Alternator Wiring Challenges on the CAT 955L and Retrofit Solutions

[MikePhua]

Retrofitting a new alternator into a 1976 CAT 955L track loader often reveals compatibility issues between modern excitation systems and legacy wiring. Without a keyed ignition circuit, proper excitation must be engineered using oil pressure switches, starter feeds, or diode-protected jumpers.
CAT 955L Background and Electrical System Design
The Caterpillar 955L was introduced in the mid-1970s as a robust track loader for construction, forestry, and quarry applications. Powered by a 3304 diesel engine, the 955L featured mechanical fuel injection, direct drive transmission, and a basic 12V electrical system. Caterpillar Inc., founded in 1925, sold thousands of these machines globally, with many still in service due to their mechanical simplicity and rebuildable architecture.
The original electrical system included a momentary starter switch, ammeter, glow plug circuit, and a non-self-exciting alternator. Over time, corrosion and heat damage often degrade the wiring harness, requiring full replacement or custom fabrication.
Terminology Note

• Excitation Post: A terminal on the alternator that receives voltage to energize the rotor field.
  
• Self-Exciting Alternator: A unit with an internal regulator that begins charging without external voltage.
  
• Ammeter: A gauge that measures current flow to and from the battery.
  
• Diode Jumper: A wire with a one-way electrical diode that allows current to flow to the excitation post but prevents backfeed.
  
• Oil Pressure Switch: A sensor that closes a circuit when engine oil pressure rises, often used to trigger alternator excitation.
  

Wiring Challenges and Observations
After rebuilding the wiring harness using a factory schematic, the operator installed a new alternator sourced from Romaine Electric. The unit matched the part number but featured different terminal configurations. The manufacturer confirmed it was not self-exciting and required switched power to the excitation post.
However, the 955L lacks a keyed ignition circuit—only a momentary starter switch is present. This raised questions about how to energize the alternator without draining the battery or requiring manual intervention.
Two heavy-gauge wires ran from the ammeter to the alternator’s charge post. The original setup did not include a separate excitation wire. The operator considered two options:

• Split the ammeter wires, sending one to the charge post and one to the excitation post.
  
• Install a jumper from the charge post to the excitation post, possibly with a diode to prevent backfeed.
  

Testing and Electrical Behavior
Multimeter readings showed:

• The excitation post had continuity with the alternator case—suggesting it was grounded.
  
• The charge post was isolated and showed no continuity to ground.
  
• The green wire post had 20 ohms resistance to the adjacent bare post, indicating a regulator feed.
  

These readings suggested that the excitation post was not suitable for direct 12V feed without risk of shorting. The operator planned to test the system by connecting only the charge post, then adding a jumper if necessary.
Recommended Retrofit Solutions

• Use an oil pressure switch to trigger excitation. Install it in the oil gallery near the injector pump. When the engine starts and oil pressure rises, the switch closes and sends voltage to the excitation post. This method was used successfully on older CAT machines like the D8H and 951C.
  
• Tap the starter motor crank terminal for excitation. This sends voltage only during cranking, energizing the rotor without continuous draw.
  
• Install a diode-protected jumper from the charge post to the excitation post. This allows excitation during operation but prevents battery drain when the engine is off.
  
• Upgrade to a self-exciting regulator if compatible with the alternator body. This eliminates the need for external excitation wiring.
  

Preventive Electrical Tips

• Use isolating bushings on all charge posts to prevent accidental grounding.
  
• Verify resistance and continuity before applying voltage to unknown terminals.
  
• Replace circuit breakers with correct amperage ratings—avoid oversizing to mask faults.
  
• Label all wires during harness fabrication to simplify future diagnostics.
  
• Keep a wiring diagram and alternator spec sheet in the cab for reference.
  

Conclusion
Wiring a modern alternator into a legacy CAT 955L requires creative adaptation. Without a keyed ignition circuit, excitation must be triggered by oil pressure, starter voltage, or diode jumpers. Understanding terminal behavior and grounding paths is essential to avoid shorts and ensure reliable charging. With careful testing and proven retrofit strategies, the 955L’s electrical system can be modernized without compromising its rugged simplicity.