What is Komatsu PC160LC-7 Fault Code S-4?

Fault Code S-4 on the Komatsu PC160LC-7 indicates a malfunction in the engine preheating system circuit, specifically related to the glow plug relay or its associated wiring harness. This code triggers when the Engine Control Module (ECM) detects abnormal voltage or current flow in the preheating circuit during cold start sequences.

The preheating system is critical for reliable cold-weather starting on this model's SAA4D102E-2 diesel engine. When S-4 activates, the ECM registers that the glow plug relay isn't energizing properly, preventing adequate cylinder head warming before ignition. For used excavators operating in cold climates, this fault can mean extended cranking times, rough starts, and accelerated engine wear due to insufficient combustion chamber temperature.

Common Symptoms

Operators experiencing Code S-4 typically notice:

• Prolonged cranking time during cold starts, especially below 40°F (5°C)
• Glow plug indicator light flashing or remaining illuminated on the instrument cluster
• White or gray exhaust smoke during initial startup due to incomplete combustion
• Rough idle for the first 30-60 seconds after cold starting
• Hard starting or no-start conditions in freezing temperatures despite adequate battery voltage

Potential Causes

The most frequent technical causes for Code S-4 in used PC160LC-7 machines include:

• Failed glow plug relay (Komatsu P/N 600-815-8940) due to contact corrosion or coil burnout after 5,000+ operating hours
• Damaged wiring harness between the ECM and relay, particularly at the frame rail routing point where cable ties degrade and allow harness chafing
• Corroded connector terminals at the X2 connector behind the left-side battery box—a known moisture intrusion point on older units
• Faulty individual glow plugs creating excessive current draw that trips relay protection
• ECM internal relay driver circuit failure (less common, typically requires ECM replacement)

How to Troubleshoot and Fix Code S-4

Step 1: Visual Inspection Begin by inspecting the glow plug relay mounted on the left frame rail near the fuel filter. Check for corrosion, burnt terminals, or oil contamination. On used excavators, examine the wiring harness routing for obvious damage, abrasion against frame edges, or deteriorated protective conduit—common after 8,000+ hours.

Step 2: Electrical Testing Using a digital multimeter, test relay coil resistance (should read 70-90 ohms). With ignition on but engine off, check for 12V supply voltage at relay terminal 85. Verify ground continuity at terminal 86 (less than 1 ohm resistance to chassis ground). Test relay output at terminal 87—should show battery voltage when relay activates during preheat cycle.

Step 3: Connector and Glow Plug Testing Disconnect the X2 harness connector and inspect pins for green corrosion (indicating moisture intrusion). Clean with electrical contact cleaner and apply dielectric grease. Test each glow plug resistance individually—spec is 0.5-2.0 ohms. Replace any glow plug reading open circuit or above 3 ohms, as failed units cause excessive current draw triggering S-4.

Step 4: ECM Communication Check Connect Komatsu diagnostic software (KOMTRAX Plus) to verify the ECM is commanding relay activation. If software shows proper ECM output but relay doesn't energize, suspect internal ECM driver circuit failure—requires ECM repair or replacement.

For used machines, always inspect harness routing and strain relief before replacing components, as physical wear is the leading cause of electrical faults in high-hour excavators.

---

Disclaimer: This guide provides general troubleshooting information. Always consult the Komatsu PC160LC-7 shop manual and consider professional diagnostic services for complex electrical issues or ECM replacement procedures.