Front vacuum disconnect vs. NP242

Hey,

I have a 1986 XJ that I put a 1993 ZJ drivetrain into, which includes AW4 and NP231. Now my XJ has a vacuum disconnect front axle, and the old NP207 I had had internal synchronization mechanisms to enable me to kick into four wheel drive up to 60 mph according to the owner's manual.

Well, my new NP231 is made for a ZJ without the front disconnect, and therefore assumes the front driveshaft is always spinning. It doesn't have any synchronization. If I try to engage 4wd withouth standing completely still, the transfer case makes horrible scraping noises... I haven't tried that more than once.

I've just read that the NP242 uses the same housing as the 231, right? So that drive shaft lengths are the same.

Well, my question is really: Did the NP242 come with front axle disconnect? I.e. do they exist with that synchro feature? Or did only the NP207/231 have disconnect? If NP242 did come with the synchro thing, I'd like to swap to one of those in the future, perhaps...

Amund said:

Hey,

I have a 1986 XJ that I put a 1993 ZJ drivetrain into, which includes AW4 and NP231. Now my XJ has a vacuum disconnect front axle, and the old NP207 I had had internal synchronization mechanisms to enable me to kick into four wheel drive up to 60 mph according to the owner's manual.

Well, my new NP231 is made for a ZJ without the front disconnect, and therefore assumes the front driveshaft is always spinning. It doesn't have any synchronization. If I try to engage 4wd withouth standing completely still, the transfer case makes horrible scraping noises... I haven't tried that more than once.

I've just read that the NP242 uses the same housing as the 231, right? So that drive shaft lengths are the same.

Well, my question is really: Did the NP242 come with front axle disconnect? I.e. do they exist with that synchro feature? Or did only the NP207/231 have disconnect? If NP242 did come with the synchro thing, I'd like to swap to one of those in the future, perhaps...

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I have a 87 and a 88 both have the np242 and are with out the vacum disconnect.. and I believe after 1990 all cherokees regardless of Transfer case eliminated the vacumn disconnect.

So I don't think the NP242 is going to help you.

There are mods to eliminate the vacum disconnect search for them on this bbs.

As I understand it, though, NP 231's had synchros until mid-95 on XJ's. That's what the 95 FSM implies, at least. Might it be possible to use an eariler NP231 or fit yours with synchro XJ innards?

Yes, okay. Well, I quite like the disconnect feature, as I get 2wd low, and it probably saves a little wear, tear and gasolin when in 2wd. I've also wanted a NP242, and thought if they all could live together that would be great

We'll see, it'll probably just stay the way it is for now. It would be nice to have the ability to run an open center diff, but I think the disco is more important to me. If I'm ever swapping I think it would be to an earlier NP231 as you suggest, but only if having to stop before engaging 4x4 is being a big problem, I'll have to see first, and now that I do the disco manually I can just keep the axle engaged and the transfercase disengaged if I know I may need 4x4 at speed.

Matthew, I don't understand the point about synchros in the 231. Aside from the fact that I never heard that, the 231 has always been "shift-on-the-fly" whether or not equipped with an axle disconnect, and I would think that the ones with no disconnect would need the synchro more than those with a disconnect.

Amund, I think the easiest way to accomplish what you want is to buy or make up a cable actuator for the axle disconnect. A few companies sell them, for ridiculous amounts of many, but people make them up at home, too.

What a couple of guys in the North Atlantic Chapter have done is get vacuum switches from older full-size Grand Wagoneers and simply control the vacuum to the axle disconnect manually rather than using the transfer case to do it. Since you may not have a large supply of wrecked Grand Wagoneers in Norway, perhaps a visit to an industrial supply company that sells pneumatic operators would turn up some kind of universal, small air/vacuum controller. All you need is something to switch the vacuum from the "engage" port to the "disengage" port.

Eagle: The 95 FSM specifically mentions that some 231's have synchros, and some do not. I am assuming, as I think Amund is, that when the 231 was used with a disco axle, the synchro was needed to allow shift-on-the-fly because it goes into gear before the disco engages. It isn't needed with a non-disco axle, though, so presumably they dropped it.

Although it sounds from Amund's initial post as if he has the switching of the disco worked out, it would help if he could engage the disco independently before shifting. I have seen one home-made setup using a bracket and a cam lever with the standard transfer-case vacuum switch. The swtich threads into the TC, and is actuated by a round putton at its end, so all you need is something that presses that button and holds it in.

Gotcha. I was thinking in reverse. With a disconnect, the front driveshaft isn't turning at road speed.

Ain't that something?

Yes, you got it right the last time, Eagle.

Thanks for the replies, but I've already made a manual disco thingy, using the original transfer case vacuum switch, and a big power-cutoff switch for a tractor. Rather cool, if I must say so myself! I'll post a picture later, when I have it nicely installed into my dash.

