Hey all,
I have a 98 4 door blazer, 4x4.
Today i went out in a snowstorm and used 4 wheel drive for the first time in a while. I was using the defrost selection on the switch to heat my truck (as i normally do). I then parked and did my business, but when i got back to start my truck only the dash vent were blowing air, even if i have it set to just defrost. Now i notice a hissing sound that changes tones when i try and change the vents that i want air to come out of. Im not sure if using 4 wheel drive has anything to do with it, but i figured id throw it out there. It does the same if im in 2 wheel and 4 wheel.

I also noticed that i am low on antifreeze, and i usually keep tabs on how much i have (weekly).

Any suggestions would be greatly appreciated!!!

 

There are 4 vacuum actuators under the dash. These two are on the driver side, next to the gas pedal:





Some of the actuators have one vacuum line, others have two. There is another actuator behind the glove box, and one above the passengers feet. Make sure the vacuum lines are connected tightly.

 

check the vacuum lines under the hood, especially the ones in the upper right driver's side. Be sure to check that the one to the reservior is intact and the one going into the firewall.

 

I've forgotten. The pic above, which one is the defrost actuator? Yellow or blue/red?

 

This image shows each actuator, all vacuum lines, which port on the mode switch each line goes to, and when each port has vacuum or is vented to atmosphere. For example: When the mode switch is set to the BLEND position, (windshield & feet) none of the actuators have vacuum. When set to DEFROST, only the brown line has vacuum. The black line should have vacuum while the engine is running, and in theory, should hold vacuum after the engine is turned off.

 

This may be your problem, this is what happened to mine, the seal between your transmission and transfer case is leaking so the transfer case is being overfilled with transmission fluid, when you engaged the 4x4 it opened the vacuum circuit through the vacuum switch on the t-case to the front diff actuator, this vac switch will actually leak vacuum to the inside of the t-case allowing fluid to be sucked up into the lines if the t-case is overfilled above the switch, if you left the 4x4 on when you shut the engine off then there was vacuum stored on the front diff actuator, hvac actuators and vacuum reservoir, with the engine not supplying vacuum to these components they then pull vacuum back but the check valve to the engine vacuum port is supposed to seal and hold it, however with the circuit open on the t-case vac switch the suction then pulls transmission fluid from the overfull transfer case and fills the vacuum reservoir the front diff actuator and the vacuum circuits and actuators in the hvac system. The transmission fluid quickly Destroys the rubber in the vacuum lines switches and actuators and now you have a bunch of problems, soon your 4x4 may not work either because the trans fluid will wreck the diaphragm in the front diff actuator.
I hope your problem isnt this bad but the first thing you should probably check is to pull the fill (TOP) plug on the transfer case and be ready for a couple quarts of fluid to come dumping out.
I just posted about my solution to this recently so look at my other posts about 4x4 and or hvac problems.
Post back with what you find and go from there.
Good luck!

 

Thanks for all the input everyone!
so, i pulled apart the dash and i found no leaks in the vacuum lines going to the controls. So i opened the fill plug on the transfer case and some atf did come out.

I removed the tc vacuum switch and found one of the three vacuum connections is not there. Im going to replace it and see what happens.

I need a few more months out of my blazer so im hoping this fixes the issues.

 

Were you able to find the missing vacuum line? The important question is if you found any fluid in the tc vacuum lines...that is a problem.

 

What is the part number for the Yellow Brown actuator? mode valve actuator