What is Sumitomo SH220LC-6 Fault Code 91?

Fault Code 91 on the Sumitomo SH220LC-6 excavator indicates a malfunction in the Engine Speed Sensor circuit or an abnormal engine speed signal being sent to the Engine Control Module (ECM). This diagnostic trouble code (DTC) is triggered when the ECM detects inconsistent, missing, or out-of-range signals from the engine speed sensor (also called the crankshaft position sensor), which monitors the rotational speed and position of the engine's crankshaft.

This code is critical for the SH220LC-6 because the engine speed sensor provides essential data that the ECM uses to control fuel injection timing, engine idle, and overall performance. When this signal is compromised, the excavator may enter a derate mode or experience erratic engine behavior, directly impacting productivity and potentially causing further mechanical damage if not addressed promptly.

Common Symptoms

When Code 91 is active on your Sumitomo SH220LC-6, operators typically experience:

• Check Engine light or warning indicator illuminated on the instrument panel
• Rough idling or unstable engine RPM, including frequent stalling
• Loss of engine power or inability to reach maximum RPM under load
• Difficult starting or extended cranking time before the engine fires
• Intermittent engine shutdowns during operation, especially under heavy loads

Potential Causes

The most common technical reasons for Code 91 on used Sumitomo excavators include:

• Faulty engine speed sensor due to internal component wear or heat damage
• Damaged or corroded wiring harness between the sensor and ECM, particularly at connector pins
• Loose or contaminated electrical connections at the sensor plug (common in machines with high operating hours)
• Broken or damaged sensor mounting, causing improper air gap between sensor and flywheel teeth
• Worn flywheel ring gear with damaged or missing teeth preventing accurate signal generation
• ECM internal fault or corrupted software (less common but possible in older machines)
• Voltage supply issues from corroded ground connections or failing alternator output

How to Troubleshoot and Fix Code 91

Step 1: Visual Inspection Begin by locating the engine speed sensor (typically mounted on the flywheel housing near the transmission bellhousing). Inspect the sensor mounting for looseness, cracks, or excessive oil contamination. Check the wiring harness from the sensor to the ECM for visible damage, chafing from vibration, or heat damage near the exhaust manifold—a common failure point on used excavators.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. Most Sumitomo engine speed sensors should read between 150-300 ohms (consult your service manual for exact specifications). Test the voltage supply at the connector with the ignition on (typically 8-12V). Check for continuity in all wires from sensor to ECM and verify ground integrity, as corrosion on ground points is extremely common in aged machines.

Step 3: Sensor Air Gap and Flywheel Inspection Check the air gap between the sensor tip and flywheel teeth using a feeler gauge (typically 0.5-1.5mm specification). If accessible, inspect the flywheel ring gear for damaged, worn, or missing teeth. On used excavators with high hours, metal debris accumulation on the magnetic sensor tip can disrupt signal quality—clean thoroughly with electrical contact cleaner.

Step 4: Component Replacement and Code Clearing If testing confirms sensor failure, replace with an OEM or high-quality aftermarket sensor and ensure proper torque specifications. After repairs, use Sumitomo diagnostic software or a compatible scan tool to clear the code and perform a test run under load. Monitor for code recurrence, which may indicate ECM issues requiring dealer-level diagnostics.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Sumitomo service manual for your specific machine serial number and seek professional assistance from certified technicians for complex electrical diagnostics or ECM programming.