Hitachi ZH200-5A Fault Code P0687: Complete Diagnostic Guide

What is Hitachi ZH200-5A Fault Code P0687?

Fault Code P0687 indicates an ECM/PCM Power Relay Control Circuit Low voltage condition. This diagnostic trouble code (DTC) signals that the Engine Control Module (ECM) has detected abnormally low voltage in the power relay control circuit that supplies critical voltage to the ECM and fuel injection system.

In the Hitachi ZH200-5A excavator, this code specifically relates to the main power relay that energizes the ECM after the key switch is turned on. This relay acts as a gatekeeper for the engine management system, and when the ECM detects insufficient voltage (typically below 10 volts) in this control circuit, it logs P0687. This fault is critical because without proper relay function, the fuel injection solenoids, turbocharger controls, and emission system components may not receive adequate power, potentially causing the engine to run poorly or fail to start completely.

Common Symptoms

• Hard starting or no-start condition, especially during cold starts or after the machine has sat idle
• Intermittent engine stalling during operation, particularly at idle or during low-load conditions
• Check Engine light or malfunction indicator lamp (MIL) illuminated on the instrument cluster
• ECM fault indicator flashing on the multi-function display panel
• Loss of engine power or failure to reach full RPM under load conditions

Potential Causes

The most frequent causes of P0687 in used ZH200-5A excavators include:

• Failed main power relay (common failure point after 5,000+ operating hours due to contact wear)
• Corroded or loose connections at the relay socket or ECM power supply harness
• Damaged wiring harness between the key switch and ECM power relay, particularly at known rub points near the engine mounting brackets and along the right-side frame rail
• Weak or failing battery unable to maintain voltage above 12.5V during cranking (critical in older machines)
• Faulty ignition switch not providing full voltage to the relay control circuit
• ECM internal power supply circuit failure (less common, but possible in high-hour machines with voltage spike history)
• Blown fuse in the ECM power supply circuit (typically a 15A or 20A fuse in the main fuse box)

How to Troubleshoot and Fix Code P0687

Step 1: Perform Initial Voltage Verification

Using a digital multimeter (DMM), measure battery voltage with the key off (should read 12.4-12.8V) and during cranking (should not drop below 10V). If battery voltage is weak, load test the battery and charging system before proceeding. On used excavators, battery cable corrosion at terminals is extremely common—clean and tighten all connections.

Step 2: Inspect the Main Power Relay and Socket

Locate the ECM power relay in the main relay box (typically mounted on the right side of the engine compartment near the ECM). Remove the relay and inspect the socket terminals for corrosion, discoloration, or damage. Check relay resistance: with the key off, measure resistance between terminals 85 and 86 (coil side)—should read 70-90 ohms. Measure continuity between terminals 30 and 87 with 12V applied to the coil—should show less than 0.5 ohms resistance.

Step 3: Check Wiring Harness and Control Circuit

With the key in the ON position (engine not running), backprobe the relay control wire (typically pin 85) and verify it receives 12V+ from the ignition switch. Check for voltage drops across connectors—any drop exceeding 0.2V indicates resistance from corrosion. Inspect the harness routing along known wear points: on the ZH200-5A, the engine harness often rubs against the hydraulic pump mounting bracket—check for abraded insulation and exposed copper.

Step 4: Verify ECM Power Supply

With the relay installed and key ON, measure voltage at the ECM power supply pins (consult wiring diagram for exact pin locations). You should see battery voltage (12V+). If relay clicks but no voltage appears at the ECM, check the fuse link and wiring between relay and ECM connector C1. On used machines, moisture intrusion at the ECM connector is common—inspect for green corrosion on pins and apply dielectric grease after cleaning.

Step 5: Clear Code and Test

After repairs, clear the fault code using Hitachi diagnostic software (Dr.EX) or a compatible scanner. Perform a key cycle test (ON-OFF-ON three times) and monitor for code recurrence. Run the machine under load for at least 20 minutes to verify the fix holds under operating conditions.

---

Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the Hitachi ZH200-5A service manual for model-specific procedures, torque specifications, and safety precautions. If you're uncomfortable performing electrical diagnostics, contact a certified Hitachi dealer or qualified heavy equipment technician.