Hi all, As I get into this truck (2000 Jimmy SLT), I am homing in on some repairs and getting close to ordering parts. I have just about finished diagnosing the AIR system using the flow chart posted by @swartlkk and got the 4x4 lockup resolved temporarily w/ help from @LesMyer so thanks fellas.

BTW the AIR system and the 4x4 vacuum lines were switched up causing issues with both (that the AIR system uses vacuum is typically not mentioned in all of the "my 4x4 isn't working" threads).

Of course not the only problem with the AIR. My truck does have the upgraded/relocated intake so very little moisture found so far. I found the 30 amp fuse is blown but my best attempts at bench testing say the motor is locked up. The vacuum valve and the one way valves on the exhaust inlet seem to be functional. Once I replace the fuse I can check for power to the solenoid switch and the motor.

As I finish diagnosing and before I start ordering parts I have three questions:

1)Does the AIR solenoid switch get vacuum directly from the manifold or is there an in-line check valve or something?

2)When in 2x does the 4x4 firewall solenoid entirely shut vacuum to the under battery diaphragm or does it apply a little vacuum? (can't try 4x4 until I learn this or replace it).

3)Does AWD system use vacuum at all or is that all electronically controlled?

 

To what are you referring to when stating 'AIR'? To me, AIR means air injection reactor which is a system that injects air into the exhaust to facilitate more complete catalyst reaction and is completely different from vacuum that comes from the engine intake. I think you mean vacuum so I'll continue on to your questions.

Engine vacuum is supplied by a fitting on the driver side of the intake manifold near where the brake booster line attaches. Here is a schematic of the system I created a LONG time ago and I think was first posted in the 4wd Front Axle Clicking When in 2HI - Goes Away when in 4HI/4LO tech article:


As far as the operation of the 4-button (NV236) 4wd system, here is a quick synopsis:

• 2HI -
  • Transfer case only puts power to the rear output with a 1:1 input/output ratio.
  • line to front axle engagement actuator vented to atmosphere by the electronic 4wd vacuum solenoid on the firewall to disengage the front axle engagement mechanism in the front axle.
• 4HI -
  • Transfer case locks front & rear outputs to receive equal power with a 1:1 input/output ratio.
  • Line to the front axle engagement actuator is supplied engine vacuum by the electronic 4wd vacuum solenoid on the firewall to engage the front axle engagement mechanism in the front axle.
• 4LO -
  • Transfer case locks front & rear outputs to receive equal power with a 2.72:1 input/output ratio.
  • Line to the front axle engagement actuator is supplied engine vacuum by the electronic 4wd vacuum solenoid on the firewall to engage the front axle engagement mechanism in the front axle.
• Auto4HI -
  • Transfer case engages the front output, but allows for variable lockup depending on conditions (slip detected by wheel/output speed sensors) with a 1:1 input/output ratio.
  • Line to the front axle engagement actuator is supplied engine vacuum by the electronic 4wd vacuum solenoid on the firewall to engage the front axle engagement mechanism in the front axle.

Hope this helps!

 

First thanks I know it takes time to read and post a good reply so much appreciated.

Yes the Air Inj Sys is what I'm talking about. In another post (I only have 3) I mentioned the vacuum supply in eng compartment were all disintegrating (no HVAC control). The manifold supply under the air intake comes off to feed the reservoir and the HVAC sys and the 4x4 solenoid on the firewall. But it also tees off to provide a feed (I believe) to the AIR system. That's why Question #1. It definitely tees off but trying to confirm the vac supply to the AIR solenoid is direct from the manifold similar to the 4x4 solenoid.

Now previously I found this other manifold supply which headed towards the psg side was incorrectly going attached directly to the 4x4 diaphragm and the AIR solenoid supply line was going incorrectly going to the 4x4 firewall solenoid. Long story short, the 4x diaphragm was receiving full manifold vac at all times and the firewall solenoid was feeding the AIR solenoid. Those have been switched but I'm leaving the firewall solenoid to diaphragm line off for now (see below).

Yes thanks! I had previously found and used this diagram extensively. That has probably saved countless people tons of headaches!
Any idea on Question #2? In the other post @LesMyer said my lines on the firewall could be switched which was causing my wheel hop on turns. He was right they were, except the problem was worse than that because of the switch I described above. But while investigating and with dash button on 2x, I found I had good vacuum to the firewall solenoid but no matter which port on the solenoid I attached to, I got some vacuum to the other. I didn't (can't) measure it but it felt less than the supply (full manifold). Is this normal or should it shut the supply to the 4x diaphragm completely in 2x? I don't want to hook it back up until I know if the solenoid is working correctly.

