Starter Switch Failure on Onan 150kW Generator and Diagnostic Strategies

[MikePhua]

The Onan Legacy and the 150kW Generator Platform
Onan, a brand under Cummins Power Generation, has been producing industrial-grade generators since the 1920s. Known for their reliability in standby and prime power applications, Onan units are widely used in hospitals, data centers, construction sites, and municipal infrastructure. The 150kW diesel generator is part of Onan’s commercial series, typically powered by a Cummins inline-six engine and equipped with an electronic governor, automatic voltage regulation, and programmable control panels.
By the early 2000s, Onan had shipped tens of thousands of generators in the 100–300kW range, with the 150kW model becoming a staple in mid-size facilities requiring robust backup power. These units are often configured with remote start capability, transfer switch integration, and digital fault logging.
Starter Switch Role and Common Failure Modes
The starter switch is a critical component in the generator’s control circuit. It initiates the cranking sequence by energizing the starter solenoid, which in turn engages the starter motor to rotate the engine. In Onan systems, this switch may be part of a manual control panel or integrated into an automatic start module.
Symptoms of starter switch failure include:

• No response when switch is engaged
  
• Audible click but no crank
  
• Intermittent starting behavior
  
• Starter motor runs but engine fails to fire
  
• Control panel shows “Start Failure” or “No Crank” fault
  

In one emergency response center in Arizona, a 150kW Onan unit failed to start during a grid outage. Technicians found that the starter switch contacts had carbon buildup and pitting, preventing full current flow. After cleaning and reseating the terminals, the unit resumed normal operation.
Root Causes and Electrical Diagnostics
Starter switch issues can stem from:

• Worn or oxidized internal contacts
  
• Loose or corroded terminal connections
  
• Faulty wiring between switch and solenoid
  
• Low battery voltage or poor ground
  
• Failed solenoid coil or starter relay
  
• Moisture ingress causing short circuits
  

Diagnostic steps:

• Verify battery voltage (should be ≥12.6V under load)
  
• Inspect switch terminals for corrosion or looseness
  
• Use a multimeter to check continuity across switch contacts
  
• Test voltage at solenoid input during switch engagement
  
• Bypass switch temporarily to verify starter motor function
  
• Check ground path from starter to chassis
  

In one municipal water plant, a generator showed no crank despite healthy batteries. The issue was traced to a broken ground strap between the starter and frame. Replacing the strap restored full current flow and resolved the fault.
Terminology Notes

• Starter Solenoid: An electromagnetic switch that connects battery power to the starter motor when energized.
  
• Cranking Sequence: The process of rotating the engine to initiate combustion.
  
• Continuity Test: A diagnostic method using a multimeter to verify electrical connection between two points.
  
• Ground Strap: A conductive cable that ensures electrical grounding between components and chassis.
  
• Carbon Buildup: Residue formed on electrical contacts due to arcing or oxidation, reducing conductivity.
  

Preventive Measures and Component Upgrades
To avoid starter switch failures:

• Inspect and clean switch terminals every 6 months
  
• Use dielectric grease to prevent corrosion
  
• Replace switches showing signs of wear or discoloration
  
• Upgrade to sealed switches in high-humidity environments
  
• Install surge protection to prevent voltage spikes during cranking
  
• Ensure battery terminals are tight and free of oxidation
  

In one fleet of mobile generators used for disaster relief, switching to marine-grade sealed starter switches reduced electrical faults by 80% and improved reliability during rapid deployment.
Control Panel Integration and Remote Start Considerations
Modern Onan generators often include:

• Digital control panels with programmable start logic
  
• Remote start terminals for integration with transfer switches
  
• Fault logging with timestamped error codes
  
• LED indicators for system status and diagnostics
  

Starter switch faults may trigger:

• “Low Voltage Start Fail”
  
• “Crank Timeout”
  
• “Starter Lockout”
  
• “Manual Start Required”
  

Solutions include:

• Resetting control panel and clearing fault codes
  
• Verifying remote start signal integrity
  
• Checking relay logic and override settings
  
• Updating firmware if available
  

In one hospital backup system in Illinois, a firmware update resolved a false “Starter Lockout” condition caused by a timing mismatch between the transfer switch and generator controller.
Final Thoughts
Starter switch issues on Onan 150kW generators can be deceptively simple yet disruptive. Whether caused by worn contacts, poor grounding, or control logic errors, these faults demand methodical diagnostics and preventive care. In critical power applications, reliability starts with the first turn of the key—or the first pulse of the switch. Keeping that circuit clean, tight, and responsive ensures that when the lights go out, the generator doesn’t. It roars to life.