G 20 Van V6-262 4.3L VIN Z (1993)
Vehicle Speed Sensor: Description and Operation
2 Wheel Drive
VSS 2WD (A/TRANS.) VEHICLE SPEED SENSOR
4 Wheel Drive or All Wheel Drive
VSS 4WD Or AWD
PURPOSE
The PCM uses speed information from this sensor to determine the following:
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Transmission output speed.
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Vehicle speed.
^
Calculate TCC slip.
OPERATION
The vehicle speed sensor is a magnetic induction type. The vehicle sensor is accessible from the left hand side of the transmission or transfer case
near the rear. Voltage is induced in the vehicle sensor by the rotor pressed on the output shaft.
The transmission output shaft. RPM is determined differently for 2WD and 4WD vehicles. On a 2WD vehicle, the vehicle speed sensor is located
on the transmission. On a 4WD vehicle, the vehicle speed sensor and the 4WD Lo switch are located on the transfer case. The PCM uses both the
transfer case vehicle speed signal and the 4WD Lo switch to determine the transmission output shaft speed. In both applications the vehicle speed
sensors send an AC signal to the VSS buffer module which relays 2 digital signals to the PCM. This module converts the VSS into the appropriate
miles per hour on one of the signals, then relays it to the PCM.
The PCM uses this speed signal to control engine functions and DTC 16. The other signal is the transmission output speed signal. This signal is
buffered b the VSS buffer. This signal is used for shift points, line pressure control, and TCC control.
DTC 24 or 72 will set if a fault exists in the vehicle speed sinsor circuit. When the fault remains and DTC 24 or DTC 72 are set, the PCM will
only allow second gear and maximum line pressure to occur. If the fault is removed, normal operation will resume after the next ignition cycle.