Again thanks. I am currently trying to trouble shoot the AIR system see orig post. I think I may be one of the lucky ones who just have to replace the motor. Everything else seems to check out. After pulling the AIR pump/motor I cannot get it to run on the bench but I believe I'm getting power to it. The other components seem ok.

 

I answered questions 2 & 3 in the descriptions of the 2wd/4wd modes... To reiterate, when in 2HI the line to the front axle actuator should be vented to atmosphere (i.e. it should not have any vacuum being applied to it). If it does, then there is an issue with the 4wd vacuum solenoid either in having the hoses installed improperly or physical issue in the solenoid. Auto4wd will result in vacuum being sent to the front axle engagement actuator by the vacuum solenoid just like all of the other 4wd modes.

As far as question 1 is concerned, I'm not entirely sure where exactly the vacuum solenoid on the AIR pump gets it's vacuum source from, but it could just tee into the same line as the vacuum source to the 4wd vacuum solenoid as it should get full manifold vacuum. I have never had a reason to dig into that side of the AIR system. I have had to replace the check valves & rubber hoses that get fried when the check valves fail (and repair wiring harness damage that occurs as a result as well --- not fun at all!).

If you have an update, it is probably best update the thread where you were discussing the topic for all those that were helping out to see more readily. Also, even though you used 'bold' on your replies it is not a good idea to put your comments within the quoted text of another member. You can either break up the quote like I have done above or simply reply below the quoted text. Another reason for not typing out your reply within the quote of another member is that it will then cause your reply to not be quotable using the normal means of quoting a post.

 

Note that these two photos don't exactly match the diagram but I can say that both my AIR system and 4wd work and that i have not replaced any hoses.


Line coming towards you goes to the front axle actuator. Note position of lines on valve. This is 4 button 4x4.

 

Near brake booster. Black line from left side of blk/wht valve goes across engine to vac source . Green line is to tee (one side to HVAC and the other to 4x4 axle switch. vacuum source. Right side of valve to HVAC vacuum reserve tank. This is a 2001.

 

Routed the way you show, I would question whether the check valve is operating properly. Vacuum source should be on the black side of the check valve with the powered accessories (AIR, HVAC, 4WD) and the reservoir on the white side of the check valve. This is to allow for the vacuum system to remain operational for a few changes after the engine is shut off as well as during low manifold vacuum instances (assuming that the system is sealed properly). Does your vacuum system hold some vacuum after the engine shuts off?

 

FYI I did update the other thread and will quote as you recommend.

@LesMyer Thanks for the pics, they confirm to me what I guessed in that the AIR system solenoid gets manifold vacuum directly. Swartlkk may have a point and I'm glad he threw in that tid bit about the vacuum check valves. It helps me understand how it all is supposed to work. My routing is that the manifold supply tee's off to supply the HVAC and AIR system and each have a check valve with black side of the valve to the source. IDK how critical it is but always (well usually) good to have things set up as intended.

 

Unknown on if the check valve is operating properly. I'll investigate the check valve the next time I'm under the hood. I can say that I have never touched the check valve plumbing or ever noticed any loss of HVAC vacuum duing hard acceleration - I assumed that was the purpose of the vacuum reserve tank in the fender. In reality, all I truly know is that I never changed anything on mine until I got the hoses switched on the front axle valve when replacing the engine. Can't speak for anything the previous owner did with the check valve, but I think the hose size is different on the black side of the check valve - so I imagine would be difficult to plug hose on it the wrong way. Still worth looking at. So Toxyco you may want to wait a bit before making any changes to check valve based on my photos.

After the engine replacement back in 2014 my 4WD light was coming on and it was crab-hopping when turning after coming out of 4WD into 2WD. Then I pulled off the line from the front axle valve going to actuator and it released the vacuum to the front axle. I found that the front axle valve vented one port to atmosphere when in the off position. This port has to be the port going to the actuator so it will release.

How about another set of photos from another later model Blazer. Anyone have such a Blazer? 4WD/ 4 button 4WD.

 

Dealing with a little bit of this myself. That and disintegrated lines laying in the engine compartment. I think I'm on the right track now though

Was your posts that put me onto the possibility of a switch on the 4x actuator so thanks again. I fabbed up a vacuum gauge and I'm getting about 20 in/Hg at the vacuum source leading into the actuator and about 10-12 on the diaphragm side in 2X. I unplugged the actuator to see if maybe it was being activated in error but no change so I think I'm calling it bad.