I have a 1999 Blazer with the 4 button Auto 4WD NP5 Transfer Case. From the past year of ownership, it has had a vibration when cutting the wheels all the way either direction. Come to find out, both front tires are locked when jacked up. This happens when I use 4WD and it usually take it a couple days to fully disengage.

I have changed all of the vacuum lines along with the TCCM and actuator cable. Now only the driver wheel is locked up after switching it to 2WD after using 4x4.

I have noticed that when enagaging 4 High or 4 Low it takes 3-5 blinks to engage and disengage.

I do not know if this can be a correlation, but on the 4WD dash switch, the 2HI light will randomly turn off when driving and will randomly turn on or take a couple clicks of the button to light it back up.

Please note that even with when the front axle is "locked" in 2WD the vehicle only spins the rear tires, the fronts have no power whatsoever. They function as they should in 4 High and Low range gears.

Do I try replacing the solenoid valve by the firewall or the switch?

Thank you

 

My front axle doesn't disengage I think because of the vacuum switch by the firewall. What I'm doing till I get into the auto store is just unplug the vacuum line to the vacuum actuator (not the one leading in to the vacuum switch) and just put a piece of tape over it. Put a piece of tape over the vacuum line too for peace of mind. If this doesn't solve your issue then it probably has something to do with the transfer case motor. Before I changed out the motor it would take a long time to get in and out of 4wd also.

Also, does your "Servide 4WD" light come on?

 

No my service 4WD light does not come on. I wonder how you can test the switch by the firewall

 

If it's putting vacuum through when it's not supposed to then it might need to be replaced. Pull the one going to the 4wd actuator with the car on and feel for suction. You might be able to hear a hissing and you might be able to feel it pull your skin in.

i just realized I had my vacuum lines reversed! The top one should be the one leading in to the switch and the bottom one should be the only leading out! There's a relief hole on top. I guess if it's working correctly then the aor will be sucked in from there when the 4wd is off.