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NP 231 neutral research

Matthew Currie

Matthew Currie

NAXJA Member #760
Location
Vermont, land of big clay
This question keeps coming up with regard to how one should or should not dolly-tow an XJ with NP 231, so I checked mine, and found that there must indeed have been a change of design somewhere along the way.

The NP 231 out of an 89 XJ definitely, and definitively, locks both output shafts together in neutral. This is as expected, and as all the owners' manuals I've seen imply.

But I just tested my 99, by jacking up only the left front wheel, and just as definitely, the wheel turns, indicating that the output shafts are not locked together. In 4 high and 4 low, the wheel does not turn. In N, it does. The transfer case is also definitely not broken.

Some time when it's not dark and snowing outside, I'll try my 95 and see which type it has.

I know that some time during the 95 model year, they dropped the synchromesh from the NP231, and wonder if this is also the point at which the neutral behavior changed.

In any case, it's easy to test, if you have a non-disconnect axle. Just jack up one front wheel, leaving the other three on the ground, and try turning it with the TC in N. If it turns, you know it's turning the driveshaft, and thus it must be uncoupled from the other driveshaft. I'd be interested to see what results come from non disco models prior to '99.
 
Matthew Currie said:
This question keeps coming up with regard to how one should or should not dolly-tow an XJ with NP 231, so I checked mine, and found that there must indeed have been a change of design somewhere along the way.

The NP 231 out of an 89 XJ definitely, and definitively, locks both output shafts together in neutral. This is as expected, and as all the owners' manuals I've seen imply.

But I just tested my 99, by jacking up only the left front wheel, and just as definitely, the wheel turns, indicating that the output shafts are not locked together. In 4 high and 4 low, the wheel does not turn. In N, it does. The transfer case is also definitely not broken.

Some time when it's not dark and snowing outside, I'll try my 95 and see which type it has.

I know that some time during the 95 model year, they dropped the synchromesh from the NP231, and wonder if this is also the point at which the neutral behavior changed.

In any case, it's easy to test, if you have a non-disconnect axle. Just jack up one front wheel, leaving the other three on the ground, and try turning it with the TC in N. If it turns, you know it's turning the driveshaft, and thus it must be uncoupled from the other driveshaft. I'd be interested to see what results come from non disco models prior to '99.
Click to expand...

In N both outputs are free to turn. In 2H the front is free. In 4H or 4L both shafts are coupled together.
That's normal, I've done it with the TC on the bench. With a 242 you also have FT, in which case they both turn, if one has some drag the other speeds up due to the internal differential.
I don't believe that the operation has changed over the years.
 
In 2H the front has always been free. The problem is that you can't tow it that way. In that mode the output shaft from the tranny will be spinning. Since the pump on the auto is run from the input shaft, there will be no oiling of the output shaft bearing on the tranny and it will burn up if towed at speed for any appreciable distance.
 
falcon556 said:
In N both outputs are free to turn. In 2H the front is free. In 4H or 4L both shafts are coupled together.
That's normal, I've done it with the TC on the bench. With a 242 you also have FT, in which case they both turn, if one has some drag the other speeds up due to the internal differential.
I don't believe that the operation has changed over the years.
Click to expand...

It has changed. That's the point of my post. The earlier NP231 definitely locks the two output shafts together. I have an 89 example here. 2WD is the only position of the four that decouples the two output shafts. This is not the case with later ones. I checked the 95 this evening and that too uncouples the shafts in N.

And it leaves unanswered the question of why the 99 owner's manual still warns of transfer case damage if towed in N with a dolly or a sling that prevents the front wheels from turning! Were the people who wrote the manual unaware of the change?

And when did this change occur? Does it coincide with the elimination of the synchromesh, or some other point?
 
Matthew Currie said:
It has changed. That's the point of my post. The earlier NP231 definitely locks the two output shafts together. I have an 89 example here. 2WD is the only position of the four that decouples the two output shafts. This is not the case with later ones. I checked the 95 this evening and that too uncouples the shafts in N.

And it leaves unanswered the question of why the 99 owner's manual still warns of transfer case damage if towed in N with a dolly or a sling that prevents the front wheels from turning! Were the people who wrote the manual unaware of the change?

And when did this change occur? Does it coincide with the elimination of the synchromesh, or some other point?
Click to expand...

One other change was the disconect front axle. That might be the reason for change of the TC.
 
MY 89 YJ with 231 and disco axle locks the front and rear driveshafts together with the t-case in neutral, my 87 XJ with 231 and disco axle locked them together as well. My 94 XJ with 231 and non-disco axle does NOT lock the driveshafts together (true neutral).

While this ensures there is no binding while flat towing and would allow it to go on a dolly with the t-case in neutral without locking up, I don't know enough about the 231 to say for certain whether extended towing on the dolly would be a good idea.
 
scoobyxj said:
I have always been under the inpression that it is an oiling issue with the newer T/C.
Click to expand...

