Usage:
3406B (PEEC) Truck Engines ( 8TC)
It is possible for the truck speed shown by the ECAP (Electronic Control Analyzer Programmer) to be different than the actual speed on trucks powered by 3406B Truck Engines equipped with PEEC (Programmable Electronic Engine Control). One cause of this can be an incorrect vehicle speed calibration parameter, which is measured in pulses per mile (PPM). PEEC uses this parameter to determine the truck ground speed.
PPM is determined by: tire revolutions per mile, rear axle ratio, and number of chopper teeth on the drive shaft where the sensor is mounted. (See formula at end of chart.) Thus, if any one of these three variables is not the same as what has already been used to predetermine the PPM, then the PPM data used by PEEC will not be correct. It has been learned that the number of chopper teeth can vary even if the transmission model is the same. Tire revolutions and axle ratios can be obtained, but the chopper wheel is internal.
Any incorrect PPM can be corrected without knowing the number of chopper teeth.
1. Determine the actual ground speed with a wheel dynamometer, or stop watch, or an accurate speedometer.
2. Divide the speed displayed by ECAP by the actual speed, and multiply this answer times the speed calibration parameter already programmed in the electronic control module (ECM). This is the corrected PPM that should be used for the new parameter.
For example, The ECAP speed reading is 55 mph, the actual speed is 65 mph, and the programmed speed calibration parameter is 29836 PPM.
Correct PPM = (55 divided by 65) times 29836. Correct PPM = 25246 PPM.
Program 25246 PPM into the ECM with the ECAP and the correct speed should be displayed.
Calculating The Pulses Per Mile (PPM) When All The Variables Are Known
If you know the rear axle ratio and the number of chopper teeth, the only other variable needed to determine PPM is tire revolutions per mile. The following table provides this for various tire sizes. The radii listed are an average of several major tire manufacturers. Revolutions per mile = 10085 divided by tire radius.
PPM = Rear Axle Ratio times No. of Chopper Teeth times Tire Revolutions per Mile.