What is Komatsu HB205-1 Fault Code H-21?

Fault Code H-21 on the Komatsu HB205-1 hybrid excavator indicates a malfunction in the swing motor resolver circuit, specifically detecting an abnormal voltage signal from the resolver position sensor. This code triggers when the Electronic Control Module (ECM) receives out-of-range voltage readings from the swing motor's resolver, which is responsible for detecting the precise rotational position and speed of the swing motor.

The resolver is a critical component in the HB205-1's hybrid system, acting as a rotational feedback device that allows the ECM to accurately control the electric swing motor's performance. When this sensor fails or provides faulty signals, the system cannot properly manage swing function regeneration and electric motor assistance, directly impacting the machine's fuel efficiency and operational smoothness. For hybrid excavators like the HB205-1, resolver accuracy is essential for the seamless transition between electric motor assist and hydraulic power.

Common Symptoms

When fault code H-21 is active, operators typically experience:

• Swing function operating in limited or backup mode, with noticeably reduced smoothness and slower swing speeds
• Hybrid system warning light illuminated on the operator display panel
• Loss of electric motor assist during swing operations, forcing the machine to rely solely on hydraulic power
• Increased fuel consumption due to the hybrid regeneration system being disabled
• Error message displayed on the monitor indicating swing motor resolver malfunction

Potential Causes

The most common technical causes for H-21 code on used HB205-1 machines include:

• Resolver sensor internal failure due to age, moisture ingress, or mechanical wear from vibration
• Damaged or corroded wiring harness between the swing motor resolver and the hybrid ECU, particularly at connection points near the swing circle where flexing occurs
• Loose or corroded electrical connectors at the resolver plug or ECU terminals, especially common in machines with high operating hours
• Contamination or moisture inside the resolver housing affecting signal transmission
• ECU communication issues or internal ECU faults affecting the resolver input circuit
• Damaged swing motor assembly where physical impact has affected resolver alignment or mounting

How to Troubleshoot and Fix Code H-21

Step 1: Initial Inspection and Code Verification Connect Komatsu diagnostic software (KOMTRAX Plus or compatible scan tool) to verify the fault code is current and not intermittent. Record any freeze-frame data showing operating conditions when the fault occurred. Visually inspect the swing motor resolver area for obvious physical damage, oil leaks, or housing cracks.

Step 2: Electrical Connector and Harness Inspection On used excavators, this is critical—locate the resolver connector at the swing motor (typically mounted on the swing motor housing). Disconnect the connector and carefully inspect pins for corrosion, bent contacts, or moisture. Check the wiring harness along its entire path from the swing motor to the hybrid ECU, paying special attention to areas where the harness passes through the swing circle, as this is a known wear point for the HB205-1. Look for wire insulation damage, chafing, or exposed conductors.

Step 3: Voltage and Resistance Testing Using a digital multimeter, measure the resolver excitation voltage from the ECU (typically 5-8V AC) at the harness side of the disconnected resolver connector with ignition on. Consult the service manual for exact specifications. Then measure resolver output signals (usually three wires: excitation, sine output, and cosine output) for proper resistance values (typically 50-150 ohms between signal wires, but verify with Komatsu specifications for the HB205-1).

Step 4: Resolver and Component Replacement If electrical tests indicate resolver failure, replacement of the swing motor resolver assembly is required. For used machines with high hours, consider inspecting resolver mounting alignment and swing motor bearing condition, as excessive play can cause resolver signal errors. After replacement, clear fault codes using diagnostic software and perform a swing function calibration procedure as specified in the Komatsu service manual to ensure proper ECU learning of the new resolver's characteristics.

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Disclaimer: This guide provides general troubleshooting information for experienced heavy equipment technicians. Always consult the official Komatsu HB205-1 service manual for precise specifications, torque values, and safety procedures. If unfamiliar with high-voltage hybrid systems, contact a certified Komatsu technician, as hybrid excavators contain dangerous electrical components requiring specialized training.