Troubleshooting Starting Failures on the JLG 450AS Series II Boom Lift

[MikePhua]

The JLG 450AS Series II and Its Electrical Start System
The JLG 450AS Series II is a mid-size articulating boom lift designed for aerial access in construction, maintenance, and industrial applications. With a platform height of 45 feet and a horizontal outreach of over 25 feet, it balances maneuverability with reach. The Series II variant introduced refinements in control logic, engine integration, and safety interlocks.
Its starting system is electrically controlled, relying on a combination of relays, solenoids, safety switches, and battery voltage. When the operator turns the key or activates the start switch, a signal is sent to the starter solenoid, which engages the starter motor. If any part of this chain is interrupted—by low voltage, faulty relays, or safety lockouts—the engine will not crank.
Symptoms of a No-Start Condition
Operators may encounter:

• No crank when the start switch is pressed
  
• Audible click from the solenoid but no starter engagement
  
• Starter spins but engine fails to fire
  
• Intermittent starting depending on platform or ground control
  
• No response from either control station
  

These symptoms suggest a fault in the electrical start circuit, possibly compounded by safety interlock behavior or low battery voltage.
Battery Voltage and Ground Integrity
The first step in diagnosis is verifying battery health. The JLG 450AS requires a minimum of 12.4 volts under load to engage the starter reliably. Weak batteries may show 12.6 volts at rest but drop below 10 volts during cranking, causing the solenoid to disengage.
Checkpoints include:

• Battery terminals for corrosion or loose connections
  
• Ground strap from battery to frame
  
• Voltage drop across starter cable during cranking
  
• Battery load test using a carbon pile or electronic tester
  

In one case, a technician found that the ground cable had frayed internally, causing intermittent starting. Replacing the cable restored full function.
Starter Solenoid and Relay Behavior
The starter solenoid receives a low-current signal from the start switch and closes a high-current circuit to the starter motor. If the solenoid clicks but the motor does not spin, possible causes include:

• Burned solenoid contacts
  
• Weak signal voltage from the control switch
  
• Faulty relay upstream of the solenoid
  
• High resistance in the starter cable
  

Use a multimeter to test voltage at the solenoid trigger wire during start attempt. If voltage is present but the solenoid fails to engage, replace the solenoid. If no voltage is present, trace back to the relay and switch.
Safety Interlocks and Control Station Logic
The JLG 450AS includes multiple safety interlocks:

• Platform and ground control selector switch
  
• Emergency stop buttons at both stations
  
• Tilt sensor and load sensor inputs
  
• Neutral start switch on the drive circuit
  

If any of these inputs are out of range or misaligned, the start circuit may be disabled. For example, if the platform control selector is set to platform but the emergency stop is engaged, the machine will not start from either station.
To test:

• Cycle the emergency stop buttons
  
• Toggle the control selector switch
  
• Verify tilt sensor status on the display (if equipped)
  
• Check for fault codes using the JLG analyzer tool
  

One operator reported that his lift would not start from the platform but worked from the ground. After inspecting the selector switch, he found a broken contact inside the switch housing. Replacing the switch resolved the issue.
Starter Motor and Engine Cranking
If the solenoid engages and voltage reaches the starter motor but the engine does not crank, inspect:

• Starter motor brushes and armature
  
• Flywheel engagement and gear teeth
  
• Engine seizure or hydraulic lock
  

Remove the starter and bench test it with jumper cables. If it spins freely, reinstall and check flywheel clearance. If the engine is locked, remove glow plugs or injectors and attempt manual rotation.
Recommendations for Field Technicians
To resolve starting issues on the JLG 450AS Series II:

• Begin with battery voltage and ground checks
  
• Test solenoid and relay function with a multimeter
  
• Inspect control station switches and interlocks
  
• Use the JLG analyzer to check for fault codes
  
• Clean all terminals and apply dielectric grease
  

For machines in rental fleets or exposed to weather, corrosion is a common culprit. Regular inspection and preventive maintenance can avoid downtime.
Conclusion
Starting failures on the JLG 450AS Series II often stem from electrical faults, safety interlock misalignment, or battery degradation. By methodically testing each component in the start circuit—from battery to solenoid to control logic—technicians can isolate the fault and restore reliable operation. In aerial work platforms, uptime is safety—and every start begins with clean voltage and clear logic.