Diagnosing No-Start Issues on a JLG 330CRT After Alternator Replacement

[MikePhua]

JLG 330CRT Overview and Electrical System Design
The JLG 330CRT is a compact scissor lift designed for indoor and outdoor use, equipped with a dual-fuel Kubota 3-cylinder engine capable of running on gasoline or propane. Its electrical system includes a starter circuit, alternator, ignition control, and dual control panels—one at the platform and one at the base. The alternator not only charges the battery but also plays a role in system readiness and voltage feedback to the control modules.
Terminology Note

• Alternator: A rotating electrical generator that charges the battery and powers electrical systems while the engine runs.
  
• Voltage Regulator: A component inside the alternator that controls output voltage to prevent overcharging.
  
• Ignition Wire: Supplies voltage to the alternator when the key is turned on, enabling charging.
  
• Lamp Wire: Connects to the charge indicator light and may be used for excitation voltage.
  
• T-Plug Connector: A molded plastic plug with two small wires, typically used for ignition and lamp terminals.
  

Symptoms and Initial Observations
After replacing the alternator, the JLG 330CRT failed to start from either the top or bottom control panels. Reinstalling the old alternator restored starting function, though it no longer charged due to internal damage. The new alternator was bench-tested and confirmed functional, but when installed, the machine remained unresponsive.
Key details:

• Three wires connect to the alternator: one large battery wire and two small wires in a T-plug.
  
• Wiring diagram identifies the small wires as ignition and lamp.
  
• Old alternator had a cracked case and a worn tab on the voltage regulator.
  
• New alternator passed bench testing but caused a no-start condition when installed.
  

Root Cause and Electrical Interaction
The issue likely stems from how the new alternator interacts with the ignition circuit. On some machines, the alternator’s internal regulator requires a specific excitation voltage from the ignition wire to activate. If the alternator draws excessive current or fails to isolate the ignition circuit properly, it can prevent voltage from reaching the starter relay or control module—effectively disabling the start function.
Possible causes:

• Incorrect internal wiring or regulator configuration in the replacement alternator
  
• Misidentified ignition and lamp terminals in the T-plug
  
• Voltage feedback loop interfering with control panel logic
  
• Grounding issue or short within the alternator housing
  

Recommended Diagnostic Steps

• Use a multimeter to confirm voltage at the ignition wire with key ON
  
• Disconnect the T-plug and attempt to start the machine—if it starts, the issue is within the excitation circuit
  
• Swap the two small wires in the T-plug to test for reversed polarity
  
• Check for continuity between alternator case and ground
  
• Inspect starter relay and control panel fuses for voltage drop when alternator is connected
  

In New York, a technician encountered a similar issue on a Genie lift where a replacement alternator caused the ignition circuit to collapse. The fix involved installing a diode in the excitation wire to prevent backfeed and isolating the lamp circuit with a resistor.
Solutions and Workarounds

• Install a diode (e.g., 1N5408) in the ignition wire to block reverse current
  
• Use an alternator with an external regulator if internal configuration is incompatible
  
• Confirm alternator part number matches OEM spec for dual-fuel Kubota engines
  
• If necessary, rebuild the old alternator with a new regulator and case
  

Conclusion
A no-start condition following alternator replacement on a JLG 330CRT is often caused by electrical interference between the new alternator’s regulator and the ignition circuit. Careful testing of the excitation and lamp wires, along with voltage isolation techniques, can restore functionality. Matching the alternator’s internal configuration to the machine’s control logic is essential—especially on dual-fuel systems with sensitive panel integration.