That would make sense, since the manual does permit towing with the front wheels up as long as they're not contstrained (in other words, a tow truck with an axle/bumper sling is OK, but a tow truck with wheel lifts is not), but if the shafts aren't coupled, it isn't going to turn anything else anyway, is it? So I'm still puzzled.
 
Hey Matt,
How about a small apology. ;) (relating to another thread concerning this). xjnation owes me a big apology because he specifically said his 99 did not have a true N.;)
You had me thinking I was nuts or something was wrong with my Jeeps. I even went out a retested and every time I had a true N yet no one believed me. I just gave up the arguement knowing I was right and thats not easy for me to do.
 
Last edited:
Ray H said:
Hey Matt,
How about a small apology. ;) (relating to another thread concerning this). xjnation owes me a big apology because he specifically said his 99 did not have a true N.;)
You had me thinking I was nuts or something was wrong with my Jeeps. I even went out a retested and every time I had a true N yet no one believed me. I just gave up the arguement knowing I was right and thats not easy for me to do.
Click to expand...

Well I was hoping you wouldn't notice, but yes. I read the effin' manual, and went out and turned shafts on an NP231 and I was still wrong! Apology here::anon:
 
Matthew Currie said:
That would make sense, since the manual does permit towing with the front wheels up as long as they're not contstrained (in other words, a tow truck with an axle/bumper sling is OK, but a tow truck with wheel lifts is not), but if the shafts aren't coupled, it isn't going to turn anything else anyway, is it? So I'm still puzzled.
Click to expand...

There was a recent thread regarding towing and I can't remember, I searched, best I can remember is that the AW4 must be in P and TC in N.
That would prevent the AW4 from running out of lube and keep the TC lubed.
Don't anybody try it. I don't claim it is correct, all I'm saying is that I think I read that on a previous topic recently.
 
Matthew Currie said:
Well I was hoping you wouldn't notice, but yes. I read the effin' manual, and went out and turned shafts on an NP231 and I was still wrong! Apology here::anon:
Click to expand...

Thats understandable. I read all three of my manuals (96XJ, 99TJ & 95YJ). and none of them made much sense. If I recall correctly, the XJ said not to dolly it and both Wranglers said it was ok to dolly. Thats one reason it didnt make any sense to me that a 99 XJ (xjnation said his 99 was connected) could not be dollied. XJ and TJ transfer cases are identical, and the fact that I tested my XJ more than once and then checked my wifes TJ just to be sure.
I guess they never updated that part of the XJ manual.

As to the question about why you leave the transmission in P. I think its to insure that it doesnt spin while the engine isnt running. If the engine is off, the pump in the tranny doesnt pump fluid. Maybe theres enough friction in the bearings inside the tcase that some movement would make it up to the input shaft and to the tranny at highway speeds.
 
Someone on the other thread about dolly towing just mentioned something that I had not considered. They said the 242 is lubed by the front shaft spinning. That may explain why the XJ manual states not to dolly while the Wrangler manuals say its ok to dolly. The XJ manual doesnt differenciate between the 231 or 242 and the 242 was never available in a Wrangler.
 
RickyN29 said:
Very interesting. I was also following the other thread. I guess it is because I have the vacuum disco axle that it is alright to tow (short distance) in P and Neutral on TC.
Click to expand...

One thing to remember is that on disco axle Jeeps, the front driveshaft is not usually balanced. If you don't have true neutral in the t-case, the unbalanced shaft always turns and can tear itself and other things up if run at speed for extended periods. I wiped out the u-joints in the front driveshaft of my YJ before I reallized what was going on.
 
What makes you say the disco equipped do not have a balanced front DS? Every disco axle driveshaft I have seen has weighs on it from the factory.

Are you positive about this? Because I am running a front driveshaft (from a disco axle) in my rear for my SYE. I better change it out if it is not balanced.
 
In my YJ, all I have to do is put it in 4WD at any speed above 35 mph and it's VERY obvious that things aren't balanced :) XJ's may have had the fronts balanced, espcially if the part is common to both the 231 and 242 t-case configurations. I assumed I'd have to go looking for a non-disco front drive shaft when I do my SYE. I hope your right and all of the XJs were balanced :)
 
cwstnsko said:
In my YJ, all I have to do is put it in 4WD at any speed above 35 mph and it's VERY obvious that things aren't balanced :) XJ's may have had the fronts balanced, espcially if the part is common to both the 231 and 242 t-case configurations. I assumed I'd have to go looking for a non-disco front drive shaft when I do my SYE. I hope your right and all of the XJs were balanced :)
Click to expand...

I don't think the same problem occurs with the XJ. My disco-equipped 87 certainly could go much faster than 35 in 4WD without signs of imbalance.
 
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