Sumitomo SH80-6B Fault Code P1620-12: Complete Diagnostic Guide

What is Sumitomo SH80-6B Fault Code P1620-12?

Fault Code P1620-12 indicates a short circuit to ground in the Engine Control Module (ECM) power relay circuit. This code specifically signals that the ECM relay coil or its associated wiring has detected an abnormal electrical path directly to ground, preventing proper voltage supply to critical engine control systems.

In the Sumitomo SH80-6B excavator, this fault affects the Isuzu diesel engine's electronic control system. The ECM power relay serves as the primary switching device that energizes the entire engine management system. When this circuit shorts to ground, the ECM may lose power intermittently or completely, causing immediate engine performance issues or preventing startup altogether. This is critical because without proper ECM voltage, fuel injection timing, throttle response, and emissions controls cannot function correctly.

Common Symptoms

• Engine cranks but fails to start or starts then immediately shuts down
• Check Engine Light (CEL) illuminated on the instrument cluster with no engine response
• Complete loss of electronic throttle control, making the excavator unresponsive to accelerator input
• Intermittent power loss during operation, especially when traveling over rough terrain (indicating vibration-related wiring issues)
• Battery drain when the machine is parked, as the shorted relay may remain energized

Potential Causes

The most common causes of P1620-12 on used SH80-6B excavators include:

• Damaged wiring harness near the engine firewall where the relay harness passes through metal grommets (known rub point on this model)
• Corroded or moisture-damaged relay connector pins in the engine compartment fuse box
• Failed ECM power relay with internal coil short to the mounting bracket
• Chafed wires along the right-side engine mount area where harnesses contact the frame during engine movement
• Aftermarket electrical accessories improperly tapped into the relay circuit
• Water intrusion into the main engine harness connector behind the battery box (common in machines used in wet conditions)

How to Troubleshoot and Fix Code P1620-12

Step 1: Visual Inspection Begin with the fuse/relay panel located under the right-side access cover. Remove the ECM power relay (typically a 4-pin or 5-pin relay labeled "ENG" or "ECU") and inspect the connector for green corrosion, bent pins, or moisture. Check the relay mounting area for ground shorts caused by damaged relay cases.

Step 2: Electrical Testing Using a digital multimeter, test the relay coil resistance. With the relay removed, measure between terminals 85 and 86 (coil circuit). You should see 60-120 ohms. Next, check for short to ground: with the ignition OFF and relay removed, measure resistance from relay socket terminal 85 to chassis ground. You should read infinite resistance (open circuit). Any reading below 10k ohms indicates a harness short.

Step 3: Harness Inspection for Used Machines Trace the relay control wire (usually a thin blue/white or yellow wire) from the fuse box toward the ECM. On used excavators, carefully inspect where this harness passes through the firewall grommet and near the engine mounts. Look for wire insulation wear, melted spots, or exposed copper. Flex the harness while monitoring resistance to ground—intermittent shorts will show changing readings.

Step 4: Repair and Verification Repair any damaged wiring using heat-shrink solder connectors and protective loom. If the relay tested faulty, replace with OEM Sumitomo/Isuzu relay (part number should match existing relay). After repairs, clear the code using diagnostic software or by disconnecting the battery for 10 minutes, then test-run the machine under load.

For used excavators, always apply dielectric grease to repaired connectors to prevent future corrosion, especially in high-moisture operating environments.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Sumitomo service manual for your specific serial number range and consider professional diagnosis for complex electrical issues. Improper electrical repairs can cause ECM damage or safety hazards.