Komatsu PC110-7 Fault Code S-9: Complete Diagnostic Guide

What is Komatsu PC110-7 Fault Code S-9?

Fault Code S-9 on the Komatsu PC110-7 indicates a malfunction in the engine control system, specifically related to the engine controller (ECM) detecting an abnormal signal from the throttle motor or its associated circuit. This code triggers when the ECM cannot properly control engine speed due to communication errors, electrical issues, or mechanical failures within the throttle motor assembly.

The throttle motor plays a critical role in governing engine RPM and load management on the PC110-7's SAA4D95LE-5 engine. When S-9 activates, the excavator's performance and fuel efficiency are directly compromised. This code is particularly important for used machines, as wear on electrical connectors and aging wiring harnesses commonly trigger false positives or legitimate failures in older equipment.

Common Symptoms

When Code S-9 is active, operators typically experience:

• Erratic engine speed: RPM fluctuations or inability to maintain consistent idle
• Reduced power output: Engine may enter derate mode, limiting maximum RPM to protect systems
• Warning light activation: Engine malfunction lamp illuminates on the instrument panel
• Poor throttle response: Delayed or unresponsive reaction to control inputs
• Intermittent stalling: Engine may shut down unexpectedly, especially under load conditions

Potential Causes

The most common technical causes for Code S-9 on used PC110-7 excavators include:

• Throttle motor failure: Internal motor winding breakdown or mechanical binding from accumulated debris
• Wiring harness damage: Chafing at known rub points near the engine mounting brackets or along the right-hand frame rail
• Connector corrosion: Moisture intrusion in the 13-pin throttle motor connector causing high resistance
• ECM voltage issues: Low battery voltage or poor grounding at the ECM mounting location
• Sensor feedback failure: Throttle position sensor (TPS) internal to the throttle motor providing inconsistent signals
• Software glitches: Rare ECM firmware issues requiring reprogramming with Komatsu KOMTRAX or diagnostic software

How to Troubleshoot and Fix Code S-9

Step 1: Visual Inspection Begin by examining the throttle motor assembly mounted on the fuel injection pump. Check for obvious physical damage, oil contamination, or loose mounting bolts. Inspect the wiring harness from the ECM to the throttle motor, paying special attention to areas where the harness contacts metal surfaces or moves with engine vibration.

Step 2: Electrical Testing Using a digital multimeter, disconnect the throttle motor connector and check for supply voltage (should read approximately 8-12V with key on, engine off). Test ground continuity to the ECM—resistance should be less than 1 ohm. Measure throttle motor winding resistance (typically 10-30 ohms between pins); values outside this range indicate motor failure.

Step 3: Signal Analysis Connect Komatsu diagnostic software (or compatible diagnostic tool) to monitor live throttle motor position feedback. Command the throttle to various positions and verify smooth, proportional response. Erratic readings or failure to respond confirms throttle motor or TPS malfunction.

Step 4: Component Replacement For used excavators, cleaning corroded connectors with electrical contact cleaner and applying dielectric grease often resolves intermittent S-9 codes. If testing confirms failure, replace the throttle motor assembly as a complete unit (Komatsu Part# 7834-27-2002). After replacement, perform ECM relearn procedures to calibrate the new motor.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number and consider professional diagnosis for complex electrical issues. Improper repairs may cause additional damage or safety hazards.