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Possible Causes - Engine ECU not calibrated; 2-speed axle switch engaged.
Procedure:
1. If the vehicle is equipped with a 2-speed axle and switch, check the 2-speed input
to the SMC as described in Section 5.4.5; if not, continue with next step.
2. Connect the HDT to the J1708 diagnostic connector.
3. Road-test the vehicle and compare the speedometer reading with the HDT reading.
a.
If the readings are about the same and you suspect they do not reflect the
vehicle's true speed, follow the engine manufacturer's procedures to
check the Engine ECU calibration.
b. If the readings are different, replace the SMC.
Possible Cause - A loose J1708 connection is causing odometer messages to the SMC
to be lost; Engine ECU not calibrated correctly.
Procedure:
Check the calibration of the Engine ECU by following the engine manufacturer's pro-
cedures. If the Engine ECU is calibrated properly, replace the SMC.
Note - A slight error in calibration parameters such as rear axle ratio and/or tire
revolutions could cause the odometer to increment slightly higher or lower
with little or no difference in the SMC pointer accuracy.
The Engine ECU odometer reading and the SMC odometer reading will be
different if the SMC or Engine ECU has ever been replaced, or if the vehicle
is equipped with a two-speed axle or a PTO option.
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