Troubleshooting No Spark from Magneto on Caterpillar D6-9U

[MikePhua]

Overview
One common ignition-related issue in the Caterpillar D6-9U bulldozer concerns the magneto failing to produce spark, leading to engine start problems. Diagnosing and fixing magneto spark issues requires a systematic approach focusing on ignition components and internal timing.
Magneto Function

• The magneto generates high voltage to fire the spark plugs, independent of battery power, vital for diesel engine ignition.
  
• It is mechanically timed to the engine via a drive gear and cam, ensuring spark delivery at precise piston positions.
  

Common Causes of No Spark

• Faulty magneto points or condenser due to wear or corrosion.
  
• Broken or worn impulse coupling spring delaying or preventing spark generation.
  
• Loose, corroded, or broken high-tension wires and spark plug connections.
  
• Incorrect magneto timing caused by wear on cam followers or incorrect installation.
  
• Faulty magneto switch or wiring issues.
  

Step-by-Step Diagnostic Procedures

• Manually turn the magneto coupling clockwise and observe for spark at the points.
  
• Check the coil brace plate for spark by placing a finger briefly to sense voltage arcing (exercise caution).
  
• Remove and inspect spark plugs for fouling: lead, fuel, or oil contamination can inhibit spark.
  
• Perform a cold cylinder test by warming the engine, running at rough idle, and feeling cylinder head temperatures to locate non-firing cylinders.
  
• Measure resistance of spark plugs with an ohm meter; values above recommended thresholds indicate faulty plugs.
  
• Inspect ignition harness and coil springs for signs of arcing, wear, or damage.
  
• Remove and temporally disable the p-lead (magneto energizing wire) to isolate magneto from control switches.
  
• Check magneto to engine timing using manufacturer specifications and timing marks.
  
• Examine impulse coupling for spring integrity and function.
  

Maintenance Recommendations

• Regularly clean and gap spark plugs to manufacturer specs (.015” to .019” is common).
  
• Inspect and replace magneto points, condensers, and coils as necessary.
  
• Apply proper lubrication to cam and cam follower to prevent premature timing wear.
  
• Use high-quality replacement parts, preferably OEM.
  
• Avoid running the engine with defective ignition to prevent damage.
  

Terminology

• Magneto: Engine-driven device producing ignition spark independent of battery power.
  
• Impulse Coupling: A spring-loaded mechanism causing the magneto to fire at low engine speeds.
  
• P-Lead: Wire supplying magneto power contact controlled by ignition switch.
  
• Points: Electrical contacts within the magneto that open and close to generate spark.
  
• Cold Cylinder Test: Technique to identify misfiring cylinders by temperature differences.
  

Real-World Example
An operator struggling with no-start issues on a D6-9U found that the magneto's impulse coupling spring was broken, causing severely retarded ignition timing and no spark at low RPMs. After replacing the spring and adjusting magneto timing, the bulldozer started reliably and performed smoothly.
Conclusion
Diagnosing no spark problems in the Cat D6-9U magneto system requires careful inspection of ignition components, wiring, and timing settings. Preventive maintenance including spark plug care and magneto servicing significantly reduces unexpected downtime. Proper understanding and following manufacturer procedures ensure long-term ignition reliability in historic crawler tractors.