C65-85 CAT Engine Speedometer Fluctuate Cold Start - kw MD DIC diesel LG5 Allison 2400 transmission MTW fluctuation indicator check light flash

The following diagnosis might be helpful if the vehicle exhibits the described symptoms in the PI.

Some dealers and customers have reported that the speedometer indicator needle fluctuates at cold start up on some 2004 C65/75/85 models with CAT Engine Option LG5 and the Allison 2400 Series Automatic Transmission Option MTW. The speedometer is erratic and the check engine light flashes when the vehicle speed reaches 48 to 50 MPH after a cold start up. This condition will continue until the engine coolant temperature reaches approximately 150 degrees F; then the speedometer will read accurate and the check engine light stops flashing. In order to correct speedometer fluctuation on cold start up, it will be necessary to reprogram the vehicle speed input calibrations in the CAT Engine Control Unit -ECU using the following process.

Connect the CAT Electronic Technician -CAT-ET diagnostic tool to the diagnostic link connector. Refer to SI Document # 954278 for the location of the CAT-ET diagnostic link connector under the dash.

1. Using the CAT-ET go to Vehicle Speed Parameters from the programming menu select the Vehicle Speed Calibration line and record the number of Pulses Per Mile -PPM under the Value column.
2. Next go to Input Selections from the programming menu and select the Vehicle Speed Input line under the Description column.
3. Choose the Change button to reprogram the Vehicle Speed Input Parameter Value. The Change Parameter Value box should display the Current Value as J1/P1:32-33.
4. Choose the New Value: down arrow select J1939 - Trans from the drop down menu and click on OK.
5. The Vehicle Speed Input line should now display J1939 - Trans in the value column.

NOTE: Now that the Vehicle Speed Input source has been changed to the J1939 - Trans input, it will be necessary to reprogram the vehicle speedometer calibration for driveline revolutions per mile in the CAT-ECU.

1. Calculate the driveline revolutions per mile using the following formula.

Original Vehicle Speed Calibration / Divided by 40 # of VSS teeth

Pulses Per Mile recorded above on Allison Trans. Output Shaft = Driveline Revs. Per Mile

EXAMPLE: 89174 #PPM / 40 #TEETH = 2229.5 DRIVELINE REVOLUTIONS PER MILE

2. Go to Vehicle Speed Parameters from the programming menu and select the Vehicle Speed Cal J1939-Trans line under the Description column.
3. Choose the Change button to display the Change Parameter Value box. Enter the driveline revolutions per mile number calculated from the above formula in the New Value: box and click on OK.
4. The Vehicle Speed Cal J1939-Tran line should now display the driveline revolutions per mile number in the Value column.

Reprogramming the CAT-ECU to eliminate the speedometer fluctuation on cold start up is complete. Contact GMICT Med. Duty Truck Technical Assistance at 1-800-633-4863 Prompt #3 if additional information is required.

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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.

(2004 Chevrolet GMC Medium Duty Conventional C65 75 85 Models with Caterpillar Engines Option LG5 and Allison 2400 Series Automatic Transmission Option MTW.)

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