Diagnosing No-Start Conditions in JLG Telehandlers with Cummins QSB4.5 Engines

[MikePhua]

The JLG Telehandler and Its Engine Integration
JLG Industries, founded in 1969 and now a subsidiary of Oshkosh Corporation, is a global leader in aerial work platforms and telehandlers. Their heavy-duty telehandlers are widely used in construction, agriculture, and logistics. The integration of Cummins diesel engines, particularly the QSB4.5, into JLG units reflects a commitment to reliable power and emissions compliance. The QSB4.5 is a 4.5-liter inline-four turbocharged diesel engine, part of Cummins’ B-series, known for its electronic control, high-pressure common rail fuel system, and robust diagnostics.
By 2020, JLG had delivered tens of thousands of telehandlers globally, with the QSB4.5 powering models such as the 1055 and 1255 series. These machines rely heavily on electronic engine management, making them vulnerable to sensor faults, wiring issues, and moisture intrusion—especially after aggressive cleaning procedures.
Terminology Annotation

• ECM (Engine Control Module): The onboard computer that manages fuel injection, air intake, and engine timing based on sensor inputs.
  
• MAF Sensor (Mass Air Flow): A sensor that measures the volume and density of air entering the engine, critical for fuel-air ratio calculations.
  
• Injector Driver Fault: An error indicating that the ECM cannot properly control one or more fuel injectors, often due to electrical or grounding issues.
  
• Cranking RPM: The rotational speed of the engine during starter engagement, typically 150–200 rpm for diesel engines.
  
• Fuel Rail Pressure: The pressure within the common rail fuel system, which must reach a minimum threshold (often 4,000–5,000 psi) for injection to occur.
  

Sequence of Failure and Initial Observations
The issue began after the engine radiator was pressure washed. The machine started briefly, moved a short distance, then shut off. It restarted intermittently but would stall under load. Eventually, it failed to start altogether. Diagnostic tools revealed active fault codes for all injector drivers and the mass air flow sensor. During cranking, the engine reached 160 rpm and built approximately 5,000 psi of fuel pressure—suggesting that the mechanical fuel system was functional but electronic control was compromised.
This pattern points to moisture intrusion affecting electrical connectors, particularly those linked to the ECM and sensor harnesses. Water ingress can cause short circuits, signal loss, or corrosion, leading to cascading faults across multiple systems.
Troubleshooting Strategy and Field Techniques
To resolve the no-start condition, a structured approach is essential:

• Disconnect and inspect all engine-related connectors, especially those leading to the ECM, injectors, and MAF sensor.
  
• Use contact cleaner to remove moisture and debris from terminals. Allow connectors to dry thoroughly before reassembly.
  
• Attempt to clear fault codes using a diagnostic tool. If codes persist, they may be latched due to unresolved electrical faults.
  
• Crack injector lines during cranking to verify fuel delivery and purge trapped air. This can help re-prime the system after fuel interruption.
  
• Check ECM grounding and power supply. A weak ground or intermittent voltage can prevent injector drivers from functioning.
  

In this case, moisture was found in multiple connectors. Cleaning and reseating did not immediately resolve the issue, and fault codes remained active. This suggests either persistent electrical damage or a compromised ECM.
A Story from the Field
In 2023, a rental fleet in Nairobi experienced a similar issue after pressure washing a JLG telehandler. The machine failed to start, and diagnostics showed injector driver faults. Technicians discovered that water had entered the ECM connector through a cracked seal. After replacing the connector and resealing the harness, the machine started normally. The incident led the company to revise its cleaning protocols, banning high-pressure washing near sensitive electronics.
Industry Trends and ECM Reliability
Modern diesel engines rely heavily on electronic control, and ECMs are increasingly sealed and vibration-resistant. However, moisture remains a leading cause of failure. In 2024, Cummins introduced a new ECM design with integrated moisture sensors and self-diagnostic routines. These systems alert operators before faults occur, reducing downtime.
JLG has also begun integrating CAN bus diagnostics into their telehandlers, allowing real-time monitoring of engine parameters and fault codes via onboard displays. This trend reflects a broader shift toward predictive maintenance and remote troubleshooting.
Preventative Measures and Recommendations
To prevent no-start issues:

• Avoid pressure washing near ECMs, sensors, and harness connectors
  
• Use dielectric grease on connector terminals to repel moisture
  
• Inspect harness routing for abrasion and pinch points
  
• Replace damaged seals and grommets during routine service
  
• Keep a diagnostic tool on hand to monitor fault codes and sensor values
  

For machines operating in humid or tropical environments, consider installing moisture barriers or relocating sensitive components away from high-risk zones.
Conclusion
No-start conditions in JLG telehandlers powered by Cummins QSB4.5 engines often stem from electrical faults triggered by moisture intrusion. While the mechanical systems may remain intact, compromised sensor signals and injector driver errors can prevent ignition. By following a methodical troubleshooting process and adopting preventative maintenance practices, operators can restore functionality and protect their machines from future failures. As electronic integration deepens, safeguarding the ECM and its network becomes as critical as maintaining the engine itself.