While I have a vacuum pump handy, I decided to check out my 4wd central axle disconnect. (My transfer case does engage, but I am pretty sure my central axle disconnect is not engaging. My 4wd light does not come on when I put it in 4wd, and the front tires don't throw any gravel)
The central axel disconnect has 2 ea vacuum lines running to it, a red one and a black one. They come from somewhere above the tranfer case on the back of the transmission.
(Note: all the vacuum lines on this vehicle appear to be in pretty good shape.)
I connected to both the red and black lines going to the central axle disconnect (one line at a time) and pumped them down with the hand pump. they both hold a vacuum. I then independently connected to the lines going back to the transmission and started the truck, Neither line pulled a vacuum. The truck was in 2wd during this test.
Now that I think, maybe I should try that again in 4wd. But intuition tells me it shouldn't make any difference.
I assume the one of the vacuum lines engages the central axle disconnect and the other disengages it, and I also assume they both should have vacuum all the time, and an electrical signal at the central axle disconnect activates one or the other vacuum lines. (but that is all guesswork on my part.)
So I assume I do have a vacuum problem here. I may have two problems because I think the 4wd light should come on even if I don't have a vacuum. Am I correct in any of my assumptions? Where do those vacuum lines at the tranny come from? What else is on them?

One theory question while we are talking about the front wheel drive. I see that there is only one central axle disconnect. I can see how that disengages the 4wd and would prevent anything from getting driven. But isn't the left tire (the one without the disconnect) still going to spin a lot of the mass of the front differential gears (the left axle, the spider gears, and half the right axle)? Seems like a disconnect on both axles would be considerably better? The whole idea of disconnecting hubs is to keep all that stuff from being driven, I thought. Am I missing something?
Thanks again for any ideas.
Cougsfan

 

my truck is not engaging to 4 wheel drive,we are changing torque converter when we have it torn out i am going over all lines to tranny,i hope its a vacuum line bad.

 

DELETE the CAD and go to Posi-Lock !
Brandon

 

for the price, Posi-lock is the only way to go. When I had trouble with mine the mechanic had reconnected everything backwards. Light wouldn't come on and it certainly wasn't in 4 wheel drive. At least you weren't stuck. I discovered mine wasn't working when we had 17" of snow on the ground back in January.

 

Your guesswork is a little off. Vacuum runs to a vacuum switch atop the transfer case. The switch then routes vacuum to ONE of the two lines connected to the CAD motor; which one depends on the transfer case lever position.

The electrical wires on the CAD assembly do not control the shifting, rather they run from the position sense switch in the CAD motor up to the indicator light in the dash. The light in the dash is thus designed to indicate the ACTUAL state of the CAD, not the position of the shift lever. So if the CAD won't engage, due to (say) no vacuum, the light won't come on, no matter what position the transfer case shift lever is in.

As for the theory of the CAD -- most of the friction in the front axle assembly is in the ring and pinion gears. By disconnecting one axle, these are prevented from turning.

 

If you dont use 4x4 often (such as our other truck) sometimes the cad gets sticky and dosent want to engage. Also I noticed on my truck that one of the vaccume lines poped off once. If you follow the steel lines above the front axel they run along the front of your trans crossmember(at least on a manual) and then point up on the driver side of the truck. There they attach again to rubber lines that run up and over to the vaccume switch on top of the T-case. That switch has a 4 hose connector plug. The other 2 run up to the engine compartment.

 

Dont want to be an "alarmist" here, but mine would not go into 4wd, yet the light would go on. It got me into some very serious trouble, and hating "dash switch 4wd", I changed to Dynatrac and manual hubs. Now I get all the 4wd I want, but my front passenger seal leaks even though I have changed it three times. The oil in the diff is 3/4" below the plug. I am thinking that my housing is bent. I have a truss on it, but it is too late.
Before, the bend would prevent the collar from sliding and properly engauging the axles, hence no 4wd. Now, with the straight solid axle, I have the 4wd, but the bend distorts the seal and now I have a leak. Since you are a new owner, and these front 3 piece axles are very weak, maybe, just maybe, it might not be straight and the collar is not sliding properly. I hope I am wrong though.