Troubleshooting Transmission Issues on the JLG 644E Telehandler

[MikePhua]

The 644E and Its Transmission Architecture
The JLG 644E telehandler was introduced under the Lull brand before JLG consolidated its material handling lineup. Designed for mid-range lifting tasks, the 644E features a 6,000 lb capacity and a 42-foot reach, making it popular among framing crews and masonry contractors. Its drivetrain includes a torque converter, a four-speed powershift transmission, and a Dana Spicer axle set—components known for durability but sensitive to fluid quality and electrical control.
The transmission is electronically controlled, with solenoids managing gear selection and clutch engagement. A selector lever in the cab communicates with the transmission control module (TCM), which then actuates hydraulic valves to shift gears. When problems arise, symptoms often include gear slipping, failure to engage, or erratic shifting.
Common Symptoms and Initial Checks
Operators experiencing transmission issues on the 644E often report:

• Machine starts and runs but won’t move in forward or reverse
  
• Transmission engages briefly then disengages
  
• Gears shift inconsistently or not at all
  
• Warning lights may or may not be present
  
• Hydraulic functions remain operational
  

These symptoms suggest a fault in the transmission control circuit, solenoid operation, or fluid pressure. In one case from Ohio, a framing crew found their 644E would only move in reverse, and only intermittently. After checking the selector switch and wiring harness, they discovered a corroded connector at the transmission housing.
Electrical Control and Solenoid Function
The transmission relies on solenoids to engage clutch packs for each gear. These solenoids are energized by signals from the TCM, which receives input from the gear selector and other sensors.
Diagnostic steps:

• Check voltage at solenoid terminals during gear selection
  
• Inspect wiring harness for abrasion or corrosion
  
• Test gear selector switch continuity
  
• Verify ground connections at chassis and transmission
  

A technician in Alberta found that the forward solenoid wire had rubbed against the frame and shorted. After repairing the harness and resealing the connector, the transmission resumed normal operation.
Fluid Pressure and Filter Condition
Transmission performance depends on hydraulic pressure generated by the pump and regulated by internal valves. Low pressure can prevent clutch engagement, causing the machine to remain stationary.
Recommended checks:

• Inspect transmission fluid level and condition
  
• Replace filter and check for metal debris
  
• Test pressure at diagnostic port (typically 200–250 psi at idle)
  
• Verify pump output and relief valve settings
  

In a fleet in Georgia, a telehandler showed delayed engagement and gear slippage. The technician discovered that the filter had collapsed internally, restricting flow. After replacing the filter and flushing the system, pressure returned to spec.
Torque Converter and Stall Behavior
The torque converter allows smooth power transfer from the engine to the transmission. If it fails, symptoms may include:

• Engine revs but machine doesn’t move
  
• Transmission engages briefly then stalls
  
• Excessive heat buildup in transmission housing
  

Solutions:

• Check stall speed (engine RPM at full throttle with brakes applied)
  
• Inspect converter for leaks or overheating
  
• Replace converter if internal damage is suspected
  

A contractor in Pennsylvania replaced the torque converter after noticing that stall speed had dropped below 1,200 RPM. The new unit restored full drive power and eliminated stalling.
Valve Body and Internal Wear
The valve body directs fluid to clutch packs and regulates shift timing. Wear or contamination can cause erratic behavior.
Inspection tips:

• Remove valve body and inspect spool movement
  
• Clean passages with solvent and compressed air
  
• Replace worn seals and gaskets
  
• Use torque specs during reassembly
  

In one rebuild in Texas, a technician found that the valve body had scoring on the shift spool, causing delayed engagement. After polishing the spool and replacing seals, the transmission shifted smoothly.
Preventive Maintenance and Operator Tips
To avoid transmission issues:

• Change fluid and filter every 500 hours or annually
  
• Use OEM-spec transmission fluid (typically Dexron III or equivalent)
  
• Inspect wiring harness during routine service
  
• Avoid aggressive gear changes under load
  
• Monitor for early signs like hesitation or noise
  

Some operators install inline pressure gauges to monitor clutch engagement in real time. Others retrofit diagnostic ports for easier access during service.
Conclusion
Transmission problems on the JLG 644E telehandler often stem from electrical faults, fluid pressure loss, or internal wear. With methodical diagnostics—starting from solenoids and wiring to fluid and valve body inspection—technicians can restore drive function and prevent costly downtime. Whether lifting trusses or placing block, a healthy transmission ensures the 644E performs with precision and reliability.