Caterpillar Excavator Fault Code 677-5: Complete Diagnostic & Repair Guide

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What is Caterpillar Fault Code 677-5?

Caterpillar Fault Code 677-5 indicates a "Engine Prelube Motor Relay — Current Below Normal," meaning the Engine Control Module (ECM) has detected an open circuit or insufficient current flow in the prelube motor relay control circuit.

This fault targets the engine prelube system, which pressurizes engine oil before cranking to protect critical internal components from dry-start wear. On used Caterpillar equipment, this system is especially vital — neglecting it accelerates bearing and camshaft wear significantly.

The ECM commands the prelube relay to energize before each start cycle. When current draw falls below the expected threshold, the ECM logs a 677-5 active fault and may inhibit normal engine start sequences.

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Common Symptoms

Operators and technicians typically report the following when this code is active:

• MID/Warning lamp illuminated on the instrument cluster or Cat ET diagnostic screen
• Engine fails to crank or delays cranking due to prelube cycle interruption
• Active fault logged in the ECM event history with no physical prelube motor operation
• Intermittent starting issues, particularly in cold ambient temperatures
• Possible derate or no-start condition depending on machine configuration and ECM programming

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Potential Causes

The most common technical causes behind a 677-5 fault include:

• Open circuit in the prelube relay control wiring between the ECM and the relay coil
• Failed or burned-out prelube motor relay with an open coil internally
• Corroded or damaged connector pins at the relay socket or ECM harness connector
• Broken or chafed wiring harness, particularly common where harnesses route near frame edges or engine mounts on older machines
• Faulty ECM output driver, though this is the least common cause and should only be suspected after all wiring is verified

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How to Troubleshoot and Fix Code 677-5

Step 1 — Gather Tools & Access Schematics Use Cat Electronic Technician (Cat ET) software to confirm the active fault and check ECM parameter data. Pull the machine's wiring schematic for the prelube relay circuit. A quality digital multimeter is essential throughout this process.

Step 2 — Inspect the Harness and Connectors On used excavators, harness wear and corrosion are primary suspects. Physically trace the wiring from the ECM connector to the prelube relay socket. Look for chafing, heat damage, or cracked insulation at known rub points near the engine block and firewall bulkhead. Clean all connector pins with electrical contact cleaner and inspect for bent or corroded terminals.

Step 3 — Test the Relay and Circuit Remove the prelube relay and measure coil resistance across terminals — a healthy relay coil typically reads 70–90 ohms. An open reading confirms relay failure. Next, with the relay removed, back-probe the ECM output wire and measure voltage during a commanded prelube cycle via Cat ET. You should see battery voltage (approximately 24V) if the ECM output is functioning correctly.

Step 4 — Repair or Replace Replace any failed relay, repair damaged wiring, or re-pin corroded connectors. Clear the fault using Cat ET, perform a prelube function test, and confirm the code does not return.

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Disclaimer: This guide is intended for qualified heavy equipment mechanics and experienced technicians. Always consult official Caterpillar service documentation and a certified Cat dealer for complex repairs. Improper diagnosis or repairs may result in equipment damage, personal injury, or voided warranties.