4x4 won;t ingage,2000 chevy tracker

Hi Wayne how's it going ? Here is a Transfer Case Circuit Description ! then I will get into some possibility's of what could be wrong !
There are two main components for the four wheel drive. The powertrain control module (PCM) and the front drive axle actuator pump. The pump is controlled by the PCM. The pump compresses air to engage the front axle. When the transfer case is in four wheel drive (4H or 4L), the four wheel drive switch closes providing a ground signal to the PCM. The PCM actuates the pump which pressurizes the actuator in the front axle. This pressure engages the axle hub which connects the front drive shaft to the axle shafts, allowing the transmission/transfer case to turn the front wheels. The PCM will not operate the pump for more than 10 seconds in order to protect it. The PCM will also provide a path to ground for the 4WD indicator in the instrument cluster. The indicator is provided battery voltage when the ignition switch is in the ON or START position through the IG fuse. With power and ground supplied, the indicator lights.
When the transfer case is in 4L (with auto trans), the four wheel drive low switch sends a ground signal to the PCM. The PCM will not engage the torque converter clutch while this signal is being received.
When the transfer case is returned to the two wheel drive mode, the release valve in the pump opens and releases the pressure in the actuator. Without pressure, the hub disengages.The PCM also releases the ground to the 4WD indicator which causes the indicator to turn off.
Does the four-wheel drive indicator light with 4x4 engaged ? DEFINITION: The 4WD indicator does not illuminate with 4WD engaged. All other engine and transmission functions are normal. Refer to: Electric Motor and Actuator Check in Driveline/Axle-Front Axle (w/diff).
The next step of testing the electric motor an actuator involve a lot of steps an a special tool -
J 43846 Actuator Pressure Gauge
This a factor diagnostic flow chart , I'm not familiar with the in's an out's of this system , the most probable thing's that go wrong with this system. If you want I can send you the diagnostic flow chart !
Electric Motor and Actuator Check
Tools Required J 43846 Actuator Pressure Gauge

1. Raise the vehicle. Support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
2. If the vehicle is equipped with an engine skid plate, remove the four bolts that hold the skid plate in place. Remove the front skid plate from the vehicle.
   
   
4. Install theJ 43846 with the air hose connecting between the air pump assembly and the front differential actuator.
5. Tighten the adjusting knob as far as it stops.
Important: Be careful not to bend any part of the hose.

6. Turn on the ignition switch and shift transfer lever from 2H to 4H range.
7. Make sure to check that the motor of the actuator starts running (a running sound can be heard) within 1 second and it stops when the actuator pressure gauge indicates specified pressure. Check for air leakage during the procedure. Pressure specification is: 38.0 kPa (5.40 -- 8.25 psi).
8. Check that the pressure indicated on the gauge drops as soon as the transfer lever is shifted to the 2H position.
9. Start the motor of the actuator pump by shifting the transfer lever to the 4H position again. Loosen the adjusting knob to lower the pressure on the gauge. The actuator motor should start to run again and stop in about 4 seconds.
10. With the adjusting knob of the actuator pressure gauge loosened, shift the transfer shift lever to neutral or the 2H position and then 4L or 4H position. Pump motor should stop in about 10 seconds after the motor st4x4 won;t ingage,2000 chevy tracker - showgif.do?image=461445g src="/uploads/images/showgif.do?image=461445" alt="showgif.do?image=461445" class="h_mi" />
11. 
12. Disconnect the air hose (2) from the actuator pump assembly and connect it the front differential air hose inlet. Loosen the adjusting knob.
13. Connect the air compressor air hose to the actuator pressure gauge.
14. Compress air into the actuator pressure gauge and turn knob to obtain specified pressure. Specified pressure : 38.0 kPa (5.40 psi)
15. While both wheels are lifted off the ground, turn one of the front wheels by hand and check the other wheel for the following:

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When the actuator pump is not working (by not applying pressure) the other front whee4x4 won;t ingage,2000 chevy tracker - showgif.do?image=461486ary while the wheel turned by hand is turning.

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Install the actuator pressure gauge to the actuator pump assembly (1) .

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Install a voltmeter and the battery to the pump assembly (1) .

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Tighten the adjusting knob as far as it will stop and the air check side opening by a hose as a blind plug.

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Check that the actuator assembly motor starts to run (a running sound can be heard) when the battery is connected and when it stops when specified pressure is obtained. Specified pressure is: 38.0- 58.0 kPa (5.40 -- 8.25 psi).

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Ensure that the voltmeter is within specifications when the motor is running should be about 0v and when the motor has stopped it should be about 10 -- 14v
good luck !