System Operation Description:
The ECM uses vehicle speed information for the following features:
• Cruise control
• Fast idle control
• Idle speed control
• PTO operation
• Vehicle speed limiting
• Speedometer
• Progressive shift
• Idle shutdown
• Operation of the air inlet heater
• Secure idle theft deterrent
• Trip data
• Maintenance data
There are three different methods that can be used to obtain the information for the ECM to use for calculating vehicle speed. The ECM can calculate vehicle speed by either of the following means:
• The ECM monitors the signal from a vehicle speed sensor in pulses per kilometer (mile). The vehicle speed sensor is connected to the vehicle speed input circuit (terminals 32 and 33 of the ECM).
This is the “Vehicle Speed Calibration” option.
• The ECM receives a message over the J1939 data link from the Transmission ECU that indicates revolutions per kilometer (mile) of the transmission output shaft. This is the “Vehicle Speed Cal (J1939-Trans)” option.
• The ECM receives a message over the J1939 data link from the Anti-lock Brake System (ABS) that contains information about the wheel speed.
This is the “Vehicle Speed Cal (J1939-ABS)” option.
Each of these methods require a specific calibration number to be programmed into the ECM to calculate vehicle speed.
For the “Vehicle Speed Calibration” option that uses the vehicle speed sensor, the ECM must know the number of pulses that are produced by the vehicle speed sensor during each kilometer (mile) of travel by the vehicle. In order to find the number of pulses that are produced by the sensor, multiply the tire size by the axle ratio. Multiply the number that was obtained in the previous calculation by the number of teeth on the output shaft that is used by the sensor to obtain the pulses.
For the “Vehicle Speed Cal (J1939-Trans)” option, the ECM must know the number of times that the output shaft revolves per kilometer (mile) of travel by the vehicle. This can be calculated by one of the following methods:
• Divide the pulses per kilometer (mile) of the sensor on the transmission output shaft by the number of teeth on the transmission output shaft.
• Multiply the axle ratio by the number of revolutions per kilometer (mile) of the tire.
For the “Vehicle Speed Cal (J1939-ABS)” option, the ECM must know a calibration number. The number is calculated by dividing the actual tire revolutions per kilometer (mile) by the tire revolutions per kilometer (mile) that is programmed into the ABS.
The ECM calculates vehicle speed by converting the vehicle speed signal to kilometers per hour (km/h) or miles per hour (mph). The conversion factor is customer programmable and the conversion factor is stored in pulses per km (PPKM) or pulses per mile (PPM).
The variable for the tire revolution could be affected by changing from bias ply tires to radial tires and vice versa. Also, changing to radial tires that have a low profile could affect the variable for the tire revolution. This result may change the actual vehicle speed which could result in performance complaints or complaints about fuel consumption. If the tire configuration has been changed, determine the new variable for the tire revolution. Enter the new value in pulses per km (PPKM) or pulses per mile (PPM). The preferred method is obtaining the data from the individual tire manufacturers for the specific tire that is being used.
Note: The ECM speedometer signal is set to 18,600 pulses per km (PPKM) or 30,000 pulses per mile (PPM). This setting cannot be changed. All speedometers that are driven by the ECM output (J1/P1 terminal 36 and terminal 37) must match the fixed output of the ECM for proper operation.
Test Step 1. Use the Electronic Service Tool to Determine the Configuration for the Vehicle Speed Input to the ECM
A. Connect the electronic service tool to the cab data link connector.
B. Access the “Configuration Parameter List”. View the “Vehicle Speed Parameters”. Determine if the vehicle is configured to use the “Vehicle Speed Calibration” option, the “Vehicle Speed Cal (J1939-Trans)” option, or the “Vehicle Speed Cal (J1939-ABS)” option.
Expected Result:
RESULT 1 The ECM is configured for the “Vehicle Speed Calibration” option.
RESULT 2 The ECM is configured to use the “Vehicle Speed Cal (J1939-Trans)” option.
