3126B and 3126E On-highway Engines Caterpillar

Vehicle Speed Circuit - Calibrate

Usage:

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

With SEP00 and newer software, there are two 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 ECM vehicle speed input circuit (terminals 32 and 33). This is the "Vehicle Speed Calibration" option.
The ECM receives a J1939 Datalink Broadcast Message from the Transmission ECU that indicates revolutions per kilometer (mile) of the transmission output shaft. This option is only available with SEP00 and newer software. This is the "Vehicle Speed Cal (J1939-Trans)" 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.

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 tire revolution variable could be affected by changing from bias ply tires to radial tires, and vice versa. Also, changing to low profile radial tires could affect the tire revolution variable. This result may change the actual vehicle speed which could result in performance complaints or fuel consumption complaints. If the tire configuration has been changed, determine the new tire revolution variable. 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). For "GM Truck Manufacturer" configurations with SEP00 or newer software, the output of the speedometer is set to 2,485 PPKM (4,000 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. Determine if the ECM Receives the Vehicle Speed Input from the "Vehicle Speed Calibration" Option or from the "Vehicle Speed Cal (J1939-Trans)" Option.

Connect Caterpillar Electronic Technician (ET) to the cab data link connector. If the software for the ECM is older than SEP00, proceed to Test Step 3.

If SEP00 or newer software is used in the ECM, access the "Configuration Parameter List". View the "Vehicle Speed Input" parameter. Determine if the vehicle is configured to use the "Vehicle Speed Calibration" option, "J1/P1:32&33" or the "Vehicle Speed Cal (J1939-Trans)" option.

Expected Result:

The ECM is using software that is dated prior to SEP00 or the ECM is configured to use the "Vehicle Speed Calibration" option.

Results:

Yes - Proceed to Test Step 3.
No - The ECM is configured to use the "Vehicle Speed Cal (J1939-Trans)" option. Proceed to Test Step 2.

Test Step 2. Calculate the Revolutions Per Kilometer (Mile) of the Transmission Output Shaft

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.

Program 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 4.
No - Recalculate the value and re-enter the correct value into the ECM.STOP

Test Step 3. Calculating Pulses Per Kilometer (Mile) When all the Variables are Known.

Calculate pulses per kilometer (PPKM) or pulses per mile (PPM).
PPKM = K × Ra × N.
PPM = M × Ra × 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.

Use ET 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 4.
Not OK - Recalculate the value and re-enter the correct value into the ECM.STOP

Test Step 4. Inspect the Speedometer Calibration.

Determine the speedometer's vehicle speed calibration setting. You may be required to contact the OEM dealer or you may be required to send the vehicle to the OEM dealer for this information.

Expected Result:

The speedometer must be calibrated to one of the following values:

For "Other Truck Manufacturer" configurations, the value is 18,600 pulses per kilometer (30,000 pulses per mile).
For "GM Truck Manufacturer" configurations that have SEP00 or newer software, the value is 2,485 pulses per kilometers (4,000 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:Send the vehicle to the OEM dealer or reprogram the OEM speedometer to the correct setting for the pulses per kilometer (mile).
STOP