I got a 2000 Oldsmobile Bravda and has smart track that works when it wants to the service light comes on once and awhile. When I really need 4wd it doesn't kick in Help

There are three different transfer case's ! Which of these does your olds have ?

Transfer Case - NVG 136-NP4 (One Speed Automatic)
Transfer Case - NVG 233-NP1 (Two Speed Selectable)
Transfer Case - NVG 236 (Two Speed Automatic)

All three have an electronic control module ! When the light is on DTC'S are stored in the transfer case shift control module .

SERVICE indicator (AWD/4WD) lamp
The SERVICE indicator (AWD/4WD) lamp is an integral part of the instrument cluster and cannot be serviced separately. This lamp is used to inform the driver of the vehicle of malfunctions within the automatic transfer case (ATC) system. The SERVICE indicator (AWD/4WD) lamp is controlled by the transfer case shift control module via a Class 2 serial data message.

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 speed sensors on the rear output shaft is the vehicle speed sensor (VSS). It is an input to the vehicle control module (VCM). The VCM 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 propshaft speed sensor to a Rear Propshaft Speed in RPMs 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 front propshaft speed sensor to Front Propshaft Speed in RPM to be used for calculations, and to monitor the difference between the front and rear propshaft sensor speed. It is also used in the AUTO 4WD 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.

Transfer Case Encoder Motor
The transfer case encoder motor is a permanent magnet D.C. motor and gear reduction assembly. The encoder motor is controlled with a Pulse Width Modulated (PWM) signal provided by the transfer case shift control module. It is located on the left hand side (drivers side) of the transfer case. When activated it turns the sector shaft of the transfer case to shift the transfer case. This circuit consists of a power supply relay (Motor Feed A circuit ) that supplies voltage to the motor. The Motor Control B circuit is a PWM driver that varies the duty cycle to control the amount of current through the motor to ground.

There are a number of codes that can set .
DTC C0300 Rear Speed Sensor Malfunction
DTC C0305 Front Speed Sensor Malfunction
DTC C0308 Motor A/B Circuit Low
DTC C0309 Motor A/B Circuit High
DTC C0310 Motor A/B Circuit Open
DTC C0315 Motor Ground Circuit Open
DTC C0374 General System Malfunction
DTC C0376 Front/Rear Shaft Speed Mismatch
DTC C0550 ECU Malfunction
DTC C0611 VIN Information Error

You don't want to just start replacing parts , that could cost $$$$$$ . You could check connections at all the sensors an the control module , but other then that ,well . Your best bet , take it to a qualified repair shop .