Hi there. Long time lurker first time poster.
I have an issues. I have a 1998 3rd gen 3.4 auto LTD
4wd engages just fine but when I disengage nothing happens even if I am in neutral or back up or forward. All the tricks.
IF I turn the vehicle off and then immediately back on the light goes off and I get back into 2wd

I have tested both VSV's for the 4wd. Both resistance and making sure they are getting power to when switched.
I have checked the front diff sensor and that checks out.
What am I missing here?
Picture for Reference of the vehicle driving me mad

 

Perhaps one of the VSV is testing OPK electrically, but is physically stuck open? And that's keeping it stuck in 4WD.

Symptoms don't seem quite 100% right for that scenario, though.

 

I am leaning towards an issue with those but I sucked on them and they seem to hold pressure. I just can’t think why cycling the ignition off and on would change anything. Maybe the one way vacuum valve?

 

One of the position sensors on the transfer case is causing the error.

 

What makes you say that ?

 

While most people get freaked out by the complexity of the m/m 4wd and transfer case, there really isn’t a whole lot to them. There is an actuator in the transfer case and in the front diff, and then a bunch of position switches to tell the computer where everything is. VSVs don’t often go bad because they are always under vacuum, they sometimes “show” being bad because a position switch is telling the computer to do something opposite of what you are trying to accomplish. Meanwhile the position switches rarely get used and moved since most people seldom shift into and out of 4wd.

 

Awesome I’ll pull those and test them. Are they just open closed switches ?

 

Yup. That’s it.

 

Is there two? Or 3? Parts diagram looks like 3

 

There are possibly 4. Hi/lo/neutral/4wd. 2 are down low off the actuator and 2 are up high near the floorpan.

 

Well i tested two switches up high on the transfer case one with a dark coloured connector and one with a grey. They are both fine.
I tested the switch on the back near the actuator. and that tests fine too. Is there any way to test the actuator?

 

I have also noted that when the truck is in 2wd you cannot spin the front drive shaft by hand

 

I seem to have fixed it. I pulled the transfer case motor out. And as I did the selector shaft pushed the motor out also. It seems that it has been off before and not reinstalled correctly. For reinstalling I removed the triangular cover with 8mm bolt heads. Manually engaged 4wd by pushing the selector shaft all the way in. then with the gear out of the triangular port and the motor connected to the wiring harness I engaged 4x4 with the button. Then installed the actuator onto the back of the t-case. Then inserted the gear and re fitted the triangular plate. All tight and reinstalled seems to now engage and disengage fine now