JLG G1255A Telehandler Misses Under Load Due to ECM Confusion and Cab Vibration

[MikePhua]

Quick answer
A 2009 JLG G1255A telehandler showing a miss under load was ultimately traced to cab window vibration and ECM mismatch. The engine, rebuilt by Pacific Power Products, used a Perkins 1104D-E44TA platform with a CAT-labeled injection pump, leading to diagnostic confusion. The miss disappeared after resealing the cab glass.
JLG G1255A background and engine configuration
The JLG G1255A is a high-capacity telehandler designed for rough terrain lifting and material placement. It features:

• 12,000 lb lift capacity
  
• 55 ft boom reach
  
• Four-wheel drive and steering
  
• Diesel engine options including CAT and Perkins platforms
  

In this case, the unit was rebuilt with a Perkins 1104D-E44TA engine, which shares architecture with CAT’s 3044C but uses different ECM logic and injector trim protocols. The rebuild tag listed engine family BPKXL04.4NJ1, rebuilt in 2012.
Symptoms and initial diagnosis
The operator reported:

• A miss or hiss under driving load
  
• No audible issue from the engine bay
  
• Miss only detectable from the driver’s seat
  
• No overheating or coolant loss
  
• Dirty oil but no water contamination
  
• Slight black smoke on throttle stab
  
• Miss worsened over time
  

Initial suspicion pointed to a blown head gasket or fuel delivery issue. The injection pump appeared new, with CAT USA labeling and part numbers 324-0532 and 2641A405. However, the ECM was confirmed to be Perkins, not CAT, complicating diagnostics.
ECM confusion and scan tool limitations
Perkins ECMs require injector trim codes to be programmed during replacement. These codes calibrate fuel delivery per cylinder. CAT ECMs use a different protocol, and scan tools are not interchangeable.
Recommendations:

• Use Perkins EST (Electronic Service Tool) for diagnostics
  
• Confirm ECM type before replacing injectors
  
• Avoid mixing CAT and Perkins components without ECM reprogramming
  

One technician noted that mismatched ECMs can cause timing errors, poor boost response, and false fault codes.
Cab vibration and acoustic illusion
While inspecting the machine during operation, the operator noticed that placing a finger on the side cab window eliminated the miss sound. The glass was vibrating due to degraded urethane sealant, creating a resonance that mimicked engine misfire.
After resealing the window with fresh urethane, the sound disappeared completely. This revealed that the perceived miss was an acoustic illusion amplified by cab structure.
Fault codes and further investigation
Using the key-on diagnostic flash method, the ECM displayed codes 551, 439, and 423. These likely correspond to:

• 551: Injector trim code mismatch or missing
  
• 439: Boost pressure deviation
  
• 423: Fuel rail pressure anomaly
  

These codes support the theory of ECM confusion and possible injector calibration issues.
Recommendations for resolution

• Reseal cab windows to eliminate vibration artifacts
  
• Confirm ECM type and injector trim compatibility
  
• Use Perkins EST to clear fault codes and verify boost/fuel parameters
  
• Inspect crankcase vent for blockage—can cause blue smoke and misfire
  
• Replace dirty oil and monitor for contamination
  

Conclusion
The JLG G1255A’s miss under load was a combination of cab vibration resonance and ECM mismatch following engine rebuild. While fuel and boost anomalies were present, the acoustic illusion from the window amplified the issue. With proper sealing and ECM-specific diagnostics, the machine can be restored to full performance.