What is Caterpillar Fault Code CID668?

Caterpillar CID668 indicates a malfunction in the Engine Speed/Timing Sensor Circuit, specifically detecting an abnormal frequency or signal inconsistency from the primary speed/timing sensor. This Diagnostic Trouble Code (DTC) is triggered when the Electronic Control Module (ECM) receives erratic, missing, or out-of-range signals from the sensor responsible for monitoring crankshaft position and engine timing.

This fault is critical because the speed/timing sensor provides essential data for fuel injection timing, engine speed regulation, and overall combustion efficiency. Without accurate sensor readings, the ECM cannot properly control fuel delivery, potentially causing performance issues, increased emissions, or engine protection shutdowns. On used Caterpillar excavators, this code often surfaces due to sensor degradation, wiring harness damage from vibration, or connector corrosion accumulated over thousands of operating hours.

Common Symptoms

• Check Engine Light illuminated on the instrument panel with possible derate mode activation
• Engine misfiring, rough idling, or inconsistent RPM fluctuations during operation
• Hard starting or extended cranking time, especially when the engine is cold
• Sudden engine stalling or unexpected shutdowns during normal operation
• Loss of power or failure to reach rated engine speed under load conditions

Potential Causes

The most common technical reasons for CID668 on used Caterpillar excavators include:

• Worn or failing speed/timing sensor due to heat exposure and vibration over extended service life
• Damaged wiring harness with frayed insulation, particularly at flex points near the engine block or bellhousing
• Corroded or loose electrical connectors at the sensor or ECM interface, especially in machines exposed to moisture
• Contaminated sensor face covered with metallic debris, oil residue, or dirt affecting magnetic field detection
• Incorrect air gap between the sensor tip and reluctor wheel (flywheel teeth), often from improper installation
• ECM software glitches or outdated calibration files requiring updates via Caterpillar Electronic Technician (Cat ET)

How to Troubleshoot and Fix Code CID668

Step 1: Visual Inspection and Connector Check Begin by locating the engine speed/timing sensor, typically mounted on the flywheel housing or front timing cover. Inspect the sensor connector for corrosion, bent pins, or moisture intrusion—common issues in used equipment. Check the wiring harness from sensor to ECM for chafing, cuts, or signs of rubbing against engine components or frame members.

Step 2: Sensor Resistance and Air Gap Measurement Disconnect the sensor and use a digital multimeter to measure resistance across the sensor terminals (typical specification: 200-1000 ohms, consult service manual for exact values). Verify the air gap between sensor tip and reluctor wheel teeth using feeler gauges—proper clearance is usually 0.020-0.050 inches (0.5-1.3mm). Excessive gap from worn mounts or improper installation causes weak signals.

Step 3: Live Data Analysis and Harness Continuity Connect Cat ET diagnostic software to monitor live sensor frequency during cranking (should read 100-300 Hz). Check for signal dropouts or erratic readings. Perform continuity and resistance tests on the harness wiring from sensor connector to ECM pins, looking for opens or shorts to ground. On used excavators, pay special attention to harness routing through high-vibration areas.

Step 4: Component Replacement and Verification If sensor tests fail specifications or harness damage is found, replace the faulty component using genuine Caterpillar OEM parts to ensure proper electromagnetic characteristics. After installation, clear codes with Cat ET, perform a stationary regeneration if required, and test under load conditions to confirm repair.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific model and serial number. Complex electrical diagnostics should be performed by certified technicians with proper diagnostic equipment.