TD-15 Ignition Coil Overheating and Spark Loss Diagnosis

[MikePhua]

The TD-15 and International Harvester’s Crawler Lineage
The TD-15 crawler tractor was produced by International Harvester beginning in the 1950s and continued through various iterations into the 1980s. Designed for heavy grading, logging, and construction, the TD-15 featured a robust undercarriage, torque converter transmission, and a choice of diesel or gasoline engines depending on the era. While most TD-15s were diesel-powered, some early models and retrofit projects used gasoline engines with conventional ignition systems, including coil-and-distributor setups.
International Harvester, founded in 1902, was a pioneer in agricultural and industrial machinery. The TD series became a staple in North American fleets, with the TD-15 offering a balance of power and maneuverability for mid-size dozing operations.
Ignition Coil Function and Thermal Behavior
In gasoline-powered engines, the ignition coil transforms low-voltage battery current into high-voltage pulses needed to fire the spark plugs. It operates by storing energy in a magnetic field and releasing it through the secondary winding when the primary circuit is interrupted by the breaker points or electronic control.
Key components:

• Primary winding (low voltage input)
  
• Secondary winding (high voltage output)
  
• Iron core for magnetic field generation
  
• Housing filled with oil or epoxy for cooling and insulation
  
• Terminal posts for battery and distributor connections
  

Under normal conditions, the coil may become warm during operation, but excessive heat—hot enough to burn skin or cause component failure—is a sign of electrical imbalance or internal breakdown.
Common Causes of Coil Overheating and No Spark
When a TD-15 ignition coil becomes dangerously hot and fails to produce spark, several root causes should be considered:

• Constant Voltage Supply
  • If the coil receives uninterrupted 12V without cycling, it overheats
    
  • Caused by stuck ignition switch, faulty ballast resistor, or bypass wiring
    
• Failed Ballast Resistor
  • Designed to reduce voltage after startup
    
  • If bypassed or shorted, coil receives full voltage continuously
    
• Incorrect Coil Type
  • Some coils are designed for use with resistors (e.g., 6V coils in 12V systems)
    
  • Using a resistor-required coil without one leads to overheating
    
• Shorted Primary Circuit
  • Points stuck closed or electronic module failure
    
  • Prevents coil from discharging, causing heat buildup
    
• Internal Coil Breakdown
  

• Insulation failure between windings
  
• Causes internal arcing and heat generation without spark output
  

A technician in Alberta once diagnosed a TD-15 with a coil too hot to touch and no spark. After tracing the wiring, he found the ballast resistor had been bypassed during a previous repair. Installing a proper resistor restored spark and reduced coil temperature to safe levels.
Diagnostic Strategy and Component Testing
To isolate the fault:

• Measure voltage at coil’s positive terminal with ignition on
  • Should be 6–9V if resistor is present, 12V only during cranking
    
• Check continuity across ballast resistor
  • Resistance should be 1.5–2.0 ohms
    
• Inspect breaker points or electronic ignition module
  • Points should open and close cleanly; module should pulse
    
• Test coil resistance
  • Primary: 0.5–1.5 ohms
    
  • Secondary: 6,000–12,000 ohms
    
• Look for melted insulation, oil leaks, or cracked housing
  

A restorer in Chile used a multimeter to test a TD-15 coil and found the secondary winding open. Replacing the coil with a resistor-compatible unit and verifying voltage restored full ignition function.
Replacement and Wiring Recommendations
When replacing the coil:

• Match coil type to system voltage and resistor configuration
  
• Use high-quality terminals and heat-resistant wire
  
• Mount coil away from exhaust or high-heat zones
  
• Verify correct wiring sequence:
  

• Battery → ignition switch → ballast resistor → coil (+)
  
• Coil (–) → points or ignition module → ground
  
• Coil tower → distributor cap center → spark plug wires
  

If upgrading to electronic ignition:

• Use coil rated for constant 12V input
  
• Eliminate ballast resistor if module is designed for full voltage
  
• Ensure module ground is clean and secure
  
• Test spark output with inline tester before final assembly
  

A contractor in Texas retrofitted his TD-15 with a Pertronix electronic ignition and a Flamethrower coil rated for 12V. The system ran cooler, started faster, and eliminated point maintenance.
Preventive Maintenance and Long-Term Reliability
To prevent future coil overheating:

• Inspect ignition wiring annually for corrosion or shorts
  
• Replace ballast resistor every 1,000 hours or during tune-up
  
• Use dielectric grease on terminals to prevent moisture intrusion
  
• Monitor coil temperature during operation—should be warm, not hot
  
• Keep coil mounting area clean and ventilated
  
• Document coil type and wiring diagram for future service
  

A fleet manager in Florida added coil temperature checks to his monthly inspection protocol and reduced ignition failures by 70% across his vintage equipment fleet.
Conclusion and Recommendations
Ignition coil overheating in the TD-15 is a symptom of electrical imbalance, component mismatch, or internal failure. With methodical diagnostics, proper replacement, and attention to system voltage, the issue can be resolved and prevented.
Recommendations include:

• Verify coil type and resistor compatibility before installation
  
• Test voltage and resistance across all ignition components
  
• Replace damaged or mismatched parts with OEM-grade equivalents
  
• Maintain clean, secure wiring and proper grounding
  
• Upgrade to electronic ignition if reliability is a priority
  

With the right approach, the TD-15’s ignition system can deliver consistent spark and dependable starts—keeping this classic crawler alive and working in the field.