What is Sumitomo SH210-6 Fault Code 91?

Fault Code 91 on the Sumitomo SH210-6 excavator indicates a malfunction in the throttle motor circuit or throttle position sensor system. This code specifically signals that the Engine Control Module (ECM) has detected an abnormal voltage signal, open circuit, or short circuit within the throttle actuator mechanism that regulates engine speed.

The throttle motor on the SH210-6 is a critical component that translates electronic signals from the ECM into precise mechanical adjustments of the fuel injection pump's throttle position. When Code 91 triggers, the excavator's ability to maintain consistent engine RPM and respond to operator input becomes compromised. This fault is particularly significant in used machines where connector corrosion and wiring harness deterioration are common failure points, directly impacting fuel efficiency, hydraulic performance, and overall machine productivity.

Common Symptoms

When Code 91 is active on your Sumitomo SH210-6, you may experience:

• Erratic engine RPM that surges or drops unexpectedly during operation, especially under load
• Engine warning light illuminated on the instrument cluster with possible derate mode activation
• Poor throttle response or complete loss of electronic throttle control, forcing the engine into a fixed RPM limp mode
• Reduced hydraulic power due to inconsistent engine speed affecting pump output
• Engine stalling at idle or difficulty maintaining stable idle speed

Potential Causes

The most common technical reasons for Code 91 on used SH210-6 excavators include:

• Throttle motor failure due to internal winding breakdown, carbon brush wear, or seized mechanical components from lack of lubrication
• Wiring harness damage at known rub points near the engine mount or along the chassis frame where vibration causes insulation wear
• Corroded or loose connectors at the throttle motor plug or ECM connections, especially in machines exposed to moisture or saltwater environments
• Throttle position sensor degradation providing inconsistent voltage feedback to the ECM
• ECM internal fault or software corruption (less common but possible in older machines with 10,000+ operating hours)
• Poor ground connections at the engine block or chassis ground straps causing voltage reference issues

How to Troubleshoot and Fix Code 91

Step 1: Visual Inspection and Connector Check Begin by locating the throttle motor mounted on the fuel injection pump. Disconnect the electrical connector and inspect for corrosion, bent pins, or moisture intrusion. On used excavators, this connector often shows green corrosion deposits. Clean contacts with electrical contact cleaner and dielectric grease. Inspect the entire wiring harness from the throttle motor to the ECM for chafing, especially where it crosses metal brackets or passes near hot engine components.

Step 2: Electrical Testing with Multimeter Using a digital multimeter, check the throttle motor resistance. With the connector disconnected and ignition off, measure resistance across the motor terminals—typical values range from 10-50 ohms (consult your service manual for exact specifications). Next, check for voltage supply from the ECM: turn the ignition to ON (engine off) and measure voltage at the harness connector. You should see approximately 12 volts on the power supply wire. Check for continuity to ground on the ground wire (should read less than 1 ohm resistance).

Step 3: Throttle Motor Functional Test If electrical values are correct, perform a functional test by applying 12V battery power directly to the throttle motor terminals (observe polarity). The motor should rotate smoothly without binding or unusual noise. If the motor operates correctly with direct power but Code 91 persists, suspect ECM communication issues or internal ECM failure.

Step 4: Sensor Feedback Verification Many throttle motors include an integrated position sensor that provides feedback to the ECM. Using manufacturer-specific diagnostic software (Sumitomo diagnostic tool or compatible third-party software), monitor the throttle position sensor voltage in real-time while manually moving the throttle linkage. The voltage should change smoothly from approximately 0.5V to 4.5V through the full range of motion without dropouts or erratic readings.

Step 5: Component Replacement and Verification After identifying the failed component (motor, sensor, or harness), replace with OEM or high-quality aftermarket parts. For used excavators, always replace the connector pigtail if any corrosion was present—don't rely on cleaning alone. After replacement, clear the fault code using diagnostic software, operate the machine through various RPM ranges, and verify Code 91 does not return.

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Disclaimer: This guide provides general troubleshooting information for Code 91 on Sumitomo SH210-6 excavators. Always consult the manufacturer's service manual for model-specific procedures and specifications. If you're uncomfortable performing electrical diagnostics or working with fuel system components, seek assistance from a certified heavy equipment technician to prevent further damage or safety hazards.