In order for the 4wd Allison to shift properly, it must receive a Vehicle Speed Sensor (VSS) signal. This sensor is typically located in the transfer case on a factory equipped Duramax 4×4 truck. It consists of a magnetic pickup that reads the rotational speed of a 40 tooth count reluctor wheel. The TCM needs three values programmed into it in order for it to get an approximate vehicle speed calibration:
1. Final Drive ratio
– High range total ratio from the transmission output shaft to the rear axle
– If the speed sensor is located after the transfer case low range gearing, we must also know the value of that reduction ratio (the 4wd low input wire must receive a signal when in low range to apply this ratio)
2. Speed sensor Reluctor wheel tooth Count (this is typically 40 teeth per revolution and it is recommended that this the value used if you are custom machining the reluctor for your application)
3. Tire Overall Diameter
So its important to know where the speed sensor is located (before or after the Low range of the T-case), and what the gear ratios are.
If the final drive ratio or tire size are slightly off, it will still work fine but the shifts will be slightly higher or lower than the calibrated values. However if there is any difference between the measure shaft speed and the transmission output shaft speed, the TCM will assume the clutches are slipping and put the transmission in Limp mode.
It is typical for many custom builds to locate this in the adapter on the rear of the trans which works great, the only drawback is that the speedometer will not be correct when the vehicle is in Low Range.