4 wheel drive will not engage

I have a 1997 GMC Yukon SLT. It has the buttons on the dash for automatic transfer from 2wd to 4wd HI or LO. I have read through the owners manual and it states that it can be changed from 2wd to 4HI at any speed. I have tried to make this shift from a standing position to 50miles per hour. I have tried it in drive, park, neutral, and with the tires spinning in the snow.

When I depress the 4HI switch the light blinks. It continues to blink indefinately (seemingly.)

I have also tried to switch the truck to 4LO with no prevail. The manual states that this should be done while in neutral rolling at 1-2 mph. I have tried this rolling forward and reverse, nothing.

I have checked the fuses (fuse 1 and 24 in the drivers side fuse compartment that is located on the dash left in the cab) and they are fine.

I have read that there needs to be a reason for the 4HI to engage. As I stated I have tried this with the tires spinning away in the snow. I have also tried it with the truck spinning in a circle in the snow....nothing. Just a blinkind light on the 4HI switch.

I have aftermarket wheels and tires on the truck, the truck did not come with stock wheels. I have read and find that the stock wheels of an autotransfer 4x4 do not have manual locking hubs....so I am guessing this is not the problem here.

Thanks for all of your help on this.

I'm no expert, but there is a solenoid that engages the 4wd on the front axle. I wasonce told to engage the 4wd about once a month for a few seconds, just to keep it from sticking. Maybe yours is stuck, because it sounds like the command is being given (flashing dash light) which goes solid when it's engaged. Like I say, I'm no expert, but this could be it.

Any idea where this solenoid is and what needs to be done to unstick it?

It does seems as if it is sending the commend....it just never is accomplished.

I am not positive if it is locking yet, but it seems that when I am driving, if I press the 4LO, the 4HI goes to a constant light, then the 4LO blinks for a short while. I am not positive yet, but it seems as if for this 30 second time that 4HI is engaged....will post when I know for sure

I think that the solenoid failure is common. I had to replace one on a '92 Yukon I once had. It is located on the transfer case toward the front near the starter best I remember. It is made of black polymer and looks almost like a starter solenoid. I think 2 bolts hold it in and it can be changed in about 5 minutes. Just go to the dealer and pick one up for about $40.00 bucks and then you will know what to look for when you change it out.

I think that the solenoid failure is common. I had to replace one on a '92 Yukon I once had. It is located on the transfer case toward the front near the starter best I remember. It is made of black polymer and looks almost like a starter solenoid. I think 2 bolts hold it in and it can be changed in about 5 minutes. Just go to the dealer and pick one up for about $40.00 bucks and then you will know what to look for when you change it out.
I have a 96 Yukon and the same thing happened. I went to the dealer and found nout that the wires by the transfer case had rubbed through making a short and had to be resecured and tapped. Not a big deal and haven't had a problem since 8000 miles ago.