Probable Causes
The probable root causes are listed in order below:
• Diagnostic codes
• Soft cruise control
• Poor electrical connections
• Battery power or ground to the ECM
• Air inlet system (Boost)
• Accelerator pedal position sensor
• Fuel supply
• Vehicle speed circuit
• Power train control
• Torque limit switch
Recommended Actions
Perform the following diagnostic procedures in order:
1. Certain diagnostic codes may cause poor performance. These diagnostic codes are intended to protect the engine.
a. Connect the electronic service tool to the data link connector.
b. Check for active codes and/or logged codes. Troubleshoot any codes that are present before continuing with this procedure.
2. If the problem only occurs in cruise control, check the parameter for “Soft Cruise Control”.
a. Disable the “Soft Cruise Control” parameter.
b. Verify that the problems do not return.
c. If the problems do not return, the problem was due to normal operation.
Soft cruise control provides an 8 km/h (5 mph) operating range around the cruise control set speed in order to provide a smoother cruise control.
d. Enable the “Soft Cruise Control”.
3. Check the vehicle harness and the connectors.
Refer to Troubleshooting, “Electrical Connectors – Inspect”.
4. Check the Battery power or ground to the ECM.
a. Ensure that the following connectors have been installed correctly:
• J1/P1 and J2/P2 ECM connectors
• Front valve cover connector
• Rear valve cover connector
• Primary engine speed/timing connector
• Secondary engine speed/timing connector
5. Check for an electronic system problem.
a. Monitor the fuel position, the Rated Fuel Limit and the FRC Fuel Limit.
While the vehicle is operating under full load, the parameters should meet the following conditions:
• The fuel position is equal to the Rated Fuel Limit.
• The fuel position is less than the FRC Fuel Limit.
If the fuel position is equal to the Rated Fuel Limit and the fuel position is less than the FRC Fuel Limit, the Electronics are operating correctly. Go to Step 7.
6. If the fuel position is not equal to the Rated Fuel Limit and/or the fuel position is not less than the FRC Fuel Limit, perform the following tests in order:
a. When the engine is OFF, turn the key switch to the ON position. Check the boost pressure on the status screen on the electronic service tool.
When the engine is OFF, the boost pressure should be 0 kPa (0 psi).
b. Monitor the status of the accelerator pedal position sensor. Operate the engine at high idle with the vehicle in neutral.
If the engine can not reach the programmed “Top Engine Limit”, check the duty cycle of the accelerator pedal position in order to ensure that the signal is within calibration (75 to 90 Percent at high idle).
If the accelerator pedal position is unstable, check the duty cycle of the accelerator pedal position. Check the duty cycle of the accelerator pedal position by slowly operating the accelerator pedal in order to verify that the accelerator pedal position sensor is within calibration. The accelerator pedal position sensor is within calibration when both of the following conditions are met:
• 10 to 22 percent at low idle
• 75 to 90 percent at high idle
Also, ensure that the duty cycle changes as the pedal position changes.
If a problem with the accelerator pedal position sensor is suspected, refer to Troubleshooting, “Accelerator Pedal (Throttle) Position Sensor Circuit – Test”.
Note: If the engine exceeds the Top Engine Limit, the “Top Engine Limit with Droop” is programmed to YES.
7. Monitor the vehicle speed signal. Ensure the accuracy of the vehicle speed signal by comparing the vehicle speed signal to the actual vehicle speed.
If the vehicle speed is greater than the “VSL”, the Power is limited. If a problem is discovered, refer to Troubleshooting, “Vehicle Speed Circuit – Test”.
8. Check the “Powertrain Data Link”. Ensure that the power train data link is not limiting the power.
a. Monitor the “Powertrain Data Link” status screen while the vehicle is experiencing problems.
If the “Powertrain Data Link” status screen indicates that the “Powertrain Data Link” is limiting the power or the rpm, verify that this is not normal operation. If this is not normal operation, refer to the vehicle OEM for repairs.
9. Check the fuel supply.
a. Monitor the exhaust for smoke while the engine is being cranked.
If no smoke is present, there may be a problem with the fuel quality or there may be a problem with the fuel supply.
b. Check the fuel quality. Refer to Testing and Adjusting, “Fuel Quality – Test”. Also refer to the Truck Performance and Driveability Diagnostic Guide, LEBT3477.
c. Check the fuel pressure. Refer to Testing and Adjusting, “Fuel System Pressure – Test”.
d. Ensure that the fuel system has been primed.
Refer to Testing and Adjusting, “Fuel System – Prime”.
e. Check for fuel supply lines that are restricted.
f. Check the fuel filters.
g. If the temperature is below 0 C (32 F), check for solidified fuel (wax).
h. Check for air in the fuel system. Refer to Testing and Adjusting, “Air in Fuel – Test”.
10. Check the torque limit switch.
a. Check the setting of the parameter for the “Torque Limit”.
If the “Torque Limit” is programmed below 3400 N·m (2500 lb ft), check the status screen for the torque limit switch. Ensure that the status screen indicates that the torque limit switch is OFF. If the status of the torque limit switch is ON and a torque limit is programmed, the system is operating
correctly. If the switch that is connected to this signal input is not switching ON and switching OFF, refer to Troubleshooting, “PTO Switch Circuit – Test”.
