I have a 1999 Chevy Blazer 4wd 2 door.
It will not kick from 4wd into 2wd. I replaced Transfer case control module,and Acuator in it. I hear a grinding noise when putting from 4wd to 2wd. Any ideas of what is going on?

 

Are you performing the shift in neutral, while stopped?

Which actuator did you replace - the one on the front axle or the encoder motor on the transfer case?

Have you checked the fluid level & condition in the transfer case?

 

I replaced acuator in transfer case. Yes I have shifted in neutral when stopped and fluid is good.

 

Was there any work done prior to this problem?

Are any of the indicator lights on the buttons flashing when the shift is attempted?

Have you checked for codes? Note that the CEL doesn’t have to be on for codes to be stored.

If you have access to jackstands, try getting all 4 wheels off the ground and have a helper press the shift buttons as you’re underneath - isolate whether the grinding is coming from the transfer case or front axle.

 

4 wheel drive works then when changing into 2 wd when you get to 20 mph it kicks out.
low 4 light flashes.

 

When attempting to drive in 2WD, is the engine RPM excessively high (2,500 or higher) before hitting 20 MPH?

Are you able to determine if the front axle is disengaging? Have you also checked the front axle fluid level & condition? Have you checked to see if the vacuum supply to the front axle actuator is adequate?

There’s several possible paths that diagnosis can take - I’d recommend checking for trouble codes using a scanner capable of reading codes in the PCM & transfer case control module.