There are a few questions here so you may want ot get a sandwich before reading. Don't worry they are all related to the same item.

#1 I never realized that the 4X4 system on my truck could possibly be vacuum operated. I had always thought that I had to go outside the truck and manually lock the hubs along with kicking the dash switch to 4X4 for proper operation. After reading posts about a vacuum leak I think I have discovered that I may have a vacuum operated 4x4. However, in grabbing my manual the hub looks a little different than what the manual shows. The manual shows that the hub has two dots AUTO and LOCK. My hubs have only one little red dot with no wording. Has some one put a non-vacuum lock hub in place of the factory originals? I have not removed the hubs yet to look at the units.

#2 When I lock the hubs and turn the 4X4 switch to 4X4 high or low my dampers move over to defrost. So I began to test my vacuum lines beginning by taking a MightyVac tester and testing the inlet ports at the wheels. I could not even get one ounce of vacuum to read even for a split second. So I definately have a problem there. If the answer to #1 is yes then there is not a lot I can do except plug the lines or replace the hubs to factory specs.

#3 If the answer to #1 is no, then I am assuming I need an O-ring kit for the front hubs. However, take a look at the following pic and notice the weather/rust type buildup on the vacuum fitting on the back of the front hub. The hose is snug but not as tight as I would like on that nipple. I am thinking that I would need to polish that up as part of the repair. What are your thoughts here?

 

You can just plug the vacuum lines and put in full manual hubs, like the ones in my signature. But yes, your truck has the auto and lock positions since it has the electronic shift on demand switch. If your vacuums were working ok, you should be able to engage your hubs with them in the unlocked (auto) position by just turning the switch, which throws out the vacuum and engages the hubs. Your problem is likely the yellow o-ring around main bearing assembly. Entire assembly needs to come out to replace, but it is only a $9 part. About a 2hr job per side. If you want I would try to pull vac on both the line going to the vac pump and the housing itself. I said screw it and just went manual. I did replace the yellow o ring since I was doing the needle bearings anyways though...

 

So, it is more than just the O-Ring listed in Guzzle's Hub Lube Maintenance Page. I am going to have to pull the whole wheel bearing assembly? By the way my hubs dont have both the AUTO and LOCK wordings. They just have 1 dot. Do you think someone may have changed the hubs out already? I will get a PIC to show you the outside. I guess to be sure I need to pull the hubs.

 

Mine were the same way, all the text was rubbed off. My issue was not a vacuum leak, but sticky hubs. They turned dryer than a popcorn fart. This is the o-ring that I am talking about:



I am following the steps below this weekend to replace my right outter u joint, but without breaking down the ball joints.

 

Yep, let's see a pic of the hubs before you go any further. It seems your reading and assumptions have given you a pretty good understanding of how the system works. The big question now is whether you have the stock hubs or not. You really can't come up with a diagnosis until then. I am so glad I don't live in the humid environments you guys do. My hubs don't have any of the rust or corrosion that you guys get.

 

Here we go!
Drivers Side:



Passenger Side:

 

I hear ya rad, you are definitely lucky. It is common for hubs and all ball joints to go before 100K mi. Couple the humidity with the salt on the road during winters, instant rust catalyst! Tenn isn't too bad though, odds are he just has a leak somewhere.

 

Yep, those are just like my factory ones that bound up on me.

 

ok, definitely stock hubs. Start with the vacuum nipple you showed in your pic. If you can't get a vacuum reading there, unfortunately you need to start with the yellow O ring as explained in Guzzles site which you have already found. You might as well do the needle bearing maint. while you are in there. Parts are cheap, just follow Guzzles instructions. I have a dually so that made it a little more difficult with the spacer up front, but still not too bad. OR,,,,,,,,,, you can just not worry about it and manually lock your hubs when ever you need 4wd. As long as your hubs are still turning ok.

 

Yea, they are still turning good. I just did not realize how the system worked until this week. I have always manually locked them at the hub. I didn't realize that was an override. So the hubs without the words "AUTO" and "LOCK" are still Shift on the Fly Vacuum operated hubs?

 

Yes, supposed to be anyway. Its actually kind of a complicated system the way the vacuum is distributed through the Pulse Vacuum Hublock Solenoid. Certain perameters must be met especially in 4WD Low. Brake pedal must be depressed, tranny must be in neutral, vehicle must be traveling less then 5 mph. If all these are met when you turn the ESOF switch to 4WD Low then the 4WD computer module sends a signal to the Pulse Vacuum Hublock solenoid telling it to send a pulse vacuum to the hublock. The computer sends a (as I remember) 7-10 volt signal to the PVHS in a pulse wave in order to actuate the hub and a 3-6 volt signal in order to deactivate the hub. Sometimes it takes a few seconds, up to 45 seconds as discribed in the manual, in order for these events to take place. The bottom line is you obviously have to have good vacuum to accomplish this.

 

Now here is the kicker that leaves me scratching my head. I just recently had the passenger side ball joints replaced and the driver side wheel bearings replaced. I am curious if in either situation the yellow O-Ring was replaced. I don't want to have to go that deep into the system if they are practically new. I didn't do either job myself so I wasn't sure if either side of those would require touching that yellow O-Ring. I know the other O-Ring that seals the vacuum is in the hub.

 

doesn't really matter if they were replaced or not, if they aren't holding a vacuum to specs it needs to be done again in order for the ESOF to work. Hook up your vacuum guage to the back of the hub as discribed by Guzzle and see if they are holding a vacuum. Go from there.

 

My vacuum guage will not pull anything through the back of the hub. Bet I could put a straw on it and breath through it. Guess it is coming apart. Thanks to all.

 

Another reason that you'll want to replace them is to keep water and other contamination out. I agree with rad1026 on doing your needle bearing service while you're in there. It will allow you inspect things and catch potential issues before they become wallet busters.

Make sure you do both o-rings ( yellow and black ) 2 of part F81Z-4A322-AA, and 1 of 4C3Z-1K106-AA if at least one of your hub retaining clips are in good shape.

Good luck,

Austin