03 Trailblazer won't shift into 4WD

No hubs on 2002 Chevy 2500. I'd check the front axle actuator (engages front axle when 4WD is engaged). Belive it's on the passage side axle tube of front differential. Could be wrong, but that's what it sounds like. Or the front ring/pinion is shot. They don't turn when not in 4wd.

what speed are you driving at, in 4wd ? before it does this ?

Possible the vacuum bleed-off to the switch you installed isn't allowing it to completely disengage. Pull the bleed-off line from the new switch and see if it works. could be just a tight spot/bent closed in the bleed-off line or the vent plugged.

i've been told it was the 4wd actuator located on the passenger side near were the cv joint goes in to the front axle a good test is to floor jack it up so both wheels are off the ground and have someone on the other side , put in nutrual turn both wheels one way at same time and the 4wd drive shaft should turn if not its your actuator, costs about $86 for anew actuator and takes about 5 mins to change once you have the tire off

What do the lights on the 4x4 selector switch do when you switch to A4WD or $HI or 4LO positions? Do they blink and revert to previous selection? Has the Transfer Case fluid been changed at 50K intervals?
There are actuators on the Transfer Case and Front Differential that control the application of power to the front wheels. These actuators are controlled by the TCCM (Transfer Case Control Module). It takes a qualified 4 wheel drive technician to diagnose these systems.

Did you make sure the transfer case is engaging? Have someone put it in 4wd and look at the front drive shaft see if its turning. If so that means the vaccum switch is either not engaging the lever on the axle hub. Make sure the lever on the axle punkin or gear hub is not sticking. Also check the linkage under the truck.

Have you checked if the problem is with the front hubs or with the transfer case ?
Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc ).
Key on, engage 4x4 and try to turn the front drive shaft.
If it turns it is a problem with the transfer case.
If it doesn't it is a front axle engagement problem.

Next with the truck running, have someone turn the switch to 4WD, and you should see the shift motor diaphragm move


If not, could be the shift fork is stuck ( try manually moving ) or cold be a vacuum leak back to the vacuum shift solenoids


You can remove the 4WD mode switch, and test the resistance on it ( this is also the vacuum line routing )

You might find moisture in the vacuum shift motor diaphragm, not allowing the vacuum to operate it correctly.

try putting in nuetral and then shift into high gear . if still does not work sounds like the 4x4 acctuator is bad

Your front hubs are automatic locking hubs, and the gears are hard nylon. These hubs break very easily, therefore causing you to lose your front wheel drive in 4 high, or 4 low. If you replace the hubs with "WARN (name brand) " manual locking hubs, you will never have the issue again. Of course, you will have to get out of the truck and manually lock the hubs, but they are metal gears, and much more durable. I've personally seen the issue several times, and changing the hubs has resolved the issue every time.

Look for damaged wires, or something bent, on the Auto Disconnect Device on the front axle before you do anything else. The 4WD HI light is not coming on because the front diff is not engaging.
Re 4 LO: In 4 LO, you ar NOT getting "4 LO", what you are really getting 2WD LO, because of the output gearing of the transfer case.
In any case, your front diff is not locking in, in either mode. The A.D.D. controls the 4 Hi lite.