I'm have 4x4 issues with my 01 1500. Transfer case is confirmed to engage. I'm pretty sure my issue is vacuum to the CAD on the front axels. I have the replaced vacuum lines going to the CAD. It is pulling a very weak vacuum. I haven't put a gauge on it but judging by the feel of vacuum with my finger it is very insignificant.

I want to test if the CAD is working by running a vacuum from somewhere else. I need to go buy some vacuum line before I can try. My question is does only one side of the CAD need vacuum and the other side be left open or do I need to block it off.

If that doesn't work my next step will be to take the CAD off. Is there a way to temporarily manually engage it?

 

Yes, other side needs to be left open to vent to the atmosphere, so the vacuum can do its thing.

You can manually lock the CAD, you'll just need to take it off and remove snap ring that holds the fork, then move the fork over and re-install the snap ring. Then push the collar on the axle shafts to the locked position and install the CAD.

 

Did you replace the metal lines as well? They like to rust and collapse, or break.

 

Do you have any issue with the hvac vents? If so your check valves may be bad. Check the vac before going to the t case switch then check it at the cad. Should be about the same, My line got water sucked into and clogged with rust. Definitely check the metal lines.

 

The remedy for my truck:
my axle shift motor (vacuum diaphram) was not holding vacuum. So i replaced that, but no luck. Then I replaced the control switch on the transfer case. The control switch was my issue. When I replaced the control switch the 4wd started working, the 4wd light/indicator on the dash started working and it also fixed my A/C vent selector.

 

I applied vacuum from the manifold and the CAD will engage and disengage.

The metal lines will move air from my air compressor and they look okay. I'm not getting vacuum before the metal lines either.

I will check the check valves as that could be possible. I don't know if I have HVAC issues because my blower fan isn't currently working. I used the vacuum from the HVAC line. I think my problem could also be the valve at the shifter

 

You should see vacuum on one side or the other down at the metal lines going to the cad, so long as the engine is running. If you don't have vacuum at either, then your vacuum source is the issue.

 

Your life could be made much simpler with a hand held vacuum pump for troubleshooting. Not expensive at HF -- about $26 for the cheap house brand.

Will also act as a gauge if you stick the vacuum source on the output.

Check the big rubber plug at the vac switch -- they love to fail, at least in our arid environment. You can still get the entire harness from motor to t-case and it's not even that expensive

 

The check valves will hold vac from the motor. At idle you have the most as soon as you open the throttle it drops. if you fallow the vac line from the intake(#1) to behind the fuel rail it will come to a "T". One line goes to the the hvac and the other goes to the t case. If you have good vac there and not at the cad the problem is in between somewhere. Check it a the rubber lines where they connect to metal tubes and see what you have.