What is Caterpillar Fault Code SPN679?

SPN679 indicates a problem with the Starter Motor Control Circuit, specifically detecting an abnormal voltage or current condition in the starting system. This Suspect Parameter Number (SPN) is part of the J1939 diagnostic protocol used across Caterpillar equipment and monitors the electrical pathway between the Electronic Control Module (ECM) and the starter motor relay or solenoid.

The starting circuit is critical for engine initialization on Caterpillar excavators. When SPN679 triggers, the ECM has detected either an open circuit, short to ground, short to power, or excessive resistance in the starter control wiring. This fault can prevent the engine from cranking or cause intermittent starting failures, directly impacting productivity on job sites. For used excavators, this code often points to age-related electrical degradation rather than component failure.

Common Symptoms

When SPN679 is active, operators typically experience:

• Engine fails to crank when the key is turned, or cranking is intermittent and unreliable
• Check Engine light or malfunction indicator lamp illuminated on the dashboard
• Clicking sound from the starter relay without engine turnover
• No response from the starter motor despite battery voltage being adequate
• Stored fault code in the ECM memory even after successful starts (indicating intermittent issue)

Potential Causes

The most common technical causes for SPN679 on used Caterpillar excavators include:

• Corroded or loose connections at the starter relay, solenoid terminals, or ECM connector pins (extremely common on machines with 3,000+ hours)
• Damaged wiring harness due to vibration, rubbing against frame components, or rodent damage in the engine bay
• Failed starter relay or solenoid with internal contact degradation or coil resistance issues
• ECM internal driver circuit failure affecting the starter control output (less common but possible)
• Voltage drop issues caused by poor grounding points or battery cable deterioration
• Water intrusion into connectors from pressure washing or environmental exposure

How to Troubleshoot and Fix Code SPN679

Step 1: Visual Inspection and Connector Check Begin by inspecting all connections in the starter circuit. Remove and clean the starter relay connector, solenoid terminals, and ECM harness connections related to the starting circuit. Look for green corrosion, bent pins, or moisture. On used excavators, pay special attention to harness routing near the engine mounts where vibration causes wire chafing.

Step 2: Electrical Testing with Multimeter Using a digital multimeter, check for battery voltage (typically 12-24V depending on system) at the starter relay when the key is in the start position. Test the ECM control signal at the relay coil—you should see voltage present during cranking attempt. Check ground continuity from the relay mounting point to the battery negative terminal; resistance should be less than 0.5 ohms.

Step 3: Component Testing Test the starter relay/solenoid by measuring coil resistance (typically 50-150 ohms) and performing a bench activation test. If the relay clicks but doesn't pass current, internal contacts have failed. For used machines, also inspect the starter motor draw with an inductive ammeter—excessive amperage may indicate a failing starter causing the ECM to detect abnormal circuit conditions.

Step 4: Advanced Diagnostics Connect Caterpillar Electronic Technician (Cat ET) software to monitor real-time starter circuit voltage and ECM commands. This reveals intermittent faults that basic testing misses. Check for software updates to the ECM that may address known starting circuit monitoring issues.

Step 5: Harness and ECM Verification If all components test properly, inspect the entire engine harness for hidden damage, especially at flex points and where the harness passes through bulkheads. In rare cases, internal ECM driver circuit failure requires module replacement or repair by a qualified electronics technician.

---

Disclaimer: This guide provides general diagnostic information for SPN679. Always consult the specific service manual for your Caterpillar model and consider professional diagnostic assistance for complex electrical issues, especially when working with used equipment that may have undocumented modifications.