Upgrading the Starting System on a Caterpillar D7 3T Series Dozer

[MikePhua]

The Legacy of the D7 3T Series
The Caterpillar D7 3T series was introduced in the 1940s as part of Caterpillar’s post-war expansion into heavy-duty earthmoving equipment. Built for durability and simplicity, the 3T series featured a direct-drive transmission, cable-operated blade systems, and a robust undercarriage suited for forestry, mining, and road-building. One of its most distinctive features was the use of a pony motor—a small gasoline engine used to start the main diesel engine.
While effective in its time, pony motors introduced their own set of challenges: fuel system maintenance, spark plug fouling, and cold-weather unreliability. As technology advanced, many operators sought to replace the pony motor with electric starters to simplify operation and reduce downtime.
Electric Start Conversion and Availability
Electric start kits for the D7 3T series were available as factory options in later years and became popular retrofits. These kits typically included:

• Heavy-duty starter motor
  
• Flywheel ring gear
  
• Solenoid and relay assembly
  
• Battery tray and wiring harness
  
• Modified bell housing or adapter plate
  

Finding these components today can be difficult, especially for machines with serial numbers like 3T16602. However, several sources still offer new old stock (NOS) or used parts suitable for conversion.
Recommended sourcing strategies:

• Contact vintage Caterpillar parts dealers such as General Gear or Florin Tractor
  
• Search salvage yards specializing in pre-1970s heavy equipment
  
• Network with collectors and restorers who may have donor machines
  
• Consider fabricating adapter plates if original housings are unavailable
  

Operators should verify compatibility with their specific engine model, as some early D7 units used different flywheel configurations.
Installation Considerations and Safety
Converting to electric start requires mechanical precision and electrical safety. Key steps include:

• Removing the pony motor and associated fuel lines
  
• Installing the ring gear on the flywheel (requires engine removal or partial disassembly)
  
• Mounting the starter motor with proper alignment
  
• Wiring the solenoid to a key switch or push-button starter
  
• Installing a battery tray with vibration isolation
  

Safety tips:

• Disconnect all power sources before installation
  
• Use torque specs provided in Caterpillar service manuals
  
• Test starter engagement before finalizing wiring
  
• Shield wiring from heat and abrasion
  

Some restorers choose to retain the pony motor as a backup system, especially in remote areas where battery failure could leave the machine stranded.
Anecdote from Northern British Columbia
A land surveyor in British Columbia recalled operating a D7 3T with electric start in the mid-1980s. The machine had been retrofitted by a logging crew who grew tired of coaxing the pony motor to life in freezing conditions. After installing the electric starter, they reported a dramatic improvement in cold-weather reliability and reduced maintenance. The conversion paid for itself in fuel savings and reduced downtime within a single season.
Preserving Historical Integrity vs. Modernization
While electric start conversions offer practical benefits, some collectors prefer to preserve the original pony motor setup for historical authenticity. Museums and vintage equipment shows often feature fully restored D7s with operational pony motors, showcasing the engineering of a bygone era.
For working machines, however, modernization is often the preferred path. Electric starters reduce complexity, improve safety, and align with contemporary expectations for equipment operation.
Conclusion
Upgrading a Caterpillar D7 3T series dozer from pony motor to electric start is a worthwhile investment for operators seeking reliability and ease of use. With careful sourcing, mechanical precision, and attention to electrical safety, the conversion can breathe new life into a classic machine. Whether clearing land or preserving history, the D7 remains a symbol of rugged engineering—and its evolution continues with every retrofit.