Yesterday, 06:25 PM
The JLG 660SJ telescopic boom lift is a widely used machine in construction, industrial maintenance, and utility work. Known for its long reach, strong hydraulic performance, and reliable drivetrain, it has become a staple in rental fleets and contractor yards.
However, like all aerial lifts, the 660SJ depends on a complex combination of electrical, hydraulic, and safety‑interlock systems. When the chassis refuses to drive and the boom will not lift, swing, or telescope, the issue can be alarming.
This article provides a detailed, narrative‑style exploration of the causes behind movement failure on a 2003 JLG 660SJ, enriched with terminology notes, historical context, troubleshooting strategies, and real‑world stories.
Background of the JLG 660SJ
JLG Industries, founded in 1969, pioneered the modern aerial lift. By the early 2000s, the 660SJ had become one of the company’s most successful mid‑range telescopic boom lifts, offering:
Understanding the Movement and Interlock Systems
The 660SJ uses several systems to control movement:
Terminology Note: Interlock System
A safety mechanism that prevents machine movement unless specific conditions are met, such as proper control selection, emergency stop reset, and correct sensor feedback.
Common Symptoms of Movement Failure
Operators often report:
Most Common Causes of 660SJ Movement Failure
The 2003 model year uses older wiring and relay‑based logic, making it vulnerable to several recurring issues.
Emergency Stop Circuit Issues
The machine has emergency stop buttons at both platform and ground controls. If either is engaged or partially stuck:
Control Selector Switch Problems
The key switch that selects platform or ground control can fail internally.
Symptoms include:
Hydraulic Pump Solenoid Failure
The hydraulic pump is activated by an electrical solenoid. If the solenoid fails:
An electrically controlled valve that engages the hydraulic pump when the operator activates a function.
Broken or Corroded Wiring
The 660SJ’s wiring harness runs through areas exposed to:
Failed Function Enable Switch
The platform joystick has a trigger or enable switch that must be pressed before movement is allowed.
If the switch fails:
Hydraulic Lockout Valve Issues
The 660SJ uses a hydraulic lockout valve to prevent unintended movement.
If the valve sticks or loses electrical power:
Drive and Boom Limit Switch Problems
Limit switches prevent unsafe operation, such as:
Diagnostic Approach
A structured diagnostic method helps identify the root cause efficiently.
1. Verify Emergency Stop Buttons
Reset both platform and ground emergency stop buttons.
Even a partially depressed button can interrupt power.
2. Check Control Selector Switch
Turn the key between platform and ground control several times.
If ground controls work but platform controls do not, the selector switch or platform wiring is suspect.
3. Listen for Hydraulic Pump Engagement
When a function is activated, the pump should load.
If the engine does not change tone:
4. Inspect Wiring Harness
Look for:
5. Test Function Enable Switch
Use a multimeter to verify continuity when the switch is pressed.
Replace if intermittent.
6. Check Hydraulic Lockout Valve
Ensure the valve receives power and actuates properly.
A stuck lockout valve will disable all movement.
7. Inspect Limit Switches
Check:
Real‑World Case Studies
Case 1: Corroded Ground Wire
A contractor’s 660SJ would not move. After hours of troubleshooting, a corroded ground wire near the frame was discovered. Cleaning the connection restored full function.
Case 2: Failed Pump Solenoid
A rental company reported a machine that ran but had no hydraulic movement. The pump solenoid coil had burned out. Replacing it solved the issue immediately.
Case 3: Broken Wire at Boom Pivot
A municipality’s lift lost all boom functions. A wire had broken inside the harness where it flexed during boom movement. Repairing the wire restored operation.
Case 4: Stuck Emergency Stop Button
A new operator accidentally pressed the emergency stop button halfway. The machine appeared dead. Resetting the button fixed the problem.
Case 5: Faulty Control Selector Switch
A machine would only operate from the ground controls. The selector switch had worn contacts. Replacing it restored platform control.
Maintenance Recommendations
To prevent movement failures:
Anecdotes and Industry Stories
A veteran mechanic once said, “On a JLG, nine out of ten hydraulic problems start with a bad wire.”
Another operator recalled losing half a day of work because a mouse chewed through the harness under the platform.
A rental fleet manager shared that replacing the control selector switch every few years dramatically reduced downtime.
Conclusion
A 2003 JLG 660SJ that refuses to move—either the chassis or the boom—is almost always suffering from an electrical or interlock‑related issue rather than a major hydraulic failure.
By systematically checking emergency stop circuits, control selectors, pump solenoids, wiring harnesses, function enable switches, and limit sensors, operators can identify and resolve most problems quickly.
With proper maintenance and attention to electrical integrity, the 660SJ can continue delivering reliable performance for years, proving why it remains one of the most trusted boom lifts in the industry.
However, like all aerial lifts, the 660SJ depends on a complex combination of electrical, hydraulic, and safety‑interlock systems. When the chassis refuses to drive and the boom will not lift, swing, or telescope, the issue can be alarming.
This article provides a detailed, narrative‑style exploration of the causes behind movement failure on a 2003 JLG 660SJ, enriched with terminology notes, historical context, troubleshooting strategies, and real‑world stories.
