4wd engages on its own while driving 65+mph
 

While driving 65-70 mph for an hour or so, the 4wd engages, but the buttons don't show anything but 2wd. When I slow down I have to switch it to 4wd and then back to 2wd to get it to disengage. No idea why his is happening. Any and all help will be appreciated. Thanks in advance!

Vehicle year and 4wd configuration would be a minimum for clear, applicable information on your issue.

I am going to guess that you have the NV233 transfer case with the mechanical vacuum switch located on the transfer case. There is a round tag on the back of your transfer case that can verify the model number of the case. The NV233 equipped trucks came with a 3-button 4wd switch in the dash.


Anyway, this mechanical switch can stick and cause vacuum to leak past, engaging the front axle over time.


If the switch gets real bad, it can also suck up fluid from inside the transfer case gumming up the whole vacuum system, including the HVAC side of things. This typically happens after the transfer case front seal starts to leak, allowing transmission fluid to over fill the transfer case. This can be checked fairly easily by either removing fill plug (higher of the two plugs on the back of the transfer case) or by removing the vacuum lines from the switch. If you get fluid pouring out of the fill port on the transfer case, your front seal is leaking and the transfer case will need to be removed to replace it. If you get fluid out of the vacuum line, you will then need to flush the lines out, hoping that the line that runs inside the truck to the HVAC system is still clean and dry.


The switch itself is located on the driver side of the transfer case just ahead and above the front output to the front axle. It will have 3 vacuum lines attached to a single rubber connector that then attaches to the switch. It is easiest to access from the back of the transfer case reaching forward, but is difficult to see so you'll be working mostly by feel. There are pictures in other threads showing its location. The tech article section has a few good threads on diagnosing 4wd problems.


Let us know what you find and any other supporting information that you can give so that we can do a better job of assisting you.

Thank you swartlkk
 

Thank you for your detailed reply swartlkk. Very informative and helpful. I am sorry that I failed to mention it's a 1999 blazer. There are 3 buttons on the dash, one for 2wd and then two for 4wd(hi and lo). The blazer is my parents. They are retired and I am trying to help them. Just replaced the front wheel bearing assembly this past weekend, going to replace the radiator as the right tank is leaking on that, and then this 4wd issue. I am a dialysis patient 3X a week and I work nights, so weekends is the only time I can actually work on it. I will look further into this using your info. Thanks again!!! :)