Does anybody have an idea why my service 4wd light comes on after I drive for about 3kms If I shut off the engine it is good for another 3kms. It is a 2000 Blazer. Thanks

 

The transfer case has detected a fault and it is letting you know. Get the TCCM scanned by a capable scan tool (GM Tech2 or equivalent scan tool capable of communicating with serial bus devices such as the TCCM). The code will tell you what the TCCM thinks is wrong.

 

ok thanks. I took it to Mr. Transmission and when they scanned it they said it could be a few things such as the switch on the dash or the switch by the transfer case so they didn't really help at all

 

Well, you do not have a switch per say on the transfer case (yours being the NV236 transfer case). You have the encoder motor which is what positions the transfer case for its various modes (Auto4wd, 4HI, 2HI, & 4LO in your case). Then there is the vacuum solenoid on the firewall which controls the vacuum actuator under the battery that is responsible for pulling on the cable to engage the front axle whenever you are in a 4wd mode. You then have the TCCM which orchestrates this whole symphony of actions.

If you wanted to check some things yourself before hand, you could remove the switch in the dash and test the resistance for each button push (search for "NV236+resistance"). You could also disconnect the TCCM and the encoder motor, checking the connections for signs of corrosion and cleaning as necessary. Corrosion can cause problems.