http://www.offroadtb.com/articles/tech-docs/transfer-case-tech-the-nvg-226-np8-theory-of-operation wrote:Transfer Case Speed Sensors
There are three speed sensors on the automatic transfer case (ATC), two on the rear output shaft and one on the front output shaft. Each speed sensor is a permanent magnet (PM) generator. The PM generator produces a pulsing AC voltage. The AC voltage level and number of pulses increases as speed increases.
Vehicle Speed Sensor
One of the two on the rear output shaft is the vehicle speed sensor (V SS) input to the powertrain control module (PCM). The PCM sends this information to the transfer case shift control module via the Class 2 Serial Data bus.
Rear Propshaft Speed Sensor
The transfer case shift control module converts the pulsating AC voltage from the rear transfer case speed sensor to a rear propshaft speed in RPM to be used for calculations. The rear propshaft speed can be displayed with a scan tool.
Front Propshaft Speed Sensor
The transfer case shift control module converts the pulsating AC voltage from the font transfer case speed sensor to front propshaft speed in RPM to be used for calculations, and to monitor the difference between the front and rear sensor speed. It is also used in the AUTO (Adapt) mode of operation to determine the amount of slip and the percent of torque to apply to the front axle. The front propshaft speed can be displayed with a scan tool.