What is Yanmar General Fault Code P1644-2?

Fault Code P1644-2 indicates a malfunction in the Engine Control Module (ECM) power supply circuit, specifically related to an abnormal voltage condition in the main relay circuit or ECM internal power distribution system. This code triggers when the ECM detects either excessive voltage, insufficient voltage, or an intermittent power supply that falls outside the manufacturer's specified operating range of typically 22-32 volts for Yanmar diesel engines.

This fault is critical because the ECM serves as the brain of your Yanmar excavator's engine management system, controlling fuel injection timing, throttle response, and emissions systems. When power supply integrity is compromised, the ECM cannot reliably manage engine functions, potentially leading to unexpected shutdowns, erratic performance, or complete failure to start. In used excavators, this code often surfaces due to age-related electrical degradation rather than catastrophic component failure.

Common Symptoms

• Engine cranks but fails to start, or starts then immediately shuts down after 2-3 seconds
• Intermittent loss of engine power or sudden engine stalling during operation without warning
• Check Engine Light (CEL) or malfunction indicator lamp illuminated on the instrument cluster
• Diagnostic display shows reduced engine power mode (derate), limiting machine to 50-70% throttle capacity
• Erratic gauge readings including fluctuating voltage meter or complete instrument panel blackout

Potential Causes

The most common technical causes for P1644-2 in used Yanmar excavators include:

• Main relay failure due to contact wear, corrosion, or coil resistance changes (relays typically located in the fuse/relay box near the battery)
• Corroded or loose battery cable connections at battery terminals, ground points, or ECM power harness connectors
• Damaged wiring harness particularly at flex points near the engine mount, cab pivot points, or where harnesses route along sharp frame edges
• Failing alternator producing inconsistent charging voltage (normal range: 27-29V for 24V systems)
• ECM internal power supply circuit degradation from moisture intrusion through cracked connector seals or failed internal voltage regulators
• Blown fuse or corroded fuse holder in the ECM power supply circuit (typically 15-30A rating)

How to Troubleshoot and Fix Code P1644-2

Step 1: Visual Inspection and Voltage Verification Begin with a thorough visual inspection of all battery cables, ground straps, and the main relay. Using a digital multimeter (DMM), measure battery voltage with the engine off (should read 24-26V for 24V systems). Check voltage at the ECM power supply pins with key ON/engine OFF—you should see battery voltage present. Inspect all connector pins for green corrosion, bent terminals, or moisture intrusion, especially on used machines where connector seals deteriorate over time.

Step 2: Main Relay and Circuit Testing Locate the engine control relay (consult your service manual for exact location—typically in the main fuse box). With the key in the ON position, listen for an audible "click" when the relay energizes. Using your DMM, check relay coil resistance (typically 70-90 ohms) and verify relay contacts show continuity when energized. Test the relay socket for corrosion and proper pin tension. For used excavators, swap the relay with an identical relay from another circuit (like the starter relay) to rule out relay failure before purchasing parts.

Step 3: Alternator Output and Wiring Harness Inspection Start the engine (if possible) and measure alternator output voltage at idle and at 1500 RPM—it should maintain 27-29V without fluctuation exceeding 0.5V. Inspect the engine wiring harness carefully at all flex points, especially where it passes through bulkheads or near rotating components. Look for wire insulation chafing, exposed copper, or evidence of previous repairs with electrical tape. Pay special attention to ground connections at the engine block and frame—remove, clean with a wire brush, and apply dielectric grease before reconnecting. On older machines, corrosion at ground points is a leading cause of voltage irregularities.

Step 4: ECM Connector and Advanced Diagnostics Disconnect the ECM main connector (typically a large multi-pin connector) and inspect for pushed-back pins, corrosion, or moisture. Clean contacts with electrical contact cleaner and verify proper pin seating. If available, connect Yanmar diagnostic software to monitor real-time voltage readings at the ECM and check for intermittent voltage drops during engine cranking. If all external circuits test normally but the code persists, the ECM internal power supply circuit may require professional bench testing or replacement.

Disclaimer: This guide provides general troubleshooting procedures for educational purposes. Always consult your machine's specific service manual and consider engaging a certified Yanmar technician for complex electrical diagnostics, especially when dealing with ECM replacement or programming on used equipment where previous repairs may affect system behavior.