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I would like to ask your opinion on the next step in this process. Brief explanation below.
A year ago the front drivetrain would not engage in 4H or 4L. I pulled the battery and tray and noticed the vacuum module was completely sucked in without the 4wd buttons pushed and the cable was not attached to the module. It had pulled away from the module. (I wonder how long the front end was engaged without me knowing it due to the transfer case vacuum switch being faulty).
The transfer case vacuum switch was faulty so I replaced it and the vacuum at the module worked correctly only drawing vacuum when the switch is selected.
I just replaced the vacuum module under the battery and cable. The front drivetrain is still not going into 4WD.
Prior to replacing the vacuum module and cable I left the cable disconnected from the vacuum module and pulled the cable manually and with the front tires off the ground when pulled the front driveshaft to the transferase rotated.
With the new module and cable installed the light in the dash shows it is engaged. How much credibility does this have? Would the sensor at where the cable attaches be faulty or would that matter since it seems like a mechanical connection.
Is there any adjustment within the front drivetrain itself?
If the cable and vacuum module are working correctly what other options do I have if it is not engaging in 4WD?
Good point. Good chance the hose is original. Pretty sure it is sucking the module all the way in. I will confirm this by taking the battery tray out and physically looking at it and get back to you with a photo.
Be sure to check all the vacuum hoses as the same ones that supply vacuum to the front axle actuator also supply vacuum to the HVAC system.
Do you by chance know the amount of vacuum the hose should draw. I am sure I am asking the question incorrectly by there must be a way to check if the vacuum is sufficient.
Do you by chance know the amount of vacuum the hose should draw. I am sure I am asking the question incorrectly by there must be a way to check if the vacuum is sufficient.
JM
The vacuum source is on the the intake plenum so it will be whatever the manifold vacuum is.
I created these posts to show the vacuum hoses under the hood. These are for the 2nd gen Blazers. I suspect the 3-button one is similar to what is in the 1st gen Blazers.
The vacuum source is on the the intake plenum so it will be whatever the manifold vacuum is.
I created these posts to show the vacuum hoses under the hood. These are for the 2nd gen Blazers. I suspect the 3-button one is similar to what is in the 1st gen Blazers.
I removed the battery and tray and jacked up the front end so one of the tires was free. With the car running and 4WD engaged I notice the vacuum module was not activating enough. I found a vacuum leak on one of the hoses coming from the drives side just below the air flow meter intake hose off the plenum or intake manifold. Attached is a photo. I have not had a chance to look at your links. I would like to identify the other hoses in the photo that pertain to the hose that was cracked. I would have assumed the smaller hoses in the photo that pertain to the vacuum for the 4WD would be larger. Or maybe the leak was enough to affect other vacuum lines coming off the plenum.
With the car running the vacuum module now sucks all the way in and when the 4WD button is pressed the shaft coming from the transfer case moves.
The three vacuum lines in the photo. From the plastic tee the one on the left goes to the plenum the one on the right is capped off and the one that goes straight down ties into a small hard plastic line. Are you familiar with any of these that branch off? One thing that was interesting is there was oil in the vacuum line that I replaced.
pretty sure that capped one goes to a reserve tank in the fender well
the oil is not good,pull the fill plug on the transfer case and see if you get a bath...from being way over full...bad seal between trans and tranfer case...i also changed the vacuum switch on top of the transfer case
pretty sure that capped one goes to a reserve tank in the fender well
the oil is not good,pull the fill plug on the transfer case and see if you get a bath...from being way over full...bad seal between trans and tranfer case...i also changed the vacuum switch on top of the transfer case
This is good advice. The 3-button transfer cases had issues like this.
FYI, in case you are not familiar with the jargon.
The 3-button transfer case has only the options of 2hi, 4hi, and 4lo. The RPO code is NP1 and have the designation NVG 233
The 4-button transfer cases have an additional button for the Auto 4wd. The RPO code is NP8 and have the designation NVG 236.
The Bravadas had a one-speed auto-4wd transfer case the NVG 136 with RPO code NP4. For these, the front axle is always engaged.
Last edited by christine_208; Feb 4, 2026 at 07:59 PM.
I removed the battery and tray and jacked up the front end so one of the tires was free. With the car running and 4WD engaged I notice the vacuum module was not activating enough. I found a vacuum leak on one of the hoses coming from the drives side just below the air flow meter intake hose off the plenum or intake manifold. Attached is a photo. I have not had a chance to look at your links. I would like to identify the other hoses in the photo that pertain to the hose that was cracked. I would have assumed the smaller hoses in the photo that pertain to the vacuum for the 4WD would be larger. Or maybe the leak was enough to affect other vacuum lines coming off the plenum.
With the car running the vacuum module now sucks all the way in and when the 4WD button is pressed the shaft coming from the transfer case moves.
The three vacuum lines in the photo. From the plastic tee the one on the left goes to the plenum the one on the right is capped off and the one that goes straight down ties into a small hard plastic line. Are you familiar with any of these that branch off? One thing that was interesting is there was oil in the vacuum line that I replaced.
Thank you,
John
Congrats!
The links I posted should have the information you need. To figure out the ID of the hoses, I cut my old ones away from the ends and then used my drill bit set to determine their size. The hard-plastic line is the vacuum line for the HVAC system.
As the other person noted, oil in the vacuum line is normally a bad sign. Be sure to check the fluid level in the transmission.
4WD front end not engaging?
