OK same as many on here, I'm a bit mechanically inclined. My truck is a 2001. Out this past week and went to engage the 4x4 light just flashing indicating it hasn't engaged if I wait awhile it goes back to 2wd. It will do this in auto, 4Hi, 4Lo. When this happened on my 1999 it was just a loose plug going into the front diff.

I also unplugged the computer and checked the connections there is no corrosion.

I have checked the vac actuator it seems to work I removed the battery (hooked it back using cables) I can see the actuator move. I can here the same light clunk from the transfer case as if it is moving and also the sound of the encoder motor (I think).

One thread also talked about a solenoid???

Where are the vac line I have to check.
Where is this switch on the front Front Diff can I test it?
What is the Vac switch on TC where do I find it?
Cable is not broken
I am Getting vacuum from the engine to the Vac switch on TC.

Thanks all

Also are there any good diagrams on the net showing me the different vacuum lines?

I found the line going from the actuator to a solenoid?? on the back firewall and another hose goes from there to the drivers firewall fender there is a T here one line doubles back to solenoid the other goes into the firewall where does it go? How do I test the solenoid?

Wow I know it is a Friday, but I was hoping to get a bit more help then a zero??? Guess it's time to try my hunting site I belong to. I can usually count on them.

 

sometimes it takes awhile...we all volunteer here....no mech's on duty....just saying.

there is a big thread that will help you diagnose your system.

https://blazerforum.com/forum/tech-a...e-first-30093/

that should point you in the right direction.

also...friday of a holiday week. i have been on here for several years. we all have urgent issues sometimes...its not that we dont care...nobody may be home.

 

NV236 = no vacuum lines to the transfer case. The vacuum actuator controlling the front axle is controlled by that solenoid on the firewall that you have already found. It sounds like you have already verified the entire vacuum system. Here are a few questions for you, some from tech articles:

• Have you verified that the cable is pulling in at least 1/2"? (3/4" is better)
• How have you verified that the cable is not broken?
• Have you tested the vacuum at the front axle engagement actuator while the solenoid is active with the engine running to ensure that it is nearly the same as intake manifold vacuum (which would indicate that there are no leaks or restrictions in the vacuum lines)?

The front axle engagement switch (what the TCCM uses as verification that the front axle is engaged) will be mounted in the same housing that the engagement cable mounts into where it enters the front axle on the passenger side of the differential extension housing. It will have a two pronged electrical connector. The switch should read open when the cable is not pulled and closed when the cable is pulled at least 1/2".


As far as testing the vacuum solenoid, it is just a 12V solenoid. It should be sealed in one direction while de-energized and open in that same direction once energized.

 

OK I got all the hoses connected. But something is still messed up. The actuator is now always pulled in so I'm guessing I got one of the hoses messed??? I have attached a few pics of the hose config near the drivers fire wall.

• Have you verified that the cable is pulling in at least 1/2"? (3/4" is better) It seem to be sucked in all the way
  

• How have you verified that the cable is not broken? I followed the cable from the diff up looks good. Is there a better way to check this??

• Have you tested the vacuum at the front axle engagement actuator while the solenoid is active with the engine running to ensure that it is nearly the same as intake manifold vacuum (which would indicate that there are no leaks or restrictions in the vacuum lines)? How???

 

The only real way to check that the cable is not broken is to disconnect it at the front axle.

Something looks wrong with the way you have your lines run. I can't find an NV236 specific diagram right now, but I'll see about modifying this NV233 schematic sometime:


In the meantime, basically, what you show as "goes to the solenoid" in the picture you attached, that needs to be connected to the engine. The one you have labeled as being connected to the engine needs to run to the solenoid. The other port on the solenoid should run down to the front axle actuator.

From the looks of it, you have your check valve installed backwards...the image posted above shows the check valve and the direction it should face. The inlet side is the black side and that should be connected to the line running from the engine. The outlet side is the white side which has two ports and that should run off to the fender vacuum reservoir & to another 'T' which would then split to the firewall for the HVAC & off to the vacuum solenoid.

If the front axle actuator is always pulled, then try switching the vacuum lines on the solenoid. There is only one right way to connect them since the solenoid needs to dump the vacuum pressure in the actuator when it de-energizes to release the cable. If it is connected backwards, it will result in vacuum always feeding the actuator.

As far as how to check vacuum level at various points in the system, that takes a vacuum gauge like ; a very useful tool to have even with modern vehicles.

 

Ok so I checked the wire from the actuator it's good.

Now back to the hoses. I reversed them as you said. Then the actuator doesn't pull in at all when I press the 4x4 button. Could the solenoid finally given up? I will try to get one on Monday. Now I'm going to try and find someone who has a Jimmy that I can look at their vacuum system.

If you have any more ideas for me, let me know.

Do you know of any sites online here that have the Shop manual for download?

 

The next thing you need to check is if you are getting power to the solenoid when you hit the buttons. This can be checked with a multimeter or a test light.


If you want the best information available, drop the money on the factory service manual sets. Can't get better than that. Anything else is an imitation. They come up on eBay from time to time. $75 with free shipping for the set is the best I could find with a quick search of eBay. WELL worth the money if you ask me. I have a set for every vehicle I own including many I don't own any longer.

 

OK back. I got the actuator working now, dont think I took a good look earlier.

So recap:

All new vac lines and actuator is working. But lights are still blinking. So u suggest testing if power is going to the solenoid?? Dumb question time, but do I test under the hood at the plug?

 

I'm trying to find that shop manual on ebay. Dont suppose u could post me the link? I just keep getting the owners manual.

 

If the solenoid is working and is now able to control the actuator, then there really isn't any need to test its wiring any longer and we need to move on to something else.

I would move down to the switch at the front axle to see if it is opening & closing in response to the cable being moved.I posted two links in my response above. They're the red sections of text.