What is Caterpillar 320GC Fault Code 1666-6?

Fault Code 1666-6 indicates the Engine Speed/Position Sensor 2 (Secondary) has a voltage above normal or shorted to high source. This diagnostic trouble code (DTC) specifically relates to the crankshaft position sensor circuit detecting an abnormally high voltage signal being sent to the Electronic Control Module (ECM).

On the Caterpillar 320GC excavator, this secondary speed sensor works in tandem with the primary sensor to provide redundant engine position data. The ECM uses these signals to precisely control fuel injection timing, monitor engine RPM, and regulate overall engine performance. When voltage exceeds manufacturer specifications (typically above 5 volts on the signal wire), the ECM triggers this fault code to protect the engine from potential timing errors or performance issues.

This code is critical because inaccurate crankshaft position data can lead to improper fuel delivery, reduced power output, and potential engine damage if left unaddressed.

Common Symptoms

• Check Engine Light illuminated on the instrument cluster with possible derate mode activation
• Engine performance issues including rough idle, hesitation, or difficulty starting
• Power loss or reduced hydraulic response during operation due to ECM limiting engine output
• Intermittent stalling or engine shutdown, particularly during cold starts or after extended operation
• Erratic tachometer readings or RPM gauge fluctuations on the display panel

Potential Causes

The most common technical causes for Code 1666-6 on used Caterpillar 320GC excavators include:

• Damaged or chafed wiring harness near the sensor mounting location where vibration causes wire insulation wear
• Corroded or moisture-contaminated sensor connectors, especially on machines operated in wet or humid environments
• Failed crankshaft position sensor with internal short circuit to the power supply wire
• Shorted signal wire to battery voltage anywhere along the harness route from sensor to ECM
• ECM internal fault causing false high voltage detection (less common but possible in older machines)
• Poor ground connection at the sensor or ECM causing voltage reference issues

How to Troubleshoot and Fix Code 1666-6

Step 1: Visual Inspection Begin by locating the Engine Speed/Position Sensor 2 on the flywheel housing (driver's side of engine block). Carefully inspect the sensor connector for corrosion, bent pins, or moisture intrusion. Check the wiring harness for obvious damage, particularly where it routes near moving components or sharp edges. On used excavators, pay special attention to harness chafing points near engine mounts and frame rails.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. A functioning sensor typically reads between 200-1000 ohms (consult your service manual for exact specifications). Next, check for unwanted voltage on the signal wire with the key on, engine off—it should read close to 5 volts with sensor disconnected. Any reading above 5.5 volts indicates a wiring short to power.

Step 3: Circuit and Harness Testing With the sensor disconnected, measure continuity between the signal wire and battery positive. Any continuity indicates a short to high source in the harness. Inspect the entire harness length, especially near the starter motor, alternator, and engine-to-frame flex points where wiring commonly rubs through insulation on older machines.

Step 4: Component Replacement If the sensor tests faulty, replace it with a genuine Caterpillar sensor to ensure proper voltage output characteristics. After replacement, clear codes using Caterpillar Electronic Technician (Cat ET) diagnostic software and perform a test run. If the harness is damaged, repair or replace the affected section with proper heat-shrink connectors and protective loom to prevent future issues.

Step 5: Verification After repairs, monitor the system with diagnostic software during operation to confirm voltage readings remain within 0.5-5.0 volt range during cranking and running conditions.

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Disclaimer: This guide provides general troubleshooting information for Caterpillar 320GC excavators. Always consult the manufacturer's service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs can cause additional damage or safety hazards.