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axle not engaging

R

Rob277

NAXJA Forum User
Location
Knoxville, TN
Got a '90 xj 4.0/ax-15 P/T 4wd w/vacuum actuated dana 30.

After solving a lot of other issues this week I've got one that stumps me.

When I put the truck in 4wd high or low The part time light comes on and the front drive line turns but the axle isn't turning. I've worked on and owned a ton of cars but this is my first experience with a vacuum actuated disco. It has the three vacuum lines going to the actuator so I believe that would mean if the light comes on it's got vacuum.

I'm no expert here so any help is appreciated.
 
The light coming on means the vacuum operated switch has vacuum. That's under your coolant bottle. That doesn't guarantee that the front axle shift motor has vacuum.The vac harness for the front axle shift motor frequently fails. I prefer making the mod to engage the front axle permanently like it was in later years.
 
Btw its aw4 not ax-15. No idea where i got that from. Whats involved in rigging it permenantly and is there any worry with streatability. The truck is a daily driver
 
No streetability problems. The factory quit using that system in 1991 on the Part Time CommandTrac systems and never used that system on the SelectTrac models. No worries there.

There are some write-ups. Search "CAD eliminate"
 
might do that, atleast look into it.

Did a little more wrenching and got the plate off to inspect the fork. after working it back and forth by hand a couple times i'v come up with this. I have vacuum to the single line (disengage) side, but not the double line (engage side). I was under the impression that the vacuum/electric switch connected directly to the vacuum actuator leeching the vacuum from that side to activate the switch, but I'm now thinking I was mistaken. So if I'm getting vacuum to the switch but not the actuator. Can someone correct me on this or point me to a vacuum diagram for the system. Also I have the third line coming from the vacuum switch up topside that is blue and appears to have plastic filter on the end. I'm assuming It's the vent for the system. correct me if i'm wrong
 
You're on the right track. The vacuum is routed to the light switch and the shift motor from the vac switch on the transfer case. The vac actuated light switch is under your coolant bottle. There's a mess of lines coming up from bottom to top near the starter. Look there. If it were me, I'd eliminate all that vacuum nonsense and engage the axle full time. That vac system will fail you only when you're counting on it in the middle of nowhere.
 
ok don't know how I've never stumbled over the cad delete option. Might do a full '96+ shaft swap when I can afford it. Kinda tight on funds right now. Plus I just put new u-joints in the front and would hate to have gone through all that for nothing. Believe it or not they were the stock joints with 195k miles on them. I swear i didn't find a single needle bearing left in any of the 8 caps. The noise finally drove me crazy enough to spring for new ones.

In the mean time do you think it would hurt to just hook a vacuum line directly to the engage side to try it out.
 
The colored lines are very "brittle prone". The connection area is also very "dirty prone" and I've had some trouble keeping it clean and in good shape. Although I have not perma-locked my axles in either rig (it's been working fine for me) the switch on the MJ is dirty and in need of cleaning or replacing. The Part Time light goes on and off at will, whether I've switched it or not. I have the stuff to do the posi-lock mod, just haven't got to it...that or doing the whole D30 later model non-CAD swap.

...but as said above, and as Murphy's Law dictates...it'll only fail you when you absolutely need it.
 
I'd run a vac line to try it out. But I'd still eliminate the CAD. If I were smart enough to post up a link for ya , I would, to the 20 minute mod to fix the CAD permanently. I've got it saved in my FAVORITES.
 
Btw I can't really find a picture so is the vacuum/electric switch the little green or blue cylinder mounted on a little bracket on the fender under the tank. just to make sure. Whatever route I go I'd like to run vacuum from the t-case switch to it to keep the indicator functional. It just annoys me when the the bells and whistles so to speak don't work.
 
ParadiseXJ said:
The colored lines are very "brittle prone". The connection area is also very "dirty prone" and I've had some trouble keeping it clean and in good shape. Although I have not perma-locked my axles in either rig (it's been working fine for me) the switch on the MJ is dirty and in need of cleaning or replacing. The Part Time light goes on and off at will, whether I've switched it or not. I have the stuff to do the posi-lock mod, just haven't got to it...that or doing the whole D30 later model non-CAD swap.

...but as said above, and as Murphy's Law dictates...it'll only fail you when you absolutely need it.
Click to expand...

Not to mention the fact that when the oil filter is removed, the oil spills on the vac lines and swells em up til they fall off the brittle lines that aren't worth a hoot after 20 plus years to begin with.
 
Rob277 said:
Btw I can't really find a picture so is the vacuum/electric switch the little green or blue cylinder mounted on a little bracket on the fender under the tank. just to make sure. Whatever route I go I'd like to run vacuum from the t-case switch to it to keep the indicator functional. It just annoys me when the the bells and whistles so to speak don't work.
Click to expand...


That's the deal. Got a wire running out of it and a grond wire mounted under one of the attaching screws?
 
cruiser54 said:
I'd run a vac line to try it out. But I'd still eliminate the CAD. If I were smart enough to post up a link for ya , I would, to the 20 minute mod to fix the CAD permanently. I've got it saved in my FAVORITES.
Click to expand...

I'd appreciate it if you could because everything i've found so far has been bits and pieces instead of a full write-up.
 
have you traced the lines back from the shift motor then, Im going off of memory but they kinda go up into the passenger side of the bay. as others have stated they break out easily. If you dont know... the shift motor is either on or off... no need for constant vac. are you still useing the stock vac plug? they can be leaky. I ran a piece of rubber hose from the end of each vac line to their respective plugs on the shift motor. If you know your getting "disconnect" vac. try that on the connect plug to be sure the motor is in fact working?
 
Have put vacuum to both sides and it is working both ways. Everything is stock except for the main vacuum line running between manifold and vacuum switch. I've already replaced it because it was busted. Haven't found any leaks. Just can't figure out how I could be getting vacuum to the switch but not the actuator.
 
well I tried the cad elimination method as posted on comanche club. Took all of about 10-15 minutes and works well. even did a little late night hill climbing to test it out. Put a new set of Firestone Destination A/Ts on it a back in march and two days later 4wd went out, then about a month later it was a no start. turns out after testing and replacing just about everything that has to do with a sensor or the ignition system it was a bad computer. Now it's finally up and running again and I can test out the new tires. Now just have to work out a transmission issue and a oil pressure issue and all the big stuff will be taken care of. Oh and I have to get my new lock set in tomorrow, as not a single lock matches the other and I don't have a working key for any of them. The only lock in the truck that is factory is the passenger door. Still think the truck was a good buy for $1k
 
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