I have a 2000 2 dr blazer with the 4.3 vortec and 133,000 miles on it. The 4 wheel drive worked fine last year, but when I went to put in in 4 wheel drive over the weekend, it won't engage. I have the switch with the four buttons: auto 4wd, 2hi, 4hi, and 4lo. I pushed the button for 4hi and the light blinks, then goes back to 2hi, and it does the same for auto 4wd, but when I pushed the 4lo button that light just keeps flashing and won't go back into 2hi. The 4wd is not engaging, but the 4lo keeps flashing. I know before when the 4wd would engage you could hear it inside the cab clicking then engaging, now I don't hear anything clicking or engaging. Any suggestions would be helpful thanks!!

 

Check for corrosion at the switch in the dash, at the TCCM behind the passenger side kick panel, and at the encoder motor on the transfer case. Corrosion in any one of these connectors can cause shifting problems.

Search for "NV236" and you should find some trouble shooting steps. "NV236+resistance" will turn up the resistance values for the various buttons on the switch in the dash.

Beyond that, you would need a Tech2 or equivalent full feature scan tool that can communicate with the TCCM in order to retrieve the stored codes (if any) and run other tests.

 

I am having the same problem with my blazer. Can i use a test light to check if the encoder motor is working properly?

 

I had a similar problem and found the vacuum hose that connects the vacuum control valve to the shift servo had hardened and broken right at the valve. My valve was located on the firewall and very hard so see due to an adjoining wire harness and connector. Hope your problem is this simple to fix.

 

I just fixed my 4x4 with the same problem. Part number is 15165891. I could only find it at the local dealer. Spent about $50 and it works fine now. Very simple to fix. It is located almost directly over the distributor on the firewall. It should slide right off. The vacuum hoses may be a bit hard to get off but with a little patience and gently tugging/twisting mine came right off. I think it was called a Front Axle Actuator Valve. Hope this helps. . . .