CAT 955K Starter Solenoid Troubleshooting: Bench Tests, Wiring Tips, and Field Fixes

[MikePhua]

Understanding the Starter Solenoid System
The starter solenoid on a CAT 955K track loader plays a critical role in energizing the starter motor. When the ignition switch is activated, the solenoid receives a low-current signal that triggers a high-current connection between the battery and the starter motor. If the solenoid fails, the engine won’t crank—leading to frustrating no-start conditions.
Terminology Notes

• Starter Solenoid: An electromechanical switch that connects battery power to the starter motor.
  
• Bench Test: A diagnostic procedure performed outside the machine to test electrical components.
  
• Resistance Reading: Measured in ohms, it indicates the health of the solenoid coil.
  
• Trigger Wire: The low-current wire from the ignition switch that activates the solenoid.
  
• Copper Contacts: Internal conductive pads that complete the circuit when the solenoid engages.
  

Bench Testing the Solenoid
One technician measured resistance across the solenoid coil and found values ranging from 7 to 9.5 ohms. While there’s no universal spec, readings in this range typically indicate a functioning coil. To test the solenoid:

• Secure the starter in a vise to prevent movement
  
• Connect battery power to the main terminal and ground
  
• Use a jumper wire to energize the solenoid terminal
  
• Observe whether the solenoid pulls in and engages the starter motor
  

If the solenoid clicks but the motor doesn’t spin, worn copper contacts may be to blame. These can sometimes be resurfaced to extend service life.
Starter Motor Diagnosis
If bench testing reveals no engagement, the issue may lie in the starter motor itself. Common problems include:

• Worn brushes or armature
  
• Burnt windings
  
• Faulty internal connections
  

In such cases, rebuilding the starter or replacing it with a remanufactured unit is often more cost-effective than repair.
Wiring and Installation Tips
When installing a new starter or solenoid:

• Ensure all battery cables are clean and tightly connected
  
• Use a magnetic relay switch to reduce load on the ignition circuit
  
• Verify that the trigger wire delivers consistent voltage
  
• Avoid grounding issues by checking both positive and negative paths
  

One technician recommended using a Cole Hersee 24063 relay to handle the starter solenoid’s 30–35 amp draw, reducing strain on the ignition switch.
Field Anecdotes and Practical Wisdom
A mechanic shared a story of a CAT 955K that wouldn’t start despite a functioning ignition switch. The culprit? A weak solenoid that clicked but failed to engage the starter. After bench testing and replacing the solenoid, the machine started instantly.
Another operator noted that worn copper contacts inside the solenoid caused intermittent starting. By disassembling the solenoid and resurfacing the contacts, they restored reliable operation—buying time before a full replacement.
Preventive Measures

• Periodically test solenoid resistance to detect early failure
  
• Clean and inspect battery terminals and cable ends
  
• Use dielectric grease on connections to prevent corrosion
  
• Replace aging solenoids before they fail in the field
  
• Label wiring during disassembly to ensure correct reinstallation
  

Historical Context and Design Evolution
Older CAT machines like the 955K often used inertia-style starters with external solenoids. Modern replacements feature pre-engaged starters with integrated solenoids, simplifying wiring but requiring careful compatibility checks. The shift reflects broader trends in heavy equipment toward modular, plug-and-play components.
Conclusion: A Small Part with Big Impact
The starter solenoid may be a humble component, but its failure can halt operations entirely. With proper testing, wiring, and preventive care, operators can avoid costly downtime and keep their machines running smoothly. As one veteran technician put it, “If the solenoid doesn’t sing, the engine won’t dance.”