What is XCMG QSB6.7 CM2880 B117 Fault Code 155?

Fault Code 155 on the XCMG excavator equipped with a Cummins QSB6.7 CM2880 ECM indicates a Vehicle Speed Sensor (VSS) circuit malfunction or an implausible signal error. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal voltage reading, erratic signal pattern, or complete loss of communication from the vehicle speed sensor circuit.

The QSB6.7 engine relies on accurate speed data to manage fuel injection timing, torque curve optimization, and integration with the excavator's hydraulic load-sensing systems. When Code 155 appears, the ECM cannot properly correlate engine load with ground speed, potentially causing performance inefficiencies, incorrect power derate activation, or communication errors with the machine's CAN bus network. For used XCMG excavators, this code often surfaces due to sensor degradation or wiring harness damage from years of vibration and environmental exposure.

Common Symptoms

• Check Engine Light (CEL) or malfunction indicator lamp illuminated on the instrument cluster
• Erratic speedometer readings or complete speedometer failure on machines equipped with ground speed displays
• Engine power derate or reduced hydraulic response during travel mode
• Transmission shift irregularities or failure to automatically adjust travel speed settings
• Intermittent code logging that appears and clears randomly, especially over rough terrain

Potential Causes

The most common technical failures triggering Code 155 on used XCMG excavators include:

• Failed or contaminated Vehicle Speed Sensor: Magnetic pickup sensors degrade from heat, moisture intrusion, or metal debris accumulation on the sensor tip
• Damaged wiring harness: Harness rub points near the transmission output shaft, swing bearing, or undercarriage components where vibration causes wire chafing and short circuits
• Corroded or loose electrical connectors: Moisture infiltration at the VSS connector or ECM harness pins creates high-resistance connections
• Faulty ECM speed signal processing: Internal ECM circuit board degradation (less common but documented in high-hour CM2880 units)
• Incorrect sensor air gap: On machines with adjustable VSS mounting, improper clearance between sensor and tone wheel (typically should be 0.5-1.5mm)

How to Troubleshoot and Fix Code 155

Step 1: Visual Inspection and Connector Check Begin by locating the vehicle speed sensor (typically mounted on the transmission tailshaft or transfer case). Inspect the sensor body for physical damage, oil contamination, or metal shavings. Disconnect the sensor 2-pin or 3-pin connector and examine terminals for corrosion, bent pins, or moisture. Clean contacts with electrical contact cleaner and apply dielectric grease before reconnection.

Step 2: Sensor Resistance and Signal Testing Using a digital multimeter (DMM), measure the VSS resistance across sensor terminals (specification typically 800-1400 ohms for magnetic sensors; consult CM2880 service manual for exact values). Check for AC voltage output while manually rotating the driveshaft—you should see 0.5-5V AC signal that increases with rotation speed. No signal or infinite resistance indicates sensor failure.

Step 3: Harness Continuity and ECM Pin Verification Trace the VSS harness from sensor to ECM connector, checking for wire breaks, chafing, or short to ground. Pay special attention to harness routing near pivot points and hydraulic lines where wire abrasion is common on used excavators. At the ECM connector (typically a 134-pin or similar), verify continuity from VSS pins to ECM and check for proper 5V reference voltage supply from ECM (if applicable to sensor type).

Step 4: Sensor Air Gap and Tone Wheel Inspection If sensor tests pass, inspect the tone wheel (reluctor ring) for missing teeth, cracks, or excessive runout. Verify sensor mounting bracket integrity and measure air gap using feeler gauges—adjust to manufacturer specification if outside tolerance.

Step 5: Clear Code and Road Test After repairs, clear fault codes using Cummins INSITE diagnostic software or compatible scan tool. Perform a controlled test operation, monitoring live VSS data to confirm stable signal output across varying speeds.

For used excavators, always inspect harness grommets and protective looms that may have deteriorated over time, allowing moisture and debris entry—these are frequent culprits in aging machines.

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Disclaimer: This guide provides general diagnostic procedures for informational purposes. Always consult the official XCMG service manual and Cummins CM2880 troubleshooting documentation for your specific machine serial number. If unfamiliar with high-voltage electrical systems or ECM diagnostics, seek assistance from a certified Cummins or XCMG technician to prevent equipment damage or safety hazards.