Wierd D30 disco problem

The 4wd light on my '89 XJ (4.0L, AW-4, NP231) stays on all the time, even with TC in 2WD mode. Tracing the problem down (see my cruise/vacuum thread below) I diagnosed the problem to be the axle disconnect shift motor. But as I researched the problem I became more puzzled. Most descriptions of the vacuum motor failure say that when it goes, the axle will be stuck in 2WD and unable to engage 4WD. My problem is exactly the opposite - the axle stays in 4WD and won't disconnect! Even when not running and no manifold vacuum present, the axle stays engaged - I confirmed this by jacking up the front and spinning the wheels, which caused the front driveshaft to spin as well. Is my diagnosis on target??? What could cause this kind of problem?

I'm off to the dealer to see what a shift motor will set me back, in the meantime, would this be a good opportunity to replace the vacuum system with a Posi-lok? Any opinions on how good the posi-lok is???

Thanks!

Marty

Okay, once again I have found a partial answer to my questions above, this time from a link I found in another thread here: http://www.rightcoastcrawler.com/billaVista/2low/2low.htm This contains (among other things) an excellent description of the vacuum operation of the stock system (even better than the factory service manual, IMHO). Apparently in my setup the side of the system connected to the "green" line is not receiving vacuum. Whether it is a vacuum leak, something internal to the actuator, or a problem with the transfer case switch has yet to be determined.

Anyway I'd still like opinions on the posi-lok!

Thanks!

Marty

Posi lock is good but in my opinion over priced you can do the same with bicycle brake cable and a handle. I have mine locked all the time using hose clamps in the motor(for now). I bought a old Waggy disconnect to use in my xj. That way I could have 2wd low. works the same as the posilock only uses a vaccume instead of cable.

OK, it gets wierder...

Well thanks for the opinion on the posi-lok buckyxj, you're right, I thought it was a bit overpriced too (and I'd seen the article on the vacuum switch also! ) Then tonight I stopped in at the local dealer to check on the price of the vac disconnect unit, and it was a whopping $250!!! Makes the posi-lok look really good!!!

I got ahold of a hand vaccum pump and did the tests called out in the factory service manual - turns out the vac disconnect motor is working just fine! Axle in, axle out, just like it's supposed to. SO... Vacuum problem AHEAD of that, I think. Sure enough, vacuum testing on the green and yellow lines(see this reference, thanks to www.rightcoastcrawler.com! ) showed that the "green" side (disconnected) never got vacuum, and the "yellow" side (connected) got vacuum all the time - i.e. transfer case lever in 2 and 4 WD. Therefore the shift motor never disconnected the axle!

Now here's where it gets a little wierd... on a road test, the 4WD light (which really indicates axle disconnect) stayed lit all the time, except when full throttle was applied! When really punching it, the 4WD light would go out, until letting up the throttle when it would come back on. This behavior occurred in both 2WD and 4WD positions of the transfer case lever. And I swear I could almost hear the axle switching in and out of disconnect when it happened!

So now I suspect the vacuum solenoid switch in the transfer case, but I can't find a procedure in the FSM for testing it. Anyone have any ideas for this? Maybe a vacuum check valve not working correctly??? Or a bolloxed switch??? Any help will be appreciated!!!

Thanks!

Marty

The disconnect system is really pretty simple, reason for the light going on and off is you probrably do have a bad line thats not holding vacuum but at full throttle it can create just enough to mess with ya. Replace all 4 of the lines if ya want a diagram of how every thing is set up just PM me. To test the switch hook up a vacuum source to it, remove the rest of the lines, switch from 4wd to 2wd and put your finger over the open ports on the switch, feel for vacuum, if ya never do that its shot.

Eric

The switch in the TC is relatively cheap (something like 12 bucks last time I got one), so it might be worthwhile to go ahead and replace it if it's suspect. I had one that stuck intermittently, a hard thing to test for.

Thanks, guys! I'll be getting the vac pump out again tonight and testing all the lines and the switch. Hopefully it will get sorted out. A $12 switch sounds a heck of a lot better than a $250 vacuum motor!

Marty

Solved...

Just to wrap up this thread - I tested the vac switch on the TC and it was bad. NAPA store, $16. Took about 5min. to swap. Problem solved. Thanks for the help, folks!

Now I can get back to the cruise control problem...

And there's that rear main seal thing...



Marty