What is Case General Fault Code 108?

Case Fault Code 108 indicates a Hydraulic Oil Temperature Sensor Circuit Low Voltage condition. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) or machine controller detects an abnormally low voltage signal from the hydraulic oil temperature sensor, typically below 0.2-0.5 volts depending on the specific Case model.

This sensor plays a critical role in monitoring hydraulic system health by measuring oil temperature and sending real-time data to the ECM. When hydraulic oil operates outside optimal temperature ranges, the system's efficiency degrades significantly, potentially causing component damage, reduced lifting capacity, and accelerated wear on pumps, valves, and cylinders. On used excavators, this code often surfaces due to sensor degradation, corroded wiring, or connector issues rather than actual hydraulic problems.

Common Symptoms

When Fault Code 108 is active, operators typically experience:

• Illuminated warning light on the instrument cluster, often amber or red depending on severity
• Erratic hydraulic temperature gauge readings or gauge stuck at minimum/cold reading
• Reduced hydraulic performance as the ECM may limit flow rates to protect the system
• Intermittent code activation, especially during cold starts or when operating in wet conditions
• No actual hydraulic overheating, despite the system triggering temperature-related warnings

Potential Causes

The most common technical reasons for Code 108 on used Case excavators include:

• Shorted or grounded sensor circuit caused by damaged wiring insulation, particularly at harness flex points near the hydraulic tank
• Failed hydraulic oil temperature sensor with internal short circuit or contaminated sensing element
• Corroded or moisture-infiltrated connectors at the sensor plug or ECM connection points
• Chafed wiring harness rubbing against frame members, hydraulic lines, or sharp edges—common wear pattern on machines with 3,000+ hours
• ECM internal fault or poor ground connection affecting sensor voltage supply (less common)

How to Troubleshoot and Fix Code 108

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor, typically mounted on the hydraulic tank or return line manifold. Inspect the entire wiring harness from sensor to ECM for visible damage, abrasion, pinch points, or oil contamination. On used excavators, pay special attention to areas where harnesses route near moving components or sharp metal edges.

Step 2: Connector and Resistance Testing Disconnect the sensor connector and inspect terminals for corrosion, bent pins, or moisture. Using a digital multimeter, measure sensor resistance across its terminals—typical values range from 1,000-3,000 ohms at room temperature (consult your specific Case service manual). A reading near zero ohms confirms a shorted sensor.

Step 3: Circuit Voltage Testing With the sensor disconnected and ignition on, measure voltage at the harness-side connector. You should see approximately 5 volts supply voltage from the ECM. No voltage indicates an open circuit or ECM issue. Reconnect and backprobe the circuit while monitoring voltage—readings consistently below 0.5 volts confirm the low voltage condition.

Step 4: Repair or Replace Components If wiring shows damage, repair using proper gauge wire and weatherproof heat-shrink connectors. Replace the sensor if resistance values are out of specification. Clear codes using Case diagnostic software (ServiceMaster or compatible scan tool) and verify repair by monitoring live sensor data during operation.

Disclaimer: This guide provides general troubleshooting information. Always consult your Case service manual and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with excavator systems.