RESULT 3 The ECM is configured to use the “Vehicle Speed Cal (J1939-ABS)” option.
Results:
• RESULT 1 – Proceed to Test Step 2.
• RESULT 2 – Proceed to Test Step 3.
• RESULT 3 – Proceed to Test Step 4.
Test Step 2. Calculating Pulses Per Kilometer (Mile) When all the Variables are Known.
A. Calculate pulses per kilometer (PPKM) or pulses per mile (PPM).
PPKM = K X Ra X N.
PPM = M X Ra X N.
K – This symbol represents the tire revolutions per kilometer. This is a constant that is divided by the tire static loaded radius.
M – This symbol represents the tire revolutions per mile. This is a constant that is divided by the tire static loaded radius.
Ra – This symbol represents the rear axle ratio.
The rear axle ratio can typically be found on the housing of the rear axle, or the rear axle ratio can typically be found on the specification sheet for the vehicle.
N – This symbol represents the number of chopper teeth on the transmission drive shaft.
The magnetic pickup sensor is mounted here.
The number of chopper teeth is usually 16. Some transmissions have 11 tooth chopper wheels.
B. Use the electronic service tool to enter pulses per km (PPKM) or pulses per mile (PPM) into the ECM.
Expected Result:
This value is within the valid range of the ECM.
Results:
• OK – Once the value is entered, verify that the ECM is accurately measuring vehicle speed.
Proceed to Test Step 5.
• Not OK – Recalculate the value and re-enter the correct value into the ECM. STOP.
Test Step 3. Calculate the Revolutions Per Kilometer (Mile) of the Transmission Output Shaft
A. The correct revolutions per kilometer (mile) of the transmission output shaft must be entered for the “Vehicle Speed Cal J1939-Trans” setting. If this value is unknown, either of the following calculations can be used to obtain the correct setting:
• Divide the pulses per kilometer (mile) of the sensor on the transmission output shaft by the number of teeth on the transmission output shaft.
• Multiply the axle ratio by the number of revolutions per kilometer (mile) of the tire.
B. Use the electronic service tool to enter the calculated value into the ECM.
Expected Result:
The value is within the valid range of the ECM.
Results:
• Yes – Once the value is entered, verify that the ECM is accurately measuring vehicle speed.
Proceed to Test Step 5.
• No – Recalculate the value and re-enter the correct value into the ECM. STOP.
Test Step 4. Calculate Tire Revolutions Per Kilometer (Mile) When all the Variables are Known
A. The correct calibration number must be entered for the “Vehicle Speed Cal J1939-ABS” setting. If this value is unknown, the following calculation can be used to obtain the correct setting.
Divide the actual tire revolutions per kilometer (mile) by the tire revolutions per kilometer (mile) that is broadcast by the ABS.
For example, the actual tire revolutions per kilometer (mile) could equal 400 revolutions per kilometer (mile). The tire revolutions per kilometer (mile) that is broadcast by the ABS could equal 500 revolutions per kilometer (mile).
The formula would be 400 / 500 = 0.800.
Therefore, 0.800 would be the value that would be entered into the ECM.
B. Use the electronic service tool to enter the
calculated value into the ECM.
Expected Result:
This value is within the valid range of the ECM.
Results:
• OK – Once the value is entered, verify that the
ECM is accurately measuring vehicle speed.
Proceed to Test Step 5.
• Not OK – Recalculate the value and re-enter the
correct value into the ECM. STOP.
Test Step 5. Inspect the Speedometer Calibration.
Determine the speedometer’s vehicle speed calibration setting.
Expected Result:
The speedometer must be calibrated to 18,600 pulses per kilometer (30,000 pulses per mile).
The ECM can only provide the signal that is listed above.
Results:
• OK – The speedometer is correctly calibrated.
STOP.
• Not OK
Repair: Perform the following diagnostic
procedure:
Reprogram the speedometer to the correct setting for the pulses per kilometer (mile).
STOP.