Background of the JLG 660SJ
JLG Industries, founded in 1969, pioneered the modern aerial lift. By the early 2000s, the 660SJ had become one of the company’s most successful mid‑range telescopic boom lifts, offering:
- A platform height of around 66 ft
- A horizontal outreach of approximately 57 ft
- A strong diesel powertrain
- Smooth hydraulic controls
- A robust chassis for rough‑terrain use
Understanding the Movement and Interlock Systems
The 660SJ uses several systems to control movement:
- Hydraulic pump and valve banks
- Drive motors
- Boom lift, telescope, and swing cylinders
- Electrical control circuits
- Safety interlocks
- Limit switches
- Emergency stop circuits
- Ground and platform control selectors
Terminology Note: Interlock System
A safety mechanism that prevents machine movement unless specific conditions are met, such as proper control selection, emergency stop reset, and correct sensor feedback.
Common Symptoms of Movement Failure
Operators often report:
- No drive function
- No boom lift, swing, or telescope
- Engine runs normally but hydraulics do not respond
- Platform controls dead or partially functional
- Ground controls also unresponsive
- Audible clicking but no hydraulic movement
- Warning lights or alarms
Most Common Causes of 660SJ Movement Failure
The 2003 model year uses older wiring and relay‑based logic, making it vulnerable to several recurring issues.
Emergency Stop Circuit Issues
The machine has emergency stop buttons at both platform and ground controls. If either is engaged or partially stuck:
- All hydraulic functions are disabled
- Drive and boom movement are locked out
Control Selector Switch Problems
The key switch that selects platform or ground control can fail internally.
Symptoms include:
- No response from either control station
- Intermittent operation
- Controls working only after jiggling the switch
Hydraulic Pump Solenoid Failure
The hydraulic pump is activated by an electrical solenoid. If the solenoid fails:
- The pump will not load
- No hydraulic pressure reaches the valve banks
- Boom and drive functions remain dead
An electrically controlled valve that engages the hydraulic pump when the operator activates a function.
Broken or Corroded Wiring
The 660SJ’s wiring harness runs through areas exposed to:
- Vibration
- Moisture
- UV exposure
- Hydraulic oil
- Physical abrasion
- Wires under the platform
- Harness near the boom pivot
- Ground wires on the frame
- Connectors near the control box
Failed Function Enable Switch
The platform joystick has a trigger or enable switch that must be pressed before movement is allowed.
If the switch fails:
- The joystick sends no command
- The machine appears dead
- Ground controls may still work
Hydraulic Lockout Valve Issues
The 660SJ uses a hydraulic lockout valve to prevent unintended movement.
If the valve sticks or loses electrical power:
- No hydraulic functions will operate
- The machine may idle normally but remain frozen
Drive and Boom Limit Switch Problems
Limit switches prevent unsafe operation, such as:
- Driving with the boom too high
- Over‑tilting
- Exceeding safe angles
Diagnostic Approach
A structured diagnostic method helps identify the root cause efficiently.
1. Verify Emergency Stop Buttons
Reset both platform and ground emergency stop buttons.
Even a partially depressed button can interrupt power.
2. Check Control Selector Switch
Turn the key between platform and ground control several times.
If ground controls work but platform controls do not, the selector switch or platform wiring is suspect.
3. Listen for Hydraulic Pump Engagement
When a function is activated, the pump should load.
If the engine does not change tone:
- Pump solenoid may be dead
- No power is reaching the solenoid
- A relay may have failed
4. Inspect Wiring Harness
Look for:
- Broken wires
- Corroded connectors
- Loose grounds
- Pinched harness sections
5. Test Function Enable Switch
Use a multimeter to verify continuity when the switch is pressed.
Replace if intermittent.
6. Check Hydraulic Lockout Valve
Ensure the valve receives power and actuates properly.
A stuck lockout valve will disable all movement.
7. Inspect Limit Switches
Check:
- Boom angle switch
- Drive speed limit switch
- Tilt sensor
Real‑World Case Studies
Case 1: Corroded Ground Wire
A contractor’s 660SJ would not move. After hours of troubleshooting, a corroded ground wire near the frame was discovered. Cleaning the connection restored full function.
Case 2: Failed Pump Solenoid
A rental company reported a machine that ran but had no hydraulic movement. The pump solenoid coil had burned out. Replacing it solved the issue immediately.
Case 3: Broken Wire at Boom Pivot
A municipality’s lift lost all boom functions. A wire had broken inside the harness where it flexed during boom movement. Repairing the wire restored operation.
Case 4: Stuck Emergency Stop Button
A new operator accidentally pressed the emergency stop button halfway. The machine appeared dead. Resetting the button fixed the problem.
Case 5: Faulty Control Selector Switch
A machine would only operate from the ground controls. The selector switch had worn contacts. Replacing it restored platform control.
Maintenance Recommendations
To prevent movement failures:
- Inspect wiring annually
- Clean and lubricate emergency stop buttons
- Test pump solenoid resistance
- Check ground connections
- Protect harnesses from abrasion
- Avoid pressure‑washing electrical components
- Test limit switches regularly
- Keep battery terminals clean
Anecdotes and Industry Stories
A veteran mechanic once said, “On a JLG, nine out of ten hydraulic problems start with a bad wire.”
Another operator recalled losing half a day of work because a mouse chewed through the harness under the platform.
A rental fleet manager shared that replacing the control selector switch every few years dramatically reduced downtime.
Conclusion
A 2003 JLG 660SJ that refuses to move—either the chassis or the boom—is almost always suffering from an electrical or interlock‑related issue rather than a major hydraulic failure.
By systematically checking emergency stop circuits, control selectors, pump solenoids, wiring harnesses, function enable switches, and limit sensors, operators can identify and resolve most problems quickly.
With proper maintenance and attention to electrical integrity, the 660SJ can continue delivering reliable performance for years, proving why it remains one of the most trusted boom lifts in the industry.
