The Enduring Role of Pony Motors in Heavy Equipment Starting Systems

[MikePhua]

Before the age of high-torque electric starters and sealed battery banks, pony motors were the backbone of diesel engine ignition in heavy machinery. These small gasoline engines, mounted alongside larger diesel powerplants, served as mechanical starters—spinning the main engine to life through direct coupling or gear reduction. Though largely phased out by modern systems, pony motors remain in service across vintage Caterpillar fleets and remote operations where reliability trumps convenience.
What Is a Pony Motor and Why Was It Used
A pony motor is a small internal combustion engine, typically gasoline-powered, used to start a larger diesel engine. In Caterpillar equipment from the 1930s through the 1960s, pony motors were standard on dozers, graders, and loaders. They were especially common on models like the D2, D6, D7, and D8.

• Starting Sequence: The operator would start the pony motor manually (often with a pull rope or electric starter), allow it to warm up, then engage a clutch or gear to turn over the diesel engine.
  
• Compression Release: Many systems included a decompression lever to reduce resistance during cranking.
  
• Ignition Transition: Once the diesel engine reached sufficient RPM, fuel injection and compression would take over, and the pony motor would be disengaged.
  

This method was reliable in cold climates and remote areas where batteries were prone to failure or difficult to maintain.
Terminology and Component Breakdown

• Starting Engine: Another term for pony motor, often used in technical manuals.
  
• Engagement Clutch: Mechanism that connects the pony motor to the diesel flywheel.
  
• Magneto Ignition: Self-contained ignition system used in pony motors, independent of battery power.
  
• Governor Linkage: Controls pony motor RPM and prevents over-speeding during engagement.
  
• Fuel Shutoff Solenoid: Used in later pony motor designs to control fuel flow electronically.
  

Common Issues and Field Repairs
Operators and mechanics report several recurring problems with pony motors:

• Fuel starvation due to clogged lines or deteriorated tanks
  
• Weak spark from worn magnetos or fouled plugs
  
• Sticking engagement clutches that prevent proper coupling
  
• Exhaust leaks that affect performance or cause overheating
  
• Oil contamination from shared lubrication systems or poor seals
  

A retired logger in northern Wisconsin recalled using a D6 with a pony motor during winter logging. He kept a coffee can over the exhaust to prevent snow from entering overnight. On cold mornings, he’d warm the magneto with a propane torch before pulling the rope—an improvised ritual that kept the old iron running for decades.
Modern Alternatives and Retrofit Options
While pony motors are rare in new equipment, some operators still prefer them for their simplicity and independence from electrical systems. However, retrofitting is increasingly common:

• Electric starter conversions using high-torque 24V motors and gear reduction kits
  
• Battery upgrades with sealed AGM or lithium units for cold-weather reliability
  
• Starter fluid injection systems for emergency ignition in diesel engines
  

Companies like Antique Caterpillar Machinery Owners Club and aftermarket suppliers offer rebuild kits, magneto parts, and clutch components for pony motor restoration.
Best Practices for Maintenance and Operation

• Drain fuel tanks seasonally to prevent varnish and water contamination
  
• Clean spark plugs and magneto contacts every 100 hours
  
• Inspect clutch linkage and bearings for wear or misalignment
  
• Use ethanol-free gasoline to preserve seals and carburetor integrity
  
• Keep a service log of starting engine performance and repairs
  

Conclusion
Pony motors may be relics of a bygone era, but they continue to serve in legacy fleets and remote operations where simplicity and durability are paramount. Their mechanical elegance and field-serviceable design make them a favorite among old-school mechanics and equipment historians. Whether preserved for nostalgia or relied upon for daily starts, pony motors remain a testament to the ingenuity of early diesel engineering.