Pulling a trailer through the pasture the other day and hit a muddy spot. started slipping so I put it in 4Hi, 4x4 light came on after a foot or two. Rocked tried for a minute or 2, both directions and no luck. Moved back and forth about 10 feet, spinning. Put it in 4Lo and the truck pulled right through with no issue. 

 

Coming back around a bit later with the same trailer I experimented a bit. In 4Hi, the front wheels were not getting power. Played around a bit, nothing. Truck would come to a stop if I turned the wheel, rears spinning away, fronts doing nothing. 4Lo, same as before, pulled right out no issue. Fronts getting power and spinning even turned.

 

Should I be looking at the transfer case, vacuum shift, or the axle? I did have the axle shafts out and had to reinstall the vacuum disconnector. Since it works fine in 4Lo, I'm leaning towards the transfer case, but I have no idea what to check.

I would say nothing wrong with your axle, I'm not 100% sure how that vacuum crap works going through switches and other things, my actuator went bad a while back and I ended up getting a posi lock that uses a cable to lock cad in, haven't had problems since and now I get 2w low too. if you want to keep all your vacuum stuff, there is another way of having two wheel low by adding a vacuum switch. I'm guessing the switch on the transfer case might not send vacuum in 4 high but sends it in 4 low to lock cad. But I have no idea how that switch works, just a guess. 

CAD axles suxs...

 

Basically when you select 4WD then the vacuum is applied to pull the collar over the shaft ends to couple the shafts together. Then when you select N or 2WD then the collar is slid toone side to uncouple the axles. On my 1996 Dodge its temperamental, shift to 4WD it takes a bit to pull the collar over. Then when I select 2WD it will slide the collar partially but not fully till you stop and back up just a few feet then it full released. If you don't it will remain in 4WD till it wants to let go with a sudden POP and slam the collar over. Kind of violent. Then other is climbing steep grades and dropping the manifold vacuum low and it will disconnect some times. Not enough vacuum to hold the collar and floats of the one side and back to 2WD without touching the stick. (rare but happened a few times).

 

Solid axles I've never had one issues with them.  Never had any issues with shifting or anything. 

I was told about the reverse trick a while back. Seems to help. If I have room I will turn the wheels back and forth and has the same effect of it working itself loose. Seems like you guys are leaning towards a vacuum issue.

 

I will add, I even went from 4Lo once confirmed was in and spinning, to 4Hi and still no power to the fronts. 4x4 light stays on.

Could be the vacuum motor has a tear in the diaphragm. I've seen on my 1996 Dodge the vacuum motor had been struck and bent the housing and the shaft was sticking. After replacement it worked better but still hangs on disconnection the shafts. You could pull the vacuum motor and check the fork and collar shoes for wear.  

I believe one of the "shoes" was crumbled, the other was not. Its on the list but I didn't think that would affect operation. I'll put them on my next order.

Shoes typically come with a new vacuum motor. Just make sure to pay attention to the way the fork is mounted on the stem being it can be reversed and doesn't work correctly if flipped. 

On 1/24/2021 at 9:16 PM, That Guy said:

Should I be looking at the transfer case, vacuum shift, or the axle? I did have the axle shafts out and had to reinstall the vacuum disconnector. Since it works fine in 4Lo, I'm leaning towards the transfer case, but I have no idea what to check.

 

You are experiencing an unusual symptom.  I am assuming that you have a manual selector for the transfer case.  To simplify troubleshooting, I would disconnect both vacuum lines from the CAD.  Select  2 HIGH with the transfer case lever.  Turn the engine off, set the park brake, and place the transmission in Park or 1st gear (if a manual).  Go under the truck and ensure the front drive shaft rotates freely.  This confirms that the CAD is disconnected.  Select 4 HIGH and check front driveshaft rotation - it should be locked up.  If it is not locked up, then you have a transfer case shifter issue or a problem inside the transfer case.   If the front driveshaft is locked up, then you have a CAD mechanical problem or poor vacuum supply to the CAD vacuum motor.

 

To check out the CAD operation I would use a vacuum pump and whatever suitable hose I had laying around to make up a temporary vacuum line that I could bring into the cab.  I would hook one end of the line directly to the correct port on the CAD motor and leave the other port open for venting.  I would connect the other end of the line inside the cab to the vacuum pump and operate the pump to at least 20" of vacuum.  Verify the vacuum is holding and test 4 LOW and 4 HIGH operation.  If the CAD works properly, then you have a vacuum supply problem.  If the CAD doesn't work properly, then you have CAD mechanical problem.

 

- John

 I have to ask, what does CAD stand for?

It is "center axle disconnect".  I should have written the works and not assumed that everyone would know what CAD means.

 

- John

Take your mouse on hover over the dotted underlined words and the web software will display the meaning of CAD.

 

 

I've added a ton load of acronyms to the websites database just hover over over the dotted underlined words with your mouse pointer.

3 hours ago, Mopar1973Man said:

I've added a ton load of acronyms to the websites database just hover over over the dotted underlined words with your mouse pointer.

 

I didn't know that, but it's a great idea!   Would it be possible to post that information as a one liner at the top of the page for a few days so everyone would be able to see it?  Something like, "Help Hint - Hover over any acronym with a dotted underline and view the meaning."

 

- John

 Thanks guys! I tried to figure it out but I was tired and coming up empty. Makes sense now. Thanks

Guy,

 

The switch on the transfer case could be bad.   It is where the electrical switch is on a GM, ours is just vacuum.   When you select 4 hi or low, vacuum is supplied to the black line to the cad (also called axle shift motor).  The red line should have vacuum only in 2wd.   I wonder if your valve is bad or something so that the vacuum does not get there in 4 low.  You may need to play with it a bit, because it is a detent, if you are not putting the lever in the right spot it may be wonky.

 

 

One of the few reasons I've never liked the CAD axle design. Any vacuum issues can and will leave to stuck without 4WD. Trust me I've been left stuck in deep snow and fighting to figure why the axle is not locking. 

 

They are right start at the cowl and check if there is good vacuum then work your way down to the 4 port switch and check again for good vacuum. Then move to the red vacuum lead and check again at the CAD axle. I've seen weird cases where the diaphragm is torn or small pin holes which makes it weak to pull in. I've seen bent vacuum motors that bind the shaft too. Make sure everything moves freely. Test also with the CAD removed from the axles to see if the fork is moving easy.  

Alright guys, its been a bit since I've been logged in. Life got in the way.

 

Anywho, I haven't fooled with fixing it yet, but I what I have ascertained is that the CAD works and locks in. I have heard something to the effect that there is an adjustment on the side of the transfer case. 

 

I still only have power to the front axle when in 4 lo. Works very well in 4 lo. Maybe I have a bent shift for somehow?

There is no adjustment in the switch.  It screws in.   It is possible that the bushings in the linkage or like you said bent shift lever.    Here is the adjustment procedure.

 

 

GL  HTH
 

Hag