C15 – Can Not Reach Vehicle Speed Limit

Probable Causes

The probable root causes are listed in order below:
• Diagnostic codes
• Customer specified parameters (normal operation)
• Cold mode
• Electronic system problem
• Fuel supply
• Air intake system or exhaust system
• Leakage in Air-to-Air Aftercooler Core

Recommended Actions

1. Check for diagnostic codes.
a. Connect the electronic service tool to the data link connector.
b. Turn the key switch to the ON position.
c. Check for active diagnostic codes and logged diagnostic codes.
Certain diagnostic codes will limit the vehicle speed to 45 mph.
d. Troubleshoot all diagnostic codes before continuing with this procedure.

2. Check customer specified parameters.
a. Verify that the complaint is not due to normal operation (programmed parameter).
b. Access the “Configuration” screen on the electronic service tool.
c. Check the following parameters:
• “Vehicle Speed Limit”
• “Soft Vehicle Speed Limit”
• “Lower Gears Engine RPM Limit”
• “Intermediate Gears Engine RPM Limit”
• “Gear Down Protection Engine RPM Limit”
• “VSL Protection”
• “Top Engine Limit”
• Incorrect programming of the Vehicle Speed calibration (ppm).
• “PTO Top Engine Limit” with the PTO On/Off Switch turned ON.
• “Maximum PTO Vehicle Speed” with the PTO On/Off Switch turned ON.
d. Set each parameter to the parameter’s maximum value one at a time and determine whether the               problem is corrected.

Note: The “Soft Vehicle Speed Limit” will not allow the vehicle to achieve the programmed VSL under all engine load conditions. Programming the “Soft Vehicle Speed Limit” to NO may be required to eliminate the complaint.

3. Check cold mode operation.
Note: This is normal operation if the problem occurs only after start-up in cold weather.
a. Check the status screen on the electronic service tool.
“Cold Mode” is indicated in the upper corner of any status screen when cold mode is active.

4. 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 6.

5. 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               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 for the                           accelerator pedal position sensor in order to ensure that the duty cycle 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             sensor. Check the duty cycle by slowly operating the accelerator pedal in order to verify that the                 duty cycle 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.

6. 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”.

7. Check the “Powertrain Data Link”. Ensure that the SAE J1939 Powertrain 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.

8. 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”.

9. Check the air intake system for restrictions or leakage. Check the exhaust system for restrictions or         leakage. Refer to Testing and Adjusting, “Air Inlet and Exhaust System – Inspect”.

10. Check for leakage in the Air-to-Air Aftercooler Core . Refer to Testing and Adjusting, “Aftercooler –          Test”.

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