What is Hitachi General Fault Code 655-5?

Fault Code 655-5 on Hitachi excavators indicates a malfunction in the engine coolant temperature sensor circuit, specifically a signal voltage that is out of the acceptable range or intermittent connection. This code typically means the Engine Control Module (ECM) is receiving erratic or implausible data from the coolant temperature sensor (CTS), preventing accurate monitoring of engine operating temperature.

The coolant temperature sensor is critical for proper fuel injection timing, idle speed control, and preventing engine overheating or cold-start damage. When this circuit fails, the ECM may revert to default temperature values, which can lead to reduced fuel efficiency, increased emissions, and potential engine damage on used machines with existing wear patterns. On Hitachi excavators, this sensor directly influences hydraulic oil temperature management and overall powertrain protection strategies.

Common Symptoms

• Engine warning light or malfunction indicator illuminated on the instrument cluster
• Rough or unstable idle, especially during cold starts or warm-up periods
• Engine derate mode activation, limiting maximum RPM and hydraulic pump output
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Black smoke from exhaust due to improper fuel mixture compensation

Potential Causes

The most common technical causes for Code 655-5 include:

• Faulty coolant temperature sensor due to internal resistance drift or element failure (common in machines over 5,000 hours)
• Damaged or corroded wiring harness between the sensor and ECM, particularly at connector pins
• Harness chafing at known rub points near the engine block or cylinder head where vibration causes insulation wear
• Poor electrical ground connection at the sensor mounting location
• Coolant contamination or air pockets affecting sensor accuracy
• ECM internal circuit degradation (less common but possible in high-hour used equipment)

How to Troubleshoot and Fix Code 655-5

Step 1: Connect a diagnostic scanner compatible with Hitachi systems (such as Hitachi Dr.EX software) to verify the code and check live coolant temperature data. Compare the displayed temperature with actual engine conditions.

Step 2: Visually inspect the coolant temperature sensor and its wiring harness for obvious damage, corrosion, or coolant leaks. On used excavators, pay special attention to connector seals and harness routing near heat sources or moving components.

Step 3: Using a digital multimeter, measure the sensor resistance when cold (typically 2,000-3,000 ohms at 20°C) and compare to manufacturer specifications. Check for voltage supply (usually 5V reference) at the sensor connector with the key on, engine off.

Step 4: Inspect wiring continuity from sensor to ECM and verify proper ground circuit resistance (should be less than 1 ohm). For used machines, remove protective loom and check for wire insulation damage from prolonged vibration or heat exposure.

Step 5: If sensor and wiring test within specifications, clear the code and monitor for recurrence. Intermittent codes often indicate connector corrosion—clean all pins with electrical contact cleaner and apply dielectric grease.

Disclaimer: This guide provides general troubleshooting information. Always consult the manufacturer's service manual and consider professional diagnostic assistance for complex electrical issues or warranty concerns.