What is Hitachi EX200-3 Fault Code 655-3?

Fault Code 655-3 on the Hitachi EX200-3 excavator indicates a malfunction in the pump torque control system, specifically related to the electronic control unit's (ECU) inability to properly regulate hydraulic pump displacement based on engine load conditions. This code triggers when the ECU detects abnormal signal voltage or communication errors from the torque control solenoid valve or its associated wiring circuit.

This fault is critical for the EX200-3 because the pump torque control system directly manages hydraulic flow and pressure relative to engine power output. When this system fails, the excavator loses its ability to optimize fuel efficiency and prevent engine stalling under heavy loads. The proportional solenoid valve that modulates pump angle receives command signals from the ECU based on engine RPM and load sensors—any disruption in this circuit compromises machine performance and can lead to costly component damage if left unaddressed.

Common Symptoms

• Erratic hydraulic performance with sudden loss of digging force or inconsistent boom/arm speed during operation
• Engine stalling or lugging under normal working loads, particularly during combined functions like digging and swinging simultaneously
• Abnormal hydraulic pump noise (whining or cavitation sounds) indicating improper displacement control
• Warning lamp illumination on the instrument cluster, often accompanied by reduced machine responsiveness
• Black smoke from exhaust due to engine overload when the pump fails to reduce displacement appropriately

Potential Causes

The 655-3 code on used EX200-3 excavators typically stems from age-related electrical degradation. The torque control solenoid valve itself may have failed due to internal coil breakdown or contaminated hydraulic oil causing spool sticking. Wiring harness damage is extremely common on this model—the harness routing near the hydraulic pump experiences significant heat and vibration, causing insulation breakdown and short circuits.

Connector corrosion at the pump-mounted solenoid is prevalent in machines operating in wet or coastal environments. The ECU itself may have internal failures, though this is less common. Additionally, poor ground connections at the chassis grounding points can create voltage irregularities that trigger false codes. On used equipment, previous repair attempts may have introduced incorrect wiring or aftermarket solenoids with incompatible resistance specifications.

How to Troubleshoot and Fix Code 655-3

Step 1: Visual Inspection and Connector Check Begin by thoroughly inspecting the torque control solenoid valve mounted on the main hydraulic pump. Disconnect the electrical connector and examine both male and female terminals for corrosion, bent pins, or moisture intrusion. Clean contacts with electrical contact cleaner and dielectric grease. Trace the wiring harness back to the ECU, checking for abrasion points where the harness contacts metal edges or hot engine components—this is a known wear point on EX200-3 models.

Step 2: Electrical Testing with Multimeter Using a quality digital multimeter, measure the solenoid coil resistance. The torque control solenoid should typically read between 15-25 ohms (consult your service manual for exact specifications). Next, check for supply voltage at the solenoid connector with the ignition on—you should see approximately 12V DC. Test the ground circuit for continuity to chassis ground (should read less than 1 ohm resistance). If voltage or ground is absent, trace the circuit back toward the ECU, testing at intermediate connectors.

Step 3: Functional Testing and Component Replacement If electrical values are correct, perform a functional test using Hitachi diagnostic software (Dr.EX) if available, commanding the solenoid to actuate while monitoring response. On used excavators, even if resistance values seem acceptable, internal solenoid degradation may prevent proper operation—consider replacing the solenoid valve assembly with OEM or high-quality aftermarket parts. After replacement, clear codes and perform a load test. Check hydraulic oil contamination levels, as metal particles can damage new solenoids.

Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Hitachi EX200-3 service manual for exact specifications and safety procedures. Complex hydraulic and electrical diagnostics should be performed by qualified heavy equipment technicians with appropriate tools and training.