I think I know most of the answers to my questions by reading the tech help on this, but one thing still bothers me and it is not addressed.

My 2001 NP236 system appears to have a front axle that will not unlock. With the engine shut off in 2WD, and the passenger side wheel jacked up I cannot easily rotate the passenger side front wheel. It is not the brakes, as there is a certain amount of slack back and forth when moving the wheel. If I try very hard bumping against the limits, I can make the front driveshaft move (TC seems to always have a lot of tension on front shaft even in 2WD) - so I'm quite sure the front axle is still locked in. Furthermore, when driving you can feel the resistance from being locked in when turning the front wheels sharp and trying to move the vehicle. The natural state of this system is without vacuum the front axle will be unlocked.

I pulled the battery and tray and checked out the actuator and cable. Actuator has no apparent damage and the cable is not stuck. However, I only get about 1/4" -3/8" of cable movement when pulling the bellows back. It is stated in the tech section to expect 1/2" to 3/4" movement. I rolled the vehicle forward and backwad a little in hope the collar would unlock and the cable would be normal movement, but no dice. This leads me to believe that there is something internally wrong with the internal engagement parts or the return spring, leaving the front axle engaged all the time.

Now my question. Why doesn't the front axle position switch recognize this as a problem and throw up a flag or a 4WD light or a code? Is the front axle switch just detecting fork movement, or is it actually detecting movement of the collar (which I suppose could be stuck). But that would not explain short cable movement.

Really don't want to pull this thing apart needlessly. I need it to all make sense to me. Looks like it has been worked on before with some fresh RTV. Lot of things were worked on before I got this vehicle, but few things were actually fixed.

Opinions??

 

I just had a thought. If the axle is staying engaged..... if I jack up the entire front end and spin the right front tire..... the left should spin the other direction as long as the driveshaft is not turning. Correct?

 

NP236 always has some load on the front driveshaft, even in 2wd. So there may not actually be any problem with the front diff.


But because you say you can feel resistance when turning maybe there is or maybe there's something else causing the resistance.


You can try jacking both front tires off the ground, but you might need to disconnect the front driveshaft as well. I don't know how much resistance the transfer case puts on the system.

 

Les, if you pull the vacuum line off of the axle actuator, and jack up only the right front, you should be able to rotate the tire very easily with one hand. To take dragging brakes out of the equation, push the caliper piston in a bit, and pull the caliper outboard.

 

Thanks! I hadn't pulled the vacuum hose off yet, but I will do that tomorrow night and check again to make sure vacuum wasn't trapped.

Les

 

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Thanks again Captain!! Actuator was holding vacuum and keeping the front axle locked in.

Turns out the hoses on the vacuum switch (engine firewall) were switched and apparently they have to be oriented right in order to vent the front axle actuator and release the front axle from locked-in mode. The fitting opposite the electrical connector is supposed to go to the engine vacuum source and the other one in between goes to the actuator. Switched the hoses, and now when I take the transfer case in and out of 4WD, the front axle actually locks and unlocks instead of sticking in locked position. Note this is for a 2001 NP236 (4 button) transfer case.

Still don't know why the front axle switch didn't throw up a flag, but I don't really care at this point! All is working and it was an easy fix. I would have been really upset if I had torn things apart and found nothing!

 

Can anyone tell me the name or part number of that vacuum switch on the firewall?? I've looked all over online and cannot find the part in any parts store. I've looked for vacuum switch and transfer case actuator switch - but all that comes up is the switch that goes on the transfer case on other models. Can't find the firewall mounted one anywhere.

 

One such part number is Doorman 600104...but this was for my '02.

 

Thank you - that's a start! I see Advance Auto Parts carries it and so do sellers on ebay for MUCH less. It's also called an HVAC Heater Control Valve Solenoid. Maybe that's why my HVAC goes wacky sometimes under acceleration.
And evidently it's actually a Dorman 600-104 - that's what Advance corrected it to and it shows up in searches as same. THANKS again!!!

 

You are correct, the solenoid can be used on 4WD and HVAC systems. However, on GM T series vehicles, it's only used for the 4WD front axle actuator on 1999 and newer, (NV236 transfer case). Your HVAC issue is caused by a vacuum leak in the HVAC system, or the vacuum source, which is shared with the 4WD.