Starting a Vintage Caterpillar D4 Without a Pony Motor

[MikePhua]

The D4 and Its Historical Starting System
The Caterpillar D4 crawler tractor, first introduced in the 1930s, became a foundational machine in agricultural and construction sectors. Early models, particularly the 7U and 2T series, were equipped with gasoline-powered pony motors—small auxiliary engines used to crank the main diesel engine until it could fire under compression. This system was a practical solution before electric starters became reliable for large diesel engines.
Caterpillar sold tens of thousands of D4 units globally, and many remain in use today, especially in restoration circles and rural applications. The pony motor, while ingenious, is now a frequent point of failure due to age, parts scarcity, and maintenance complexity.
Terminology Annotation

• Pony Motor: A small gasoline engine used to start a larger diesel engine by spinning it up to cranking speed.
  
• Clutch Engagement: The act of connecting the pony motor’s output to the diesel engine flywheel via a mechanical clutch.
  
• Declutching: Disengaging the clutch to prevent power transfer, often used during shutdown or troubleshooting.
  

Alternative Starting Methods for a D4
When the pony motor is missing or non-functional, operators have explored several alternatives:

• Pull Starting: Using another machine to tow the D4 in gear, forcing the diesel engine to rotate and potentially fire.
  
• Electric Conversion: Replacing the pony motor with an electric starter motor and battery system.
  
• Auxiliary Engine Retrofit: Installing a modern small engine (e.g., Honda or Predator) to replicate the pony motor’s function.
  

Pull starting requires careful coordination. The D4 should be in high gear to maximize crank speed, and the operator must be ready to declutch immediately once the engine fires. This method is risky if the diesel engine has stuck clutches or unknown internal damage. A seasoned operator once recalled a D4 rolling down a hill in fifth gear and starting with just a puff of ether—proof that gravity and timing can substitute for gasoline.
Electric Starter Conversions and Retrofit Kits
Electric conversions have gained popularity among restorers and field operators. These kits typically include:

• High-torque 24V starter motor
  
• Custom flywheel adapter or ring gear
  
• Heavy-duty battery and wiring harness
  
• Starter solenoid and ignition switch
  

While not original, electric starters offer reliability and ease of use. Some conversions are even eligible for federal modernization grants in agricultural programs. A few Amish operators have creatively adapted Honda engines to serve as starter motors, preserving mechanical simplicity while bypassing the pony motor entirely.
Terminology Annotation

• Ring Gear: A toothed gear mounted on the flywheel that engages with the starter motor pinion.
  
• Solenoid: An electromechanical switch that controls high-current flow to the starter motor.
  

Safety Considerations During Pull Starts
Pull starting a D4 requires caution:

• Ensure both tracks are free and the machine rolls smoothly
  
• Use high gear to increase crank speed
  
• Have an experienced operator in the cab ready to declutch
  
• Keep a kill method ready in case the engine runs away or clutches are stuck
  

One operator emphasized the importance of multiple shutdown options—fuel cutoff, decompression lever, and clutch disengagement—especially when starting a machine with unknown mechanical condition.
Recommendations for Owners Without a Pony Motor
If your D4 lacks a pony motor and you need to start it:

• Assess whether the diesel engine rotates freely and has compression
  
• Consider electric starter conversion for long-term reliability
  
• Use pull start only if terrain and towing equipment are suitable
  
• Explore retrofit options using small gasoline engines with belt drives
  
• Consult antique Caterpillar forums and parts suppliers for rebuild kits
  

A restorer in California successfully started his hillside D4 using a short roll in fifth gear and a mist of starting fluid. He later installed a Predator engine with a belt drive to replicate the pony motor, allowing consistent starts without altering the tractor’s original look.
Conclusion
Starting a vintage Caterpillar D4 without its original pony motor is entirely feasible, but it requires mechanical creativity and respect for the machine’s design. Whether through pull starts, electric conversions, or auxiliary retrofits, operators have found ways to keep these legendary tractors running. The pony motor may be gone, but the D4’s legacy continues—one crank at a time.