hello i am new here, but not new to forums. i have done some research on my problem and it seems to be a fairly common one, except for the fact that my vacuum actuator does not seem to be the cause. the truck was switched to 4 low and will not switch out. if you press 2 hi for example it will just flash, and you will hear clicks from the tccm, and even the actuator on the t-case, yet it will not switch out. i did find some corrosion on the tccm and i have not yet tried it when cleaned. the vacuum actuator under the battery engages and disengages. i have no service 4x4 lights. i ave tried to rock the vehicle back and fourth to get it to switch but it does not work. any help would be appreciated and i will post my solution when it is solved! the truck is a 99 blazer LS and has the auto 4x4.

 

First check the vacuum switch in the center of firewall, looks like this this:





With the engine at idle, transmission in park, disconnect the vacuum connector. There should be vacuum on only one line. With the vacuum connector unplugged, select 2WD, see if it shifts out of 4LO, (may need to put transmission in neutral and roll vehicle slightly).

 

Put the shift lever in neutral & then push the 2Hi button. It worked for me.

 

i had a problem with the vacuum actuator at first and it seemed like the cable froze up, but while tracking the cable down i jacked the truck up on the drivers side, lifting up the LF tire and the vacuum actuator released. it seemed like the front diff was bound up. i then proceeded to try and switch the car into 2 hi and it worked, with the engine not running. thinking the problem was solved, i hit 4 low again and there it remains.

 

The left front axle is always locked to the differential, that's why it felt like it was locked up. The vacuum actuator under the battery engages and disengages the right front axle. Raise the right front wheel off the ground, unplug the vacuum switch on the firewall and the right front axle must disengage allowing the wheel to turn freely.

 

just tried to unplug the vacuum switch on the firewall and it did not work. i also cleaned the connections on the tccm and the only thing that changed is now the needle for my tach is now sitting on the wrong side of the gauge, not functioning.

 

UPDATE: i got it out of 4 low. i turned the ignition on but did not start the engine, put the truck in neutral, and hit 2 hi. it switched. it switches in and out of 2hi and 4hi without a hitch. i don't think i'm about to use 4 low ever again. thanks for the help.

 

I hope that works for me, I've tried everything and all but start replacing it all to get it out of low gear. I did replace the 3 combo switch and now have all my lights working, but still no switching, have clicking from the tccm and the actuator is working yet still stuck..anyhow thankyou for this info if this works for me will let you know.

 

O. Mine I switched to 2wd and backed the truck up about 5 feet in reverse..takes it it of 4 low every time.

 

2000 Chevy Blazer stuck in 4 wheel drive. Traced vacuum lines all good and no leaks. Actuator under batter is working. Replaced vacumm switch on transfer case. My buttons show lights and everything works as should. I've read everything thread everywhere and can't get it figured out .
Now when i get up to 20mph it starts whining and making a noise like it is trying to go in or out of 4wd. Like putting something in a fan to slow it down. Won't stop noise tell I completely stop.