What is XCMG XE750D Fault Code 45783 SPN655 FMI6?

Fault Code 45783 (SPN 655 FMI 6) indicates an Engine Starter Motor Speed/Position signal problem, specifically a voltage above normal or shorted to high source. This code triggers when the Engine Control Module (ECM) detects abnormal electrical voltage in the starter motor speed sensor circuit, typically reading higher than the expected operational range.

In the XCMG XE750D excavator, this fault directly affects the engine's ability to properly monitor starter engagement and cranking speed. The ECM relies on this circuit to prevent starter motor damage during start sequences and to verify proper engine cranking RPM. When voltage irregularities occur, the system may disable starting functions to protect components, making this a critical fault that can leave your machine inoperable.

Common Symptoms

• Engine cranks but fails to start or refuses to crank at all, with starter motor remaining inactive
• Check Engine Light (CEL) or malfunction indicator lamp illuminated on the dashboard
• Intermittent starting issues, particularly during cold starts or after the machine has been sitting
• Abnormal clicking sounds from the starter relay without actual starter engagement
• ECM may log multiple fault codes simultaneously related to starting system circuits

Potential Causes

The most common technical causes for SPN 655 FMI 6 on used XCMG excavators include:

• Damaged or corroded wiring harness in the starter motor circuit, especially at connection points exposed to engine heat and vibration
• Faulty starter motor speed sensor or position sensor sending erratic voltage signals
• Short circuit to battery voltage caused by chafed wires rubbing against frame components or engine mounting brackets
• Corroded or loose connector pins at the ECM or starter motor relay, common in machines with high operating hours
• Failed starter relay or solenoid creating voltage feedback into the sensor circuit
• ECM internal circuit degradation, though less common, can occur in older machines with moisture intrusion

How to Troubleshoot and Fix Code 45783 SPN655 FMI6

Step 1: Visual Inspection of Wiring Harness Begin with a thorough physical inspection of the starter motor wiring harness from the ECM to the starter assembly. On used excavators, pay special attention to areas where the harness passes near the engine block, exhaust manifold, or hydraulic lines. Look for melted insulation, abraded wire coating, or evidence of harness contact with moving parts. Check all connector boots for cracking or moisture penetration.

Step 2: Electrical Testing with Multimeter Disconnect the starter motor connector and measure voltage at the sensor signal wire with the key in the "ON" position (engine not running). You should see approximately 5 volts reference voltage from the ECM. If voltage reads above 5.5 volts, suspect a short to battery voltage somewhere in the harness. Perform a resistance check between the signal wire and ground—readings below 10,000 ohms indicate a short circuit.

Step 3: Test Starter Motor and Relay With the harness disconnected, use an ohmmeter to check the starter motor speed sensor internal resistance (consult XCMG specifications, typically 500-1500 ohms). Test the starter relay by swapping it with an identical relay from another circuit (horn or lights) to rule out relay failure causing voltage spikes.

Step 4: Connector and Pin Inspection Remove and inspect all connectors in the starting circuit. On used machines, corrosion on connector pins is extremely common and can create high-resistance connections that cause voltage irregularities. Clean pins with electrical contact cleaner and apply dielectric grease before reassembly. Check for pushed-back pins in connectors—a frequent issue after multiple service disconnections.

Step 5: ECM Circuit Verification If all external components test normally, use XCMG diagnostic software to perform an ECM output test on the starter circuit. This will help determine if the ECM itself is providing proper reference voltage. In used excavators with high hours, ECM connector corrosion at the back of the module is a known issue—remove and inspect the main ECM connectors for green oxidation or moisture.

Critical Note for Used Equipment: Before replacing expensive components like the starter motor or ECM, always verify that harness routing hasn't been modified during previous repairs, as improper routing can create new rub points causing intermittent shorts.

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Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult XCMG service manuals and qualified diesel technicians for your specific machine. Improper electrical diagnostics can cause additional system damage or create safety hazards.