SKH

I've recently bought a 1996 XJ that has some pre-existing modifications. The Jeep was advertised as "just needing the vacuum lines hooked up" to get the 4X4 working properly. This is my first 4x4 so my knowledge is limited and I should've done more research, but I bought it anyway thinking this was a simple fix. Now I'm not so sure. The previous owner mentioned that the XJ was stuck in 4x4, so he took out the front driveshaft to decouple the front axle to the transfer case.

Some more info...
1. The Jeep has a 3 inch lift
2. The previous owner installed a SYE
3. For whatever reason, the previous owner installed a 1988 NP231 Transfer case which naturally comes with a vacuum switch.
4. The XJ has the original (1996) front axle that does NOT have the center axle disconnect (CAD).
5. My speedometer does not work


I've done alot of research and I believe that I understand the problem, but I want to confirm before I swap the transfer case to one without the vacuum lines. This is my summary of the problem, please correct me if I'm wrong.

The vacuum system on the 1988 NP231 is designed to work with the CAD, the 88 NP231 is ALWAYS driving both the front and rear driveshafts, whether there is power to the front axle is a matter of whether the CAD is engaged or not which is controlled by the vacuum switch on the 88 NP231.

Because the 88 NP231 is designed to work with the CAD (which I don't have) it is impossible to disengage the front axle and thus I am forever stuck in 4x4.

However, I've read some forum posts on various websites that say that you can actually use a older NP231 with the XJ's that don't have the CAD. Though I'm unsure of how this possible.

Am I on the right track? I believe I will have to swap the 88 NP231 to a newer NP231 that does not rely on the presence of a vacuum disconnect on the front axle?

Note:My speedometer not working also seems to be related to having a much older NP231.

Thank you!

00t444e

The np231 with a vacuum switch still has 2wd and 4x4 it is no different than the later model 231s without the vacuum switch, if it is stuck in 4x4 then it has a problem. The speed sensor on the output shaft is what your speedometer goes by and if the sensor isn't installed right the gear tooth won't mesh with the gear in the transfer case causing no reading, or you could have a broken wire or a bad sensor.

TRCM

You may also have some issue with the spline counts changing between 88 & 96.....

New Process NP231 Command Trac: 2-speed, chain drive, part-time 4WD. 2.72:1 low-range. 87-01 2.5l 4cyl and 4.0l I-6 models with manual or automatic transmission. 87-89 21-spline input, 90-01 23-spline input. The older models used a vacuum switch to activate the dash indicator lamp and engage the front axle disconnect. Newer (non-disconnect axle) versions used an electrical switch to activate the dash lamp. The shift indicator pattern is 2WD > 4WD high range part-time > Neutral > 4WD low range part-time.

if the PO forced things together, you may have much bigger issues.

00t444e

He could have swapped input shafts, they can be switched easily.

SKH

If the splines didn't match, I wouldn't be getting power to the rear axle correct? Unless, it is somehow forced together? Which yes, does sound like a real problem.

Also, is my understanding of the front axle disconnect completely incorrect? In terms of it being responsible for coupling the front drive shaft to the front axle?

00t444e

The way the front axle disconnect works is when you put it in 4x4 the transfer case engages the front drive shaft In addition to sending a vacuum to the front axle disconnect. So for the 4x4 to work both the transfer case has to be in 4x4 and the cad must engage. You can actually permanently lock the cad and just use the transfer case to engage/disengage 4x4 that is basically how the non cad system works, the front axle is always spinning but not getting power until the transfer case is in 4x4.

SKH

Ahhh, gotcha.

So essentially, I almost certainly have an issue with my transfer case that is causing it to be constantly locked into 4x4?

TRCM

True, which is why I said it may be an issue....

If he did swap them, and did something wrong, that may also be why it is locked in



As for the CAD operation....

As you know, with an open diff, whichever axle is easiest to spin will spin (think least traction). So, when not in 4wd, even if power is going to the front diff from the tcase, the pass side axle (the one disconnected from the diff by the cad) will be the easiest to spin, as you are only spinning a short piece of axle shaft, not a whole tire/rim assembly. So, if your vacuum line comes off the CAD while in 4wd, you will lose 4wd (I had this happen in a 3500 ram).

When you put it in 4wd, the vacuum switch activates the CAD to slide the sleeve over & connect the pass side wheel/rim to the diff, so you now have 2 full axle shaft/rim assemblies being powered, and have '4wd'.



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