What is XCMG XE490D (CM2150C-CM2150D) Fault Code 1681?

Fault Code 1681 indicates an Engine Speed/Position Sensor 1 Circuit - Data Valid but Below Normal Operating Range. This diagnostic trouble code (DTC) means the Engine Control Module (ECM) is receiving a signal from the primary crankshaft position sensor, but the voltage or frequency is lower than the manufacturer's specified threshold during engine operation.

This code is critical for XCMG excavators equipped with the Cummins CM2150 engine platform because the speed/position sensor directly controls fuel injection timing, cylinder identification, and engine speed regulation. When the ECM detects abnormally low signal strength, it cannot accurately determine crankshaft position, leading to performance issues or complete engine shutdown to prevent damage.

Common Symptoms

Operators may experience the following when Code 1681 is active:

• Hard starting or extended cranking time, especially when the engine is cold
• Rough idle or engine misfiring during low-RPM operation
• Check Engine Light (CEL) illuminated on the instrument panel
• Engine derate mode limiting maximum RPM to 1500-1800 range
• Intermittent stalling during idle or when hydraulic loads are applied
• Loss of power during digging or lifting operations due to ECM fuel limiting

Potential Causes

The most common technical reasons for Code 1681 on used XCMG excavators include:

• Worn or damaged crankshaft position sensor (CKP sensor) due to heat exposure or vibration fatigue
• Corroded or loose electrical connections at the sensor harness connector
• Damaged sensor wiring harness, particularly at rub points near the flywheel housing or engine mount brackets
• Excessive air gap between the sensor tip and reluctor wheel caused by sensor mounting looseness
• Contamination buildup (metal shavings, oil residue) on the sensor face reducing magnetic field strength
• Failing ECM internal circuitry (less common but possible on high-hour machines)
• Low supply voltage to the sensor due to corroded ground connections or damaged power supply wires

How to Troubleshoot and Fix Code 1681

Step 1: Visual Inspection Begin by locating the crankshaft position sensor on the engine's flywheel housing (driver's side, mid-engine). Inspect the sensor, wiring harness, and connector for physical damage, oil contamination, or corrosion. Check for harness chafing at known rub points where the harness passes near the engine mount or hydraulic lines—common on used machines with 5,000+ hours.

Step 2: Check Sensor Air Gap and Cleanliness Remove the sensor and verify the air gap specification (typically 0.020"-0.040" for CM2150 engines). Clean the sensor tip with electrical contact cleaner and inspect the reluctor wheel teeth for damage or metal debris buildup. Reinstall with proper torque (typically 89 lb-in).

Step 3: Electrical Testing Using a digital multimeter (DMM), measure sensor resistance between the signal pins (specification typically 800-2500 ohms at room temperature). Check supply voltage at the harness connector with key on/engine off (should read approximately 5V or 8V depending on ECM configuration). Inspect ground continuity to chassis—resistance should be less than 1 ohm.

Step 4: Dynamic Signal Test Connect Cummins INSITE or compatible diagnostic software while cranking the engine. Monitor live sensor frequency output—it should show 200-400 Hz during cranking. If readings are erratic or below 100 Hz, replace the sensor with a genuine Cummins 3408529 or OEM-equivalent part.

Step 5: Harness and ECM Verification If sensor tests pass, inspect the entire sensor harness from connector to ECM for internal wire breaks using a continuity test. On used excavators, pay special attention to connectors that may have been previously repaired. If all wiring checks good, the ECM may require replacement or professional bench testing.

---

Disclaimer: This guide provides general troubleshooting information. Always consult the official XCMG service manual and Cummins Troubleshooting & Repair Manual (Bulletin 4021595) for your specific engine serial number. If you lack proper diagnostic tools or experience with high-pressure fuel systems, consult a certified Cummins or XCMG technician to avoid safety hazards or further equipment damage.