Glow Plug Circuit Failure and Diagnostic Strategy on the Caterpillar 248 Skid Steer

[MikePhua]

The Cat 248 and Its Electrical Starting System
The Caterpillar 248 skid steer loader, introduced around the year 2000, was part of Cat’s compact equipment lineup designed for high-performance operation in tight spaces. With a rated operating capacity of approximately 2,200 lbs and a turbocharged diesel engine, the 248 was built for contractors, landscapers, and municipal fleets. Like many diesel-powered machines of its era, it relied on glow plugs to preheat the combustion chamber during cold starts.
Glow plugs are critical in low-temperature environments, especially for indirect injection engines. The Cat 248 uses a bus-bar-connected glow plug system activated by a relay and protected by a main breaker. When the glow plug circuit shorts, it can trip the breaker and prevent starting altogether.
Terminology Annotation
- Glow Plug: A heating element installed in diesel engines to warm the combustion chamber for improved cold starting.
- Bus Bar: A metal strip that distributes electrical current to multiple glow plugs simultaneously.
- Main Breaker: A protective device that disconnects power when excessive current is detected, preventing electrical damage.
- Ohm Reading: A measurement of electrical resistance used to identify faulty components.
Symptoms and Initial Troubleshooting
A common symptom of glow plug failure in the Cat 248 is the tripping of the main breaker when the glow plug circuit is activated. This indicates a short circuit, typically caused by a failed glow plug or damaged wiring. The diagnostic approach involves isolating the fault by disconnecting the bus bar and testing each plug individually.
Steps include:

• Disconnecting the power feed to the bus bar
  
• Removing the bus bar and testing each glow plug one by one
  
• Using an ohmmeter to measure resistance across each plug
  
• Identifying any plug with significantly lower resistance (e.g., 0.1 ohms vs. 0.3 ohms)
  
• Inspecting for physical damage such as loose electrodes or wobbling terminals
  

In one case, the second glow plug showed mechanical instability, with its internal electrode visibly loose and shorting against the housing. This confirmed the source of the breaker trip.
A Story from Upstate New York
In LaGrangeville, a technician working on a 20-year-old Cat 248 encountered persistent breaker trips during cold starts. After isolating the glow plug circuit, he found that one plug had a visibly damaged core. The electrode was wobbling inside the housing, intermittently shorting the circuit. He replaced all four plugs as a preventative measure, noting that the machine had never had them changed since new. The repair restored reliable cold starts just in time for the first snowfall.
Replacement Strategy and Cost Considerations
Glow plugs for the Cat 248 are typically threaded into the cylinder head and secured with a 10mm or 11mm nut. Removal requires cleaning the area with compressed air to prevent debris from entering the combustion chamber. A deep socket is recommended for extraction.
Replacement options include:

• OEM plugs from Caterpillar dealers
  
• Aftermarket plugs rated for the same voltage and thread pitch
  
• Sets of four ranging from $50 to $100 depending on brand and region
  
• Individual plugs priced between $30 and $50
  

While some technicians prefer replacing only the failed plug, others advocate for full-set replacement, especially on older machines. A failed glow plug can lead to hard starts, increased emissions, and in rare cases, piston damage if the tip breaks off.
Recommendations for Preventative Maintenance
To maintain glow plug performance:

• Test glow plug resistance annually before winter
  
• Replace all plugs every 2,000–3,000 hours or every 5 years
  
• Inspect bus bar for corrosion or loose connections
  
• Use dielectric grease on terminals to prevent oxidation
  
• Verify relay function and ensure proper voltage delivery
  
• Keep a spare set of glow plugs in the service kit during cold seasons
  

If the machine operates in extreme cold, consider installing a block heater or intake air heater as supplemental starting aids.
Conclusion
Glow plug circuit failure in the Caterpillar 248 skid steer is often caused by internal shorting within aged plugs, leading to breaker trips and no-start conditions. Through methodical isolation and resistance testing, faulty plugs can be identified and replaced. For machines approaching two decades of service, proactive replacement of all glow plugs is a sound investment in reliability and cold-weather performance. With proper maintenance, the Cat 248 continues to deliver dependable operation in demanding conditions.