What is Hitachi ZX240-5A Fault Code P0687 (1485-6)?

Fault Code P0687 (1485-6) on the Hitachi ZX240-5A excavator indicates an ECM/PCM Power Relay Control Circuit Low voltage condition. This diagnostic trouble code signals that the Engine Control Module (ECM) has detected an abnormally low voltage signal in the power relay control circuit, which manages the main power supply to critical engine control systems.

The power relay serves as the gateway for electrical current flowing to the ECM and its associated sensors and actuators. When the control circuit voltage drops below the manufacturer's specified threshold (typically below 10.5 volts during operation), the system triggers this fault code. For the ZX240-5A's Isuzu engine management system, this is critical because inadequate voltage compromises fuel injection timing, sensor accuracy, and overall engine performance. On used excavators, this fault often indicates deteriorating electrical connections or aging relay components that have accumulated operating hours in harsh construction environments.

Common Symptoms

• Intermittent or complete engine starting failures, particularly during cold starts or after the machine has been sitting idle
• ECM warning light illuminated on the instrument cluster, often accompanied by reduced engine power (derate mode)
• Erratic engine idle or unexpected shutdowns during operation, especially under heavy hydraulic load
• Loss of communication with diagnostic software or difficulty reading real-time engine data
• Electrical accessories (lights, HVAC, radio) functioning normally while engine control systems fail, indicating isolated ECM power issues

Potential Causes

The most common technical causes for P0687 on used ZX240-5A excavators include:

• Failed ECM power relay due to internal contact wear, carbon buildup, or coil degradation after extended service hours
• Corroded or loose connections at the relay socket, ECM power harness connectors (C101, C102), or battery terminals—particularly common in machines operating in wet or coastal environments
• Damaged wiring harness with insulation breakdown at known rub points near the engine mount brackets and along the right-side frame rail
• Weak battery voltage (below 12.4V at rest) or failing alternator unable to maintain 13.8-14.4V during operation, starving the relay circuit
• Blown fuses in the ECM power supply circuit (typically 20-30A fuses in the main relay box)
• ECM internal fault causing excessive current draw or internal short (less common but critical on high-hour used machines)

How to Troubleshoot and Fix Code P0687 (1485-6)

Step 1: Verify Battery and Charging System Begin with a digital multimeter to measure battery voltage at rest (should read 12.4-12.8V) and while running (13.8-14.4V). Test battery load capacity using a load tester (minimum 600 CCA). Inspect all battery cable connections and clean corrosion from terminals using a wire brush and battery terminal cleaner. On used excavators, weakened batteries mask relay circuit problems.

Step 2: Locate and Inspect the ECM Power Relay The main ECM power relay is located in the electrical component box under the operator's cab, typically marked as "ECM-R" or "PCM Relay." Remove the relay and inspect the socket terminals for discoloration, corrosion, or carbon tracking. Use a relay tester or apply 12V directly to the coil terminals (85 and 86) while checking continuity between power terminals (30 and 87) with a multimeter—resistance should be less than 0.5 ohms when energized.

Step 3: Test Relay Control Circuit Voltage With the relay removed and ignition key in the ON position (engine not running), backprobe the relay socket terminal 85 (control signal from ECM) and measure voltage. You should see battery voltage (12V+) when the key is turned on. If voltage is absent, trace the control wire (typically green/white stripe) back to ECM connector pin B-22 checking for breaks, particularly at the harness grommet where it passes through the bulkhead near the fuel filter.

Step 4: Inspect Wiring Harness for Damage Physically trace the ECM power harness from the relay box to the ECM (mounted behind the right-side service panel). Pay special attention to areas where the harness contacts metal edges, especially near the hydraulic pump mounting bracket and engine lifting eyes—these are known wear points on used ZX240-5A models. Look for abraded insulation, exposed copper, or green corrosion inside connectors.

Step 5: Check ECM Power Supply and Ground At the ECM main connector, measure voltage at the power supply pin (typically pin A-1, red wire) with ignition ON—should show battery voltage. Test ground circuits at pins A-20 and A-21 (black wires) for less than 0.2 ohms resistance to chassis ground. Clean and apply dielectric grease to all ECM connector pins, as corrosion buildup is common on machines with 5,000+ operating hours.

Step 6: Clear Code and Verify Repair After replacing faulty components (relay, damaged wiring, or corroded connectors), use Hitachi Dr.EX diagnostic software or a compatible J1939 scan tool to clear the fault code. Cycle the ignition three times and monitor for code recurrence. Perform a voltage drop test across the relay under load (engine running, all accessories on) to ensure less than 0.5V drop, confirming adequate circuit integrity.

---

Disclaimer: This guide provides general diagnostic information for the Hitachi ZX240-5A excavator. Always consult the official service manual for your specific machine serial number and software version. If you lack experience with electrical diagnostics or the fault persists after basic troubleshooting, consult a certified Hitachi technician or qualified heavy equipment electrician to prevent further damage to expensive ECM components.