness side.

Step 3: Component Testing If resistance is correct, perform a dynamic load test by commanding the solenoid through Komatsu KOMTRAX diagnostic software while monitoring current draw (should be 0.8-1.2A). For used machines, remove the solenoid and inspect the valve spool for varnish buildup—clean with approved hydraulic solvent if sticky.

Step 4: Harness Repair Replace chafed sections using 16-AWG marine-grade wire and heat-shrink connectors with adhesive lining. Reroute away from sharp edges using protective split loom.

Disclaimer: This guide provides general diagnostic information. Consult qualified Komatsu technicians and reference the official PC100-6 service manual for model-specific procedures and safety protocols before performing repairs.