Why Do the Drive Wheels Stop When Steering on a JLG 60H

[MikePhua]

The Issue at a Glance
When the turn switch is activated on a JLG 60H man lift, the drive wheels abruptly stop. This is typically caused by low engine idle speed, electrical voltage drop, or a failing master switch. All three factors can interact to disrupt hydraulic and drive functions.
Understanding the JLG 60H System
The JLG 60H is a telescopic boom lift designed for high-reach tasks in construction and maintenance. It features a Ford 4-cylinder gas engine retrofit in many units, hydraulic drive motors, and a platform control system with multiple levers. The machine relies on coordinated electrical and hydraulic systems to manage drive, lift, and steer functions. When one system draws excessive current or fails to respond, others may shut down as a protective measure.
Terminology Clarification

• Turn Switch: The control used to steer the machine, typically located on the platform joystick or separate lever.
  
• Master Switch: The main power switch that energizes the control system and relays.
  
• Governor: A mechanical or electronic device that regulates engine speed under varying loads.
  
• EEC-IV ECU: Ford’s electronic engine control unit used in retrofitted gas engines, managing fuel and ignition parameters.
  

Root Causes of Drive Interruption

1. Low Idle Speed
   If the engine idle is too low, activating the steering system adds hydraulic load that the engine cannot support. This causes a voltage drop and may stall the drive motors. The Ford 4-cylinder engine requires sufficient warm-up time to stabilize idle and respond to load changes. Cold starts exacerbate the issue, especially in older units with worn governors.
   
2. Electrical Voltage Drop
   Steering solenoids and drive motors draw significant current. If the master switch or wiring is compromised, activating the turn switch can cause a voltage collapse. In one documented case, the master switch had internal damage that caused high resistance and heat buildup. Replacing the switch restored stable voltage and drive function.
   
3. Governor Calibration Fault
   The governor must increase engine RPM when hydraulic demand rises. If miscalibrated, the engine remains at idle while the system attempts to steer, leading to power loss. Recalibrating the governor ensures proper throttle response during drive and steer operations.
   

Repair Strategy and Recommendations

• Replace the Master Switch: If signs of heat damage or high resistance are present, install a new switch rated for high current.
  
• Clean and Inspect Grounds: Corroded or loose ground connections can mimic electrical failure. Use a multimeter to verify continuity.
  
• Warm Up the Engine Fully: Allow 5–10 minutes of idle time before operating hydraulic functions, especially in cold weather.
  
• Recalibrate the Governor: Adjust linkage or electronic parameters to ensure RPM increases under load.
  
• Inspect ECU Wiring: Many retrofits leave the EEC-IV ECU partially connected. Ensure sensors like CTS (Coolant Temp Sensor) and TPS (Throttle Position Sensor) are functional. Add a MAP sensor if missing to improve fuel control.
  

Field Anecdote
One technician found that the engine ran rich due to missing MAP input, but the power band tolerated it once warm. After replacing the master switch and cleaning grounds, the machine operated smoothly. The experience highlighted the importance of electrical integrity in hydraulic systems.
Conclusion
The JLG 60H’s drive wheels may stop during steering due to a combination of low idle speed, electrical resistance, and governor misbehavior. Addressing these issues restores full functionality and prevents jarring interruptions. For retrofitted units, verifying ECU wiring and sensor input is essential to long-term reliability.