Resolving Fuel Pump Malfunctions on the Lull 644B-37 Telehandler

[MikePhua]

The Lull 644B-37 and Its Mechanical Foundation
The Lull 644B-37 is a rough-terrain telehandler designed for lifting and placing materials in construction and industrial settings. Manufactured by Lull Corporation prior to its acquisition by JLG Industries in 2003, the 644B-37 was built for durability and reach, offering a 6,000-pound lift capacity and a 37-foot boom extension. It was commonly powered by a Cummins diesel engine, often the 4BT or similar variants, paired with a Lucas CAV DPA rotary fuel injection pump—a system known for mechanical simplicity but sensitive calibration.
Lull telehandlers were widely adopted across North America, particularly in masonry and framing trades. Their unique horizontal boom transfer system allowed for precise load placement without repositioning the chassis, a feature that set them apart from competitors like SkyTrak and Gradall.
Identifying the Fuel Pump Configuration
The unit in question was equipped with a Cummins engine stamped “19H6” and a Lucas DES type 700 fuel pump, model A53L/800/3/2725. This pump is a variant of the CAV DPA rotary pump, which uses a cam ring and distributor rotor to meter and deliver fuel to each cylinder. The top cover houses the governor linkage, throttle lever, and shut-off mechanism—all of which must be precisely aligned for proper operation.
Terminology annotation:

• DPA Pump: Distributor-type rotary pump manufactured by Lucas CAV, widely used in diesel engines from the 1960s through the 1990s.
  
• Governor Spring: A tension spring that regulates fuel delivery based on engine speed.
  
• Metering Valve: Internal valve that controls fuel quantity delivered to the rotor.
  
• Shut-Off Lever: Mechanical linkage that cuts fuel flow to stop the engine.
  

Symptoms and Initial Repair Attempts
The operator initially addressed a fuel leak by removing the top cover and resealing it with RTV silicone. While this temporarily stopped the leak, the engine began idling at excessively high RPMs and failed to respond to throttle input. Upon re-disassembly, the gasket tore, and the spring linkage appeared misaligned.
This behavior is typical when the governor spring is installed incorrectly or the metering valve is obstructed. The DPA pump relies on precise spring placement to balance throttle response and idle control. RTV sealant, while useful for some applications, is insufficient for sealing the top cover due to fuel exposure and mechanical stress.
Correcting Spring Placement and Linkage Alignment
The spring code “3” in the pump model (A53L/800/3/2725) indicates the correct mounting points:

• Spring end should be placed in hole #3 on the throttle lever (closest to the pivot)
  
• Opposite end should be placed in hole #1 on the governor arm (topmost hole)
  

In the reported case, the spring was mistakenly installed in hole #2 on the governor, causing the metering valve to remain partially open and resulting in uncontrolled fuel delivery. Additionally, the throttle lever must pass through the center of the spring—not outside it—to ensure proper tension and movement.
Recommendations for reassembly:

• Use a genuine DPA top cover gasket from a certified injection shop
  
• Clean all mating surfaces thoroughly and avoid RTV or silicone sealants
  
• Confirm spring placement using a verified spring code chart
  
• Ensure the shut-off lever engages the internal bar correctly during installation
  

Common Pitfalls in DIY Fuel Pump Repairs
Fuel injection pumps are precision components. Even minor misalignments or contamination can lead to performance issues or complete failure. Common mistakes include:

• Using incorrect sealants that degrade in diesel fuel
  
• Reassembling linkages without referencing spring codes
  
• Leaving ports uncapped during disassembly, allowing dirt ingress
  
• Overtightening cover screws, warping the housing
  

In one documented case, a technician reused a torn gasket and applied excessive torque to the cover bolts. The result was a warped top plate that caused internal binding of the governor linkage. After replacing the gasket and realigning the spring, the pump returned to normal operation.
When to Seek Professional Help
While basic gasket replacement and spring alignment can be done in the field, deeper issues—such as worn metering valves, damaged rotors, or internal scoring—require specialized tools and calibration benches. If the engine continues to idle erratically after correcting the spring, it’s advisable to remove the pump and send it to a certified rebuilder.
Signs that professional service is needed:

• Engine surges or stalls unpredictably
  
• Fuel delivery is uneven across cylinders
  
• Pump housing shows signs of corrosion or scoring
  
• Shut-off lever fails to kill the engine
  

Master rebuilders use flow benches to calibrate fuel delivery rates and verify internal timing. These adjustments are not possible with hand tools alone.
Conclusion
Fuel pump issues on the Lull 644B-37 often stem from misaligned governor springs, improper sealing methods, or overlooked linkage details. With careful attention to spring codes, clean reassembly, and proper gasket use, many problems can be resolved without removing the pump. However, given the sensitivity of the DPA system, knowing when to call a professional can prevent costly downtime and ensure the telehandler continues to perform reliably in demanding environments.