Diagnosing J1708 Communication Failures on the Mack GU713

[MikePhua]

The Role of J1708 in Heavy Truck Diagnostics
J1708 is a legacy serial communication protocol used in commercial vehicles to link electronic control modules (ECMs) and diagnostic tools. Though largely replaced by J1939 in newer trucks, J1708 remains active in many 2010–2015 models, including the Mack GU713. This protocol operates over twisted-pair wiring and transmits data at 9600 baud, allowing modules like the engine ECM, transmission controller, and cab electronics to share fault codes and operational data.
The Mack GU713, introduced in the mid-2000s, is a vocational truck designed for dump, mixer, and heavy haul applications. Built on the Granite platform, it features Mack’s proprietary MP engines and a multiplexed electrical system. Thousands of GU713 units are still in service across North America, making diagnostic reliability critical for fleet uptime.
Symptoms of Communication Failure
In one documented case involving a 2012 Mack GU713, technicians were unable to connect to the engine ECM using diagnostic tools like JPRO or PTT. A bench harness test confirmed the ECM was functional, isolating the fault to the truck’s wiring. Voltage checks at the diagnostic plug revealed a dead short to ground on Pin G, which corresponds to ECM connector A33. Resistance between Pin G and battery negative measured 8 ohms, indicating a grounded circuit.
Despite disconnecting firewall plugs, cannon connectors, the dashboard, and the VECU (Vehicle Electronic Control Unit), the short persisted. This narrowed the fault to the cab harness or an internal module.
Understanding Cab Harness Vulnerabilities
The cab harness in multiplexed trucks like the GU713 carries both power and data signals. Over time, vibration, moisture, and heat can degrade insulation, causing wires to short against the chassis or each other. Common failure points include:

• Firewall pass-through connectors
  
• Harness bends near the fuse panel
  
• Areas behind the dashboard where harnesses are zip-tied or compressed
  

In this case, the technician suspected an internal short within the harness, possibly caused by pinched wires or corrosion. The fact that unplugging major modules did not change resistance supported this theory.
Recommended Diagnostic Strategy
To isolate the fault without dismantling the entire cab:

• Use a tone generator and probe to trace continuity along the suspect wire
  
• Inspect harness sections visually for abrasion, swelling, or discoloration
  
• Check for aftermarket modifications like added switches or lighting that may tap into the J1708 circuit
  
• Measure resistance incrementally along the harness to locate the drop point
  

If the fault cannot be isolated externally, replacing the cab harness may be necessary. This is a labor-intensive process but ensures long-term reliability.
Preventive Measures and Best Practices

• Avoid splicing into communication lines for accessories
  
• Use dielectric grease on connectors to prevent corrosion
  
• Secure harnesses with soft mounts to reduce vibration stress
  
• Label and document all wiring changes during repairs
  

A fleet in Ohio reported that 70% of their communication faults were traced to improperly installed aftermarket radios and GPS units. After standardizing installation protocols, fault rates dropped significantly.
Conclusion
J1708 faults in the Mack GU713 often stem from internal cab harness shorts, especially when diagnostic tools fail to connect despite a healthy ECM. Systematic resistance testing and harness inspection are key to isolating the issue. While cab harness replacement is a last resort, it may be the only solution when faults are buried deep within the loom. Maintaining clean, well-documented wiring practices ensures diagnostic integrity and keeps these workhorses on the road.