1992 Wrangler 4x4 Transfer Case Vacuum? - JeepForum.com
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post #1 of 3 Old 07-12-2013, 11:39 AM Thread Starter
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1998 XJ Cherokee 
Join Date: Jul 2013
Location: Alpena
Posts: 11
1992 Wrangler 4x4 Transfer Case Vacuum?

Hi everyone, i just picked up a YJ last week (first jeep i have owned!) and the 4x4 doesnt like to disengage once you put it in 4x4. so it takes some time to screw around with the 4x4 shifter to get it back into 2wd including driving forward and reverse and vise versa. Well first off the 4x4 light wouldnt come on when in 4x4 so i went underneath to the front axle and found the dash indicator sensor to be disconnected so i plugged it back in. The light stays on even when the selector is in 2wd. I went back underneath when it was in 2wd to see if the driveshaft would spin freely and it does not but the vehicle doesnt buck or anything on the pavement like it does when in 4x4. This vehicle has the 4x4 vacuum system (heard the vaccum system was pretty crappy until you get the posi lok kit) so i started checking vaccum lines. The weird thing is the lines are not color coated like they should be but instead they are all black in color. well i started tracing lines to their locations and found one of them going to the manifold which was getting manifold vaccum down to the transfer case vacuum switch so thats good, and i found the vent tube also good. So that elimnates it down to the two front vacuum axle connections, now when i check the vacuum conections up in the front when in 2wd or 4wd i find no vaccum coming from either of the vacuum tubes. I want to think its that vacuum module on the transfer case but question is does vaccum get applied all the time to the axle in 2wd or 4wd, or does it only get applied for a second when you select 2wd or 4wd? I dont get how i can drive on the pavement still when that front driveshaft is still some what engaged and not buck or anything when turning, and the 4wd works great when you use it. So another thing im wondering is if only part of the axle is engaged still in 2wd, and if its safe to drive on the pavement when the drifeshaft is like that but doesnt buck or anything unless in 4wd. i'm new to this vaccum stuff and its my first jeep ever! i am a state certified vehicle electrician mechainic (training to be a ASE certified mechanic) at only 19 years old and still learning about stuff please tread easly as im new too jeeps hahaha ! I just want to get this problem fixed until i can get enough money for the posi loc kit which i would much more prefer!

Engine: 4.0L HO
Transfer Case: NP231
Front Axle: Dana 30

Thanks for reading!
Jacob Ordway

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post #2 of 3 Old 07-12-2013, 11:58 AM
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1994 YJ Wrangler 
Join Date: Aug 2009
Location: Ripon
Posts: 20,368
Welcome Jacob.

This is probably the number one problem we have with our YJ's. There are several different avenues to consider but they all take a little work. It all depends on how much time vs money you want to spend on the situation and what usage you'll jeep will see in the end. Forgoing all the hoopla there are four major options you have sitting in front of you:

1) Replace the vacuum lines and shift fork on the cad (central axle disconnect) and utilize the vacuum system the way that jeep designed it to run.

2) Posilock cable to activate the fork independently of the transfer case shift lever. It can be useful when using a front locker or if you launch a boat on a ramp and would like 2wd low range. You can make your own which is a lot cheaper than the big box catalog offerings. However there are several that are not as reliable as you would think.

3) You can lock a new shift fork over permanently. I will let this thread speak for itself:
Read the included link inside that thread for Dodger's writeup.

4) You can replace the two piece system with one piece and the lights will work using the link above.

Luck Favours The Prepared
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post #3 of 3 Old 07-13-2013, 08:52 PM
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1994 YJ Wrangler 
Join Date: Apr 2013
Location: Memphis
Posts: 338
I found a gravel parking lot and checked for wheel spin, don't trust the light. I also used a 5' long, 3/16" hose connected to the vacuum fitting on the intake manifold closest to the firewall, then put on the CAD fittings to manually select. Driver side fitting is engaged, passenger side is disengaged. You still need to change the 4x4 shifter in the cab, too.

Following is the most descriptive "correct" test I've been able to find. I should have saved the URL, but I didn't. So to whomever it was in the interwebs that typed this, thanks:

FRONT AXEL SHIFT MOTOR PROBLEMS CHECK: make sure to raise the front of the vehicle and support it securely on jackstands.
1.) Disconnect the vacuum line from the shift motor and connect a hand held vacuum pump to the front port of the motor.
2.) Apply vacuum to the motor and rotate the right front wheel to ensure that the axel is fully disengaged. The shift motor should hold vacuum for at least 30 seconds. if it leaks, REPLACE IT.
3.) if the motor holds vacuum connect the vacuum pump to the rear port on the shift motor. plug the other ports and apply vacuum once again. it should hold vacuum for 30 seconds also. if not replace the motor.
4.) With vacuum still applied, remove the plug from the port where the vacuum line to the transfer case connects. Check for vacuum. if vacuum is not present continue on to step 5. If you have done all vacuum checks, and have passed all, but still does not engage 4x4. shift to 4x4 and drive a few feet with friend to look under to check if shaft from front of transfer case is spinning to front axel. if is, hook vacuum pump to vacuum line coming out of switch located on firewall beside the battery. apply vacuum and go drive in circles and you will be abel to tell if it is in 4wheel drive. if it works then replace the vacuum switch located at the top of the transfer case. "it has four vacuum lines coming from it". its like $12 bucks and super easy to change. this should fix the problem. if not. on to step 5.
5.)Rotate front wheel to ensure that the axel has shifted completely. If it hasn't remove shift motor and check for freeness of the sliding shift collar.
6.) inspect the vacuum harness from the shift motor to the transfer case for kinks cracks and other sorts of damage. check the vacuum harness connectors for a good tight fit on the vacuum ports.
1.) unplug vacuum harness from the shift motor.
2.) remove shift motor housing bolts and lift shift motor housing, motor and fork from housing. also remove all traces of old gasket material.
3.) Mark the relationship of the shift fork to the housing to return it to its original position upon reassembly.
4.) Rotate shift motor and remove shift fork and motor retaining snap rings. by pushing them down with two screw drivers. pul the motor out of the housing.

I sure hope this helps all those poor souls like me for the past 3 weeks that have those sleep robbing vacuum problems. i bought the jeep book, which is where the steps came in and mine passed all the checks and the book does not mention anything about the vacuum switch located at the tp of the transfer case. thats what i replaced and thats what fixed mine. and all the parts that look improvised on step four was what i did to narrow it down. GOOD LUCK
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