What is Sumitomo SH220LC-6 Fault Code 638?

Fault Code 638 on the Sumitomo SH220LC-6 excavator indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the Engine Control Module (ECM). This diagnostic trouble code (DTC) specifically relates to the coolant temperature sensor sending irregular voltage signals outside the manufacturer's specified parameters, typically 0.5-4.5 volts depending on temperature range.

This code is critical for the SH220LC-6's performance because the ECM relies on accurate coolant temperature data to optimize fuel injection timing, idle speed control, and engine protection protocols. When Code 638 activates, the system may default to a fixed temperature value, potentially causing poor fuel economy, rough idling, or premature engine wear. On used excavators, this sensor circuit is particularly vulnerable due to heat cycling fatigue and connector corrosion common in machines with 5,000+ operating hours.

Common Symptoms

• Check Engine Light or warning indicator illuminated on the instrument cluster
• Engine running excessively rich or lean, producing black smoke or hesitation during acceleration
• Difficulty starting when the engine is cold, or extended cranking times
• Engine fan running continuously at high speed regardless of actual operating temperature
• Reduced hydraulic response or engine derate mode activation to prevent perceived overheating

Potential Causes

The most common technical causes for Code 638 on used SH220LC-6 excavators include:

• Coolant temperature sensor failure due to internal resistance drift or element degradation (typical after 8,000-10,000 hours)
• Damaged or corroded wiring harness connections at the sensor plug, especially where the harness routes near the exhaust manifold
• Chafed wires causing short circuits to ground or power, common at frame mounting points
• Faulty ECM internal reference voltage circuit (rare but possible on high-hour machines)
• Low coolant level causing sensor exposure to air, creating erratic readings
• Corroded ground connections affecting sensor signal return path

How to Troubleshoot and Fix Code 638

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block near the thermostat housing. Check for physical damage, coolant leaks, or corrosion on the two-pin connector. On used excavators, examine the entire wiring harness route for rub-through points, especially where it contacts metal brackets or passes near hot exhaust components.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (consult service manual for exact specifications). If readings are infinite (open circuit) or near zero (short circuit), replace the sensor. Check connector pins for corrosion or bent terminals that prevent proper contact.

Step 3: Circuit Verification With the sensor disconnected, check the harness-side connector for proper reference voltage from the ECM (typically 5 volts). Measure continuity between the signal wire and ECM pin, and verify the ground circuit has less than 1 ohm resistance. If voltage or ground circuits fail, trace wiring for breaks or shorts before considering ECM replacement.

Step 4: Final Validation After repairs, clear the fault code using Sumitomo diagnostic software or compatible scan tool. Monitor live coolant temperature data during a complete warm-up cycle to ensure readings progress smoothly from ambient to operating temperature (180-200°F). For used machines, inspect coolant condition and flush the system if contaminated, as sediment can affect sensor accuracy.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Sumitomo service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues or if you lack proper testing equipment.