Caterpillar Excavator Fault Code E678: Meaning, Symptoms & Fix

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What is Caterpillar Fault Code E678?

Caterpillar Fault Code E678 indicates an abnormal signal detected in the Engine Speed/Timing Sensor circuit — specifically related to the secondary engine speed sensor (crankshaft position sensor) receiving an erratic or out-of-range signal.

This code is triggered when the ECM (Engine Control Module) detects that the sensor signal deviates from expected parameters during engine operation. On Caterpillar machines, the engine speed and timing sensors are critical for precise fuel injection timing and overall engine management.

When this fault is active, the ECM cannot accurately calculate crankshaft position, which directly compromises combustion efficiency, power output, and emissions compliance.

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Common Symptoms

When Fault Code E678 is active on your Caterpillar excavator, operators typically experience:

• Warning lamp illumination on the instrument cluster (yellow or red alert depending on severity)
• Engine derating — noticeable reduction in power output and throttle response
• Rough idle or erratic engine RPM, particularly during startup or low-load operation
• Hard starting or extended crank times before the engine fires
• Automatic engine shutdown in severe cases where the ECM loses signal entirely

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Potential Causes

The following are the most common technical causes for E678, especially on used or high-hour excavators:

• Failed or degraded crankshaft position sensor — common on machines with 5,000+ hours
• Damaged wiring harness — chafing, rubbing against the engine block, or heat damage near the exhaust manifold
• Corroded or loose sensor connector — moisture ingress is a leading cause on older machines
• Incorrect sensor air gap — sensor physically shifted due to vibration or improper previous installation
• ECM internal fault — rare but possible on heavily used machines with prior electrical issues

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How to Troubleshoot and Fix Code E678

Step 1 — Visual Inspection First Before using any diagnostic tool, physically inspect the crankshaft position sensor harness for signs of abrasion, melting, or connector corrosion. On used excavators, pay close attention to harness routing near the flywheel housing — a known friction point on many Caterpillar platforms.

Step 2 — Connector and Voltage Check Using a multimeter, check the sensor supply voltage at the connector. Caterpillar typically specifies 5V reference voltage to the sensor. Also verify signal voltage fluctuates appropriately during cranking (approximately 0.5V–4.5V). Resistance across the sensor itself should match OEM specifications (consult the service manual for your specific model).

Step 3 — Diagnostic Software Scan Connect Caterpillar ET (Electronic Technician) software to read active and logged fault codes, check live sensor data, and perform guided diagnostics. This confirms whether the fault is intermittent or constant — critical for used machines with unknown service histories.

Step 4 — Sensor Replacement and Gap Verification If the sensor tests out of specification, replace it with a genuine or OEM-equivalent part. Verify the air gap between the sensor tip and reluctor wheel per Caterpillar specifications (typically 0.5–1.5mm).

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Disclaimer: This guide is intended for informational purposes. Always consult a certified Caterpillar dealer or licensed heavy equipment mechanic before performing repairs. Incorrect diagnosis or repair may result in further mechanical damage or safety hazards.