Angered the 4wd Gods or My 4wd Won't Engage

1986 XJ
Disco D30
A/T
2.8 L
72,xxx miles

Ok, this is my first Jeep and I'm still learning. I've been wheeling my XJ a lot lately and the last time I went out I lost 4wd. I tooled pretty far down the trail in 2wd, but when I went into 4wd it didn't feel like it engaged. The light on the dash lit up, but it just didn't feel right going through the mud. I ended up getting buried up to my axles and it was confirmed, no 4wd. I dug around in the mud and the vac lines were still connected as far as I could feel. My buddy pullled me out and after driving home I've let it sit since the weekend.

I read a fair bit, trying to wrap my mind around Jeep axle tech and have been trying to do some diagnosing today. Thus far I've jacked it up on the pass side with 4wd engaged. The front driveshaft doesn't spin in 4wd, so good there. However, the pass tire does spin in 4wd.

OK, so from what I've read it's something with either the vacuum or the CAD assembly itself, correct?

I've been checking out the vacuum lines by eye and thus far, everything looks good. Connectors are where they should be and I can't visually see any cracks in the lines (I know, I know, I need to bust out the vac guage to verify...)

I just want to see if there's anything else I should be checking at this point or something I may have missed?

Also, would a DIY posi-lock be a good option at this point as far as being more dependable then the vacuum assist set-up? Or would I be better off shimming the fork? I'm low on cash and these seem like pretty cheap alternatives for now. Any input appreciated.

Thanks ahead of time.

it could be something as simple as your transfer case lever and linkage getting bent or broken while you were out wheeling last time.

Keekleberrys said:

it could be something as simple as your transfer case lever and linkage getting bent or broken while you were out wheeling last time.

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No, as I understood him, when he engages 4wd, the front d/s no longer freewheels, so the t-case is engaging.

Sno, you're right that it's the cad shifter. Notoriously problematic.

If you're not running a front locker, there's really no advantage to the posi-lock. You would be able to disconnect it for slightly tighter turning offroad, but not much... I wouldn't bother.

If the shift fork isn't actually broken, shimming it is really the fastest, easiest, cheapest, and surest workaround.

And fast, easy, cheap and sure are four of my favorite adjectives.

Robert

WrenchMonkey said:

No, as I understood him, when he engages 4wd, the front d/s no longer freewheels, so the t-case is engaging.

Sno, you're right that it's the cad shifter. Notoriously problematic.

If you're not running a front locker, there's really no advantage to the posi-lock. You would be able to disconnect it for slightly tighter turning offroad, but not much... I wouldn't bother.

If the shift fork isn't actually broken, shimming it is really the fastest, easiest, cheapest, and surest workaround.

And fast, easy, cheap and sure are four of my favorite adjectives.

Robert

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Thanks! I was hoping I was headed in the right direction. Just have to wait for the weekend to tear into it. And thanks again for the input on the posi-lock/shimming question. Much appreciated! :thumbup:

Zip tie the lines on there to make sure. Two of them are working since the 4wd light comes on! Maybe one doesn't??

Best way to do it is open the CAD housing up and shim that fork with washers so that it keeps the collar engaged all the time. For a few bucks in washers and an hour or so of tinkering you will eliminate the weak link in your 4wd system!

In my experience with the 87, if the 4WD light went on and stayed on it meant that the vac. motor was engaging properly. It doesn't send vacuum to the switch until the diaphragm has moved a certain distance. It was always the first warning that the vac motor was letting go, and would go off before the axle unlocked. I could be wrong about this as a diagnostic test, but if it is, then I'd suggest you take out the vac. motor and check the fork to see if it's a mechanical problem.

Matthew Currie said:

It doesn't send vacuum to the light switch until the diaphragm has moved a certain distance...check the fork to see if it's a mechanical problem.

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Yep, that.

So you may need a new shift fork before you can shim it over...

Robert

The early forks were prone to bending. Jeep came out with an updated version with more webbing on it. Probably worth taking the 15 minutes or so to remove the axle shift motor and investigating.

I would chime in that it sounds like the shift fork is broken. If you have nearby pic and pull look for older dodge dakotas with the d30 disco as these came with a cast iron fork (the "heavy duty" upgrade fork that people will try to sell you), Jeep Disco D30's came with a cast aluminum fork that is prone to failure. On my 88 I snapped two before upgrading. Both of them snapped in sub zero weather when engaging a cold axle. Another option is to search around for info on the two piece to one piece shaft swap.

The hose assy on those gets brittle and breaks. Surprisingly enough they are fairly cheap at the dealer.

The rubber ends where the harness attaches is prone to swelling also, from oil leaking on them every time you change the oil filter.

Thanks again to everyone who commented. I'm not completely sure if I found the reason why I had no 4wd. When I popped open the assembly on the axle, the fork appeared fine. However, the collar was a PITA to get to move. Without any prior experience with the CAD I'm not sure if this is normal or if that was my problem?

Either way, I shimmed the fork over, got the collar over finally and was back in business. Tried it out in the yard and she didn't skip a beat. Probably would of been better if I bothered to undo my sway bar but at least she's ready to raise hell again.



Thanks again everyone! :cheers:

Sounds like you'e good to go.

I think you guessed right. That collar should slide very easily, and the vac. motor is not capable of exerting great force.

Just had a thought. Some of the early XJs had a problem with the fork sliding the collar over so far that it stuck on the axle splines and stayed in 2WD. The vacuum wasn't strong enough to pull the fork and collar over to the 4WD position when it stuck like that. That's when they first revised the fork. Until then, we ground about 1/8" off the side of the fork closest to the diff, slid it over closer to the inner axle and added a washer on the outboard side to take up the clearance..