Komatsu PC128US-2 Fault Code E108: Technical Guide

What is Komatsu PC128US-2 Fault Code E108?

Fault Code E108 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading on the Komatsu PC128US-2 excavator. This code triggers when the Engine Control Module (ECM) detects voltage readings outside the expected range from the coolant temperature sensor, typically indicating an open circuit, short circuit, or implausible temperature signal.

The coolant temperature sensor is critical for this machine's SAA4D95LE-3 engine performance. It directly influences fuel injection timing, engine warm-up protocols, and hydraulic system efficiency. On used PC128US-2 models, this sensor failure can lead to poor fuel economy, difficulty starting in cold conditions, and premature component wear due to improper engine management.

Common Symptoms

When Code E108 is active, operators typically experience:

• Engine warning light or malfunction indicator illuminated on the monitor panel
• Hard starting conditions, especially in cold weather, with extended cranking times
• Engine running rough during warm-up phase or failing to reach optimal operating temperature
• Reduced hydraulic performance due to ECM limiting engine output in protective mode
• Excessive white smoke from exhaust during startup indicating rich fuel mixture compensation

Potential Causes

The most common technical causes for E108 on used PC128US-2 excavators include:

• Faulty coolant temperature sensor due to internal element degradation (common after 5,000+ hours)
• Damaged wiring harness at known rub points near the engine mount bracket and valve cover
• Corroded or loose connector pins at the sensor plug, especially on machines operating in wet/dusty conditions
• ECM internal fault affecting the sensor circuit reference voltage (less common)
• Coolant contamination causing sensor element coating and inaccurate readings
• Previous repair damage to harness insulation from improper routing on used machines

How to Troubleshoot and Fix Code E108

Step 1: Visual Inspection Locate the coolant temperature sensor on the cylinder head (driver's side, near thermostat housing). Inspect the connector for corrosion, bent pins, or moisture intrusion. Check the wiring harness from sensor to ECM for chafing, especially where it contacts the engine block or crosses metal edges—a common failure point on used excavators.

Step 2: Electrical Testing Disconnect the sensor connector. Using a digital multimeter, measure sensor resistance at various temperatures (approximately 2,500 ohms at 68°F/20°C). Check for 5-volt reference supply from the ECM at the harness connector with ignition on. Verify ground circuit continuity (should be less than 1 ohm resistance to chassis ground).

Step 3: Component Replacement If sensor resistance values are out of specification or wiring shows damage, replace the defective component. For used machines, always apply dielectric grease to connector terminals and ensure proper harness routing with new protective loom where friction points exist. Clear codes using Komatsu diagnostic software and perform a test cycle.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number and consider professional diagnostic services for complex electrical issues.