my 2001 blazer 4x4 isnt working. when you put it in 4 hi it makes a sound but the light doesnt come on . then when i put it back into 2 hi the light comes on but it starts whinning and grinding until i put it back into 4 h!!! does any one have any ideas where to start ?

 

if its the automatic console, replace the 4wd trasfer motor, cost 130 at Napa but, its definitly something u can do yourself. along with that "lube up" lol, the 4wd shaft and just get everything cleaned up and hopefully that will get rid of the grinding sound, it may be the motor is shot and unable to turn the shaft from 2hi to 4wd but, give that a try and let me know how that works out for ya bud

 

The whining sound leads me to believe that the axle is engaging and when you take it out of four wheel drive it is not disengaging all the way, so that would most likely be a vacuum problem, which may be as simple as the vacuum switch on the transfer case (costs like 15-20 bucks) To check this, pull the battery and battery tray and then reconnect them, if the actuator has vacuum with the truck "in" 2WD the vacuum switch is bad which would cause your whining sound.

Is the transfer case engaging? This can be checked by pressing the 4Hi button and then crawling underneath to see if you can turn the front drive shaft. If you can't that means your t-case is engaging and it could be as simple as the switch in the dash is bad. If you can still turn the drive shaft, it could be the switch, the TCCM (Transfer case Control Module) which is located behind the passenger side kick panel, or as Volff said the encoder motor.

On the topic of the encoder motor, will it go into 4Low? because if it just wont go into 4Hi but will go into 4lo and 2hi, then it could be the encoder motor. IMO, before immediately assuming its the encoder motor I'd try some of the other stuff I mentioned, much cheaper to check and your not just throwing parts at the truck.

Good luck and let us know what you figure out then we'll go from there!