Front Wheels Dont Spin in 4WD, drive shaft moves though

Okay so I never used the 4wd in this Cherokee yet and just assumed it worked. Now its a 1990 Jeep Cherokee Laredo with only 4wd part time high and low, it's not the one with the the full time option. Now I took it somewhere with the ground wet and muddy and was going to see if the front wheels would spin, I tried it in 4hi and Lo. The front driveshaft was spinning though, but it didn't help pull the jeep at all. Now when I got home I could move the driveshaft freely which I think is normal, and that was out of 4wd. Also when it was out of 4wd and moving the driveshaft didn't move so it looks like transfer case is doing what it's suppose too. Could this be a vacuum problem or something with the locking the hubs. I really have no clue when it comes to that front end. Pleaseeeee help with suggestions!!!

Yes, it's a vacuum problem. As long as it is a stock front end, your hubs are always engaged. If you have a vacuum disconnect D30 up front, like you must, then the vacuum is not doing it's thang, so in 4WD, your driveshaft is spinning only part of a front axle that is not connected to the wheel.

Do a search on vacuum disconnect repair/not working/etc on this site, and you will find threads on how to diagnose and repair the problem.

Since you say the 4WD has not been used yet by you, that makes a malfunctioning vacuum disconnect in the front end all the more likely.

Good luck!

Simple solution, pull the shift fork cover, slide the fork over and lock it in place.
Since it is OLD, you may want to check all the vacuum lines underneath.

i had this same problem in my 1990 but it turned out it was a cracked vacum hose that comes off the fitting to the axle.

Well here's one kicker I should have mentioned. All vacuum
lines were replaced by the dealer in the engine bay area, I saw the jeep when it had the garden hose under there. So maybe they didn't hook it back up correctly. Very frustrating when you get stuck in the front yard!!! Can anyone just say where I should look for that vacuum line real quick, sorry for bothering you guys. Thanks though!!

are your heater controls working? are you getting vac at the motor controls? No heater vac leak... vac at the motor on the axle but no engagement... broken shift fork. I dont remember wich color does what, but I used to... look me up. One is for the indicator on the dash, one for engage, one for disengage.

1. Look under the front bumper, passenger's side, confirm that the vacuum reservoir is there in its stock position and there is a vacuum line attached to it.

2. Examine the vacuum line around the battery--major fail point.

3. Relocate the vacuum reservoir from the bumper into the engine compartment, I put mine under the coolant pressure bottle. This eliminates about 5' of problematic vacuum line.

Here is the test procedure for the D30 CAD axle:

From the FSM 1990:

Raise and support the vehicle. Disconnect the vacuum harness from the axle shift motor and connect a vacuum pump to the vacuum shift motor front port.

Apply 15 in. Hg of vacuum to the front port and rotate the right front wheel to fully disengage the outer and intermediate axle shafts (i.e., shift to two-wheel drive operation).

The shift motor should maintain the vacuum applied to the front port for a minimum of 30 seconds. If the motor does not maintain the vacuum, replace it. If the motor maintains the vacuum proceed to the next step.

Disconnect the vacuum pump from the vacuum shift motor front port. Connect the vacuum pump to the vacuum shift motor rear port, cap the port for the indicator lamp switch, and apply 15 in. Hg of vacuum to the rear port.

The shift motor should maintain the vacuum applied to the rear port for a minimum of 30 seconds. If the shift motor does not maintain the vacuum, replace it. It the motor does maintain the vacuum proceed to the next step.

Remove the cap from the port for the indicator lamp switch and determine if vacuum was present at this port. If vacuum was present, the shift motor functions normally. If vacuum was not present, proceed to the next step.

Apply 15 in. Hg vacuum to the rear port of the shift motor. Rotate the right front wheel as necessary and ensure that the outer and intermediate axle shafts are completely engaged. The axles must be completely engaged (i.e., shifted to four wheel drive operation) to open the port for the indicator lamp switch.

Determine if vacuum is present at the port for the indicator lamp switch again. If vacuum was present at the port, the shift motor functions normally. If vacuum was not present at the port, replace the shift motor.

Ok, if at this point it still won't shift correctly you may have a worn shift fork. OEM and replacements are cast and will wear. Posi-Lok 4x4 makes a cast iron fork, about $55.

Here is a JP Magazine article on different "fixes" for the D30 CAD axle:

http://www.jpmagazine.com/techartic...154_0904_dana_30_vacume_disconnect/index.html

Post back what you find.

Thanks guys for all the help. I got under the vehicle and found the greasy mess. I guess the vacuum lines look like electrical wire almost? Anyways I think what it may be is that the rod is not sliding the piece over to engage 4wd. I have indicator light on the dash working saying part time. Which I guess is somehow connected to the vacuum lines. Just glad I know it didn't work till I was really needing it.

I see Xj's all the time with their axle vac harness hanging down. When I got my MJ the vac lines were all greased up and wouldn't pop in to their spot. I scored a full vac harness (looked new!!) from a nicely maintained XJ, cleaned the snot out of the whole axle and disco. Muuuuch better now.

I gotta work on clean-ish stuff, grease just gets in my way and makes the job a b--tch.

There is a switch on the transfer case. It sends vacuum to the motor on the axle to engage/disengage the axle collar. From the back most port on the axle motor a vacuum line runs up to the vacuum/electrical switch on the passenger's side firewall, from there its electrical to the 4x4 light.