4x4 Won't Engage Please Help

I Have A 2001 Chevy Blazer The 4x4 will not engage. I Press the 4x4 Auto, Hi or Low button on the dash they just keep flashing. you can here the transfer case engage, but It will not be in 4 wheel drive. I have tried driving around with the light flashing to see if it would engage it did not. please help me find out what this is. Thank You

On the front, there is an actuator that engages the front differential to the front drive shafts. It may not be engaging if the transfer case does. It is easy to check which is at fault by a mechanic with the truck on a lift. Likewise, if this actuator is a fault and not the one for the transfer case it’s a straight forward replacement. Either way, you need it evaluated by a mechanic on a lift.

A temporary check that you can do, is service (replace and or top off if low) the fluids in the transfer case and front differential. Warm them up for sure if it’s cold where you are. If they then work, it may just be this and using a synthetic may help in cold weather. Don’t count on it, but give it a try. Heck, it could be anything from mechanical to a loose or corroded connection. But it will have to be evaluated on a lift.

Also, use your 4 wd monthly all year long, even if you don’t need to, for at least 10 to 20 minutes. Lubrication is essential for longevity of these units and many fail for lack of use then you can believe.

First pull the switch from the dash. Then go to this link http://www.youtube.com/watch?v=JNWxhIWs8Uk It will show how to test the switch. I just went thru this with my 06 GMC Sierra 1500. My switch was bad and my shift motor was bad also. After I replaced them I also had a bad wire. It was not a problem at first. But I had pulled the shift module and finished it off. When you are on youtube there are a lot good videos on how to fix it. If your switch is bad order it from amazon. Under $40.00 there. Also shift motor is under $200 there.

I agree with @oldbodyman. The GM trucks of this era had a problem with the 4X4 switches. I had to replace the ones on my '98 Silverado and 2001 Blazer before I could sell them. I don’t use 4WD but it’s a good selling point when you decide to sell your used 4X4 vehicles.

The TCCM (Transfer Case Control Module) would lock up if I didn’t engage 4wd enough on my 2000 Blazer. Either disconnect the battery negative cable or the wiring harness from the TCCM (behind right kick panel) to reboot the module.

Also check all the vacuum hoses from the actuator (under the battery tray) to the vacuum switch (middle of firewall) back to the vacuum reservoir (inside left fender?). When my Blazer was around 10 years old, the hose ends were showing signs of deterioration.

The only item I ever had to replace was the vacuum switch (firewall mounted for AutoTrac). I determined it was bad by disconnecting the vacuum hoses on each side and connecting the hoses directly to send vacuum to the actuator.

These links were helpful when I was troubleshooting the 4wd on my Blazer. It explains the system far better than I can.


Good luck,

Ed B.

My Service light is not on

I worked last year. though I did take a while but this year nothing

"it worked last year"
This is a sure sign you should use your 4wd frequently. First, so you are sure it will work when needed, and secondly, to keep everything lubed. Regardless of the make and year, as they age they should be check when you don’t need them so they work when you do. Not using 4wd is ne of the biggest causes of failure !
Take it to a mechanic. Everything’s a guess from here on out.

@dagosa The new style transfer case’s like what in my 06 GMC, are not like the old style that you need to use once in a while. The new ones (been this way since the late 90’s) are shifted with motor on side of transfer case. They lubed as you drive all the time. No vacuum use at all , it’s all elc. now. The switch, shift motor and actuator on the front axle are prone to failure. The shift module is also a known problem with trucks. As I just found out the wiring is not great too. I do agree that you should engage the system from time to time to make sure it works, But using it from time to time wont make it keep from not working. My truck is a good example of that. I use mine at least once a week and it failed when we got a foot of snow. Give me a lever and linkage any day!! Also lock out hubs too!

I have to disagree. I have own part time 4 wd vehicles for many years. Little has changed in their requirement to be used. The front differential never turns and the gears do not get complete lubrication without use. Neither do parts in the transfer case. Likewise, the actuator rods should be use occasionally. I have yet to meet a mechanic who hasn’t told me that occasional use of the front drive train of a rwd truck or truck based SUV with part time 4 wd is beneficial.

Read an owners manual for a part time 4wd system. Though systems vary, I would be hard pressed to find one that did not recomend you engage the 4wd at regular intervals. Toyotas are 10 miles per month. Problems with shift modules and wiring may be problems associated with GMCs. It depends on the make.

OP is in trouble, because he didn’t catch it in good weather. He said it worked poorly the year before; and waited to try it the next winter? I don’t believe many mechanical components repair themselves and improve over time.

Regardless, for the third time; it needs to be looked at by a mechanic on a lift and we are just guessing ! ;=()