Komatsu PC750LC-7 Fault Code H-27: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code H-27?

Fault Code H-27 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC750LC-7 excavator. This code is triggered when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, typically indicating an open circuit, short circuit, or sensor reading outside the normal operating range (usually -40°C to 150°C).

This fault is critical for the PC750LC-7 because the hydraulic system relies on accurate temperature monitoring to prevent overheating, maintain optimal viscosity, and protect expensive hydraulic components. The ECM uses this data to adjust pump output and activate cooling systems when necessary. Ignoring this code can lead to reduced hydraulic performance, accelerated component wear, or catastrophic hydraulic system failure.

Common Symptoms

When Code H-27 is active, operators typically experience:

• Hydraulic oil temperature warning light illuminated on the monitor panel
• Reduced hydraulic response or sluggish boom/arm/bucket movements due to ECM entering protective derate mode
• Inaccurate temperature readings displayed on the monitor (showing extremely high, low, or erratic values)
• Intermittent alarm sounds when the system attempts to read sensor data
• Potential automatic shutdown in severe cases if the ECM cannot verify safe operating temperatures

Potential Causes

The most common technical reasons for H-27 in used PC750LC-7 excavators include:

• Failed hydraulic oil temperature sensor due to age, thermal cycling, or internal element degradation
• Damaged or corroded wiring harness at known rub points near the hydraulic tank or along the chassis frame
• Loose or corroded electrical connectors at the sensor or ECM interface, especially in machines exposed to moisture
• Broken or chafed wires in the sensor circuit caused by vibration or improper previous repairs
• ECM internal fault (rare) affecting the temperature sensor input circuit
• Poor ground connections in the sensor circuit causing voltage irregularities

How to Troubleshoot and Fix Code H-27

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor (typically mounted on the hydraulic tank). Inspect the sensor connector for corrosion, bent pins, or moisture intrusion. On used excavators, check the wiring harness along its entire length for abrasion points, especially where it contacts metal edges or moving components.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at various temperatures (consult Komatsu specifications, typically 200-400 ohms at 20°C). Check for continuity in the wiring from sensor to ECM. Measure voltage supply at the sensor connector (should be approximately 5V reference voltage from ECM).

Step 3: Component Replacement and Verification If the sensor tests outside specifications, replace it with a genuine Komatsu part (part number 7861-93-3320 or equivalent). For used machines, always apply dielectric grease to connectors to prevent future corrosion. Clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, then operate the machine through a full work cycle to verify the repair.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC750LC-7 service manual and consider professional assistance from certified Komatsu technicians for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.