What is XCMG XE750D Fault Code 33329 SPN91 FMI4?

Fault Code 33329 (SPN 91 FMI 4) indicates a voltage below normal or shorted to ground condition in the Accelerator Pedal Position Sensor (APPS) circuit. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally low voltage signal from the throttle position sensor, preventing accurate fuel delivery control.

On the XCMG XE750D excavator, SPN 91 specifically monitors the accelerator pedal position sensor, which communicates operator throttle input to the engine's electronic control system. FMI 4 designates a voltage issue where the signal falls below the manufacturer's specified threshold, typically under 0.5 volts when a higher reading is expected. This fault is critical because the ECM relies on precise throttle position data to regulate engine RPM, hydraulic pump output, and overall machine responsiveness. Without accurate sensor feedback, the excavator may enter a protective derate mode or experience erratic performance.

Common Symptoms

• Check Engine Light (CEL) illuminated on the instrument cluster with reduced engine power
• Engine derate mode activated, limiting maximum RPM to 1200-1500 range regardless of throttle input
• Unresponsive or sluggish throttle response when operating hydraulic functions or traveling
• Intermittent loss of power during operation, especially when machine vibration increases
• Erratic engine idle or failure to increase RPM when accelerator pedal is depressed

Potential Causes

The most common technical reasons for Code 33329 on used XCMG XE750D excavators include:

• Worn or damaged accelerator pedal position sensor due to accumulated operating hours and environmental exposure
• Corroded or loose electrical connectors at the APPS sensor or ECM harness connection points
• Chafed or pinched wiring harness between the operator cab and engine compartment, particularly at flex points near cab mounts
• Faulty ECM ground connections causing voltage reference issues throughout the sensor circuit
• Water intrusion into the pedal assembly or connector boots from cab floor leaks common in older machines
• Internal short circuit within the APPS sensor potentiometer due to wear or contamination

How to Troubleshoot and Fix Code 33329 SPN91 FMI4

Step 1: Visual Inspection Begin by thoroughly inspecting the accelerator pedal assembly and associated wiring harness. On used excavators, check for physical damage, corrosion on connector pins, and worn harness insulation. Pay special attention to wire routing where the harness passes through the cab floor—this is a known wear point on XCMG models.

Step 2: Electrical Testing Using a digital multimeter, backprobe the APPS connector while the key is in the "ON" position (engine off). Verify the 5-volt reference supply from the ECM and check that the sensor ground shows less than 0.1 volts. Slowly depress the accelerator pedal and monitor the signal voltage—it should sweep smoothly from approximately 0.5V to 4.5V. Any erratic readings, dropouts, or values outside this range indicate sensor failure.

Step 3: Component and Harness Testing Disconnect the APPS sensor and measure resistance across the sensor terminals (typically 1-5 kΩ depending on pedal position). Inspect all connector seals for moisture and apply dielectric grease to prevent future corrosion. For used machines, perform a wiggle test on the harness while monitoring voltage—intermittent faults often reveal damaged wires. If all electrical values are correct but the code persists, replace the accelerator pedal position sensor with an OEM or quality aftermarket part.

Step 4: Clear Codes and Verify Repair After repairs, use XCMG diagnostic software or a compatible scan tool to clear fault codes. Perform a complete operational test including full throttle application under load. Monitor for code recurrence during a 30-minute test cycle.

---

Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the XCMG XE750D service manual for model-specific procedures and safety precautions. If you lack proper diagnostic equipment or experience with high-voltage electrical systems, seek assistance from a certified XCMG technician or qualified heavy equipment repair facility.