The problem with the synchro is no worse than other people in other 4x4 with manual locking hubs; they have to keep them engaged all winter if they want to be able to shift on the fly. I have to stop, but I don't have to run around the car to shift them.. and I can of course disengage them at speed.

I still miss 4x4-on-the-fly a little though, like one time I was in a slow moving traffic jam while raining. Suddenly a gap in the next lane, which was moving quite fast, was opening. If I could kick in the 4x4 I could power out and into that lane before the truck behind me would run me over, but with only 2wd I'd probably just drive sideways, while spinning the rear tires. Fun, but not particularily fast.

Here it is:



Not very pretty, but most of it will hide behind the dash. Push and twist 90 degrees to engage front axle. The key can be taken out when disconnected, as a kinda safety feature.. also it needs to be pushed in to be twisted, so you can't really engage it by accident.

I like it, I like it!

A 93 NPG231 does have a synchronizer assembly in it. Sounds like the problem is since the front driveshaft isn't turning at the same time due to the disconnect axle I would say that is why you have to come to a stop to engage it. You might try engaging the front axle first and then the t-case.

No the cases are different for a 231 and 242. The 242 was never offered with a disconnect axle otherwise the full time feature would be useless. The disconnect front axle was dropped in XJ's after 91.

MJR said:

No the cases are different for a 231 and 242. The 242 was never offered with a disconnect axle otherwise the full time feature would be useless. The disconnect front axle was dropped in XJ's after 91.

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I just heard the 231 and 242 use the same housing... is that wrong?

The disconnect front axle would certainly not make the full time feature useless! On the contrary, it would make the 242 an ultimate choice, combining both fuel efficiency when in 2wd (and front disconnected) and full-time, part time and everything you'd want. But you'd need a synchro feature to be able to jump into 4wd at speed, if you want to run it disconnected while in 2wd.

Amund2 said:

I just heard the 231 and 242 use the same housing... is that wrong?

The disconnect front axle would certainly not make the full time feature useless! On the contrary, it would make the 242 an ultimate choice, combining both fuel efficiency when in 2wd (and front disconnected) and full-time, part time and everything you'd want. But you'd need a synchro feature to be able to jump into 4wd at speed, if you want to run it disconnected while in 2wd.

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Not even close. The 242 has a larger case which is most evident near the front output. I don't see the front turning as a big loss since it's free to turn.

MJR said:

I don't see the front turning as a big loss since it's free to turn.

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Well, how come Warn lives from selling free wheel hubs for most 4x4s then? (Stopping the front driveshaft will save a considerable amount of gas, actually.)

curious?

i have a motor & drivetain out a 93 zj that i got to put in my 89xj .. later i was told it wouldnt work in my jeep because of different computers ..i was just curious on how you got it to work? also the i have the front axle off the zj and it doesnt have the front vacuum disconnect and the plate on the t-case says its a np231..if that means anything..
TsfoTo-:anon:

Amund2 said:

Well, how come Warn lives from selling free wheel hubs for most 4x4s then? (Stopping the front driveshaft will save a considerable amount of gas, actually.)

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Other reasons such as being able to disconnect a side with a broken axle, for those that flat tow, those that have some front driveline vibrations at speed, etc ..... I have never heard of anyone buying a Warn kit to save gas.

Come on! All Toyotas and all the japanese 4x4's (at least in this country) come with free wheel hubs from the factory, they probably don't all have warns, but I've seen warn hubs on stock vehicles. Don't tell me they are there for towing or for breaking shafts!

Tsfoto, I got the entire wiring harness from the ZJ with the engine. All the wiring and colors are different, I don't know if you can plug a ZJ computer into a XJ harness however, they look the same, but I know the ZJ has slighly different engine parameters (more low-end torque). Your safest bet is probably to get the computer and harness with the engine, that's what I did as well as the auto and transfer case (NP231 here too). I know the ZJ never had vacuum disco front axle, ZJ came in 93 and XJ's stopped using vacuum front axles in 90 or 91, I think.

About computers. You said you have a 89 XJ, right? Well, the High Output engine came in 1991 or something, with slightly different manifolds, and probably some different electronics too.. I don't know exactly what is changed, but the wiring, sensors and computer is probably different. I'd say you'd get the best running engine by using everything for the ZJ HO engine.

Splicing the engine wiring harness with the car is really not as hard as it seems, you just need (good! Haynes is not good enough I think, lacks lots) wiring diagrams for both the XJ you have and the ZJ you get the engine and wiring from. First you connect the battery to the ZJ harness, that fixes all the charging and ignition and stuff, you need + from the battery (ZJ) to the XJ harness to get power to the ignition key, then you need + back from the ignition to the computer to know it's on. Starter needs power from the switch... and that's about it! Cut off what you don't need from the ZJ, you'll need to keep the power distribution center with all the relays and fuses for the engine... but now everything in the car automagically works, like heater, window wipers.. oh, you'll need to connect the wires for the gauges manually. Went quite smooth, what took the most time was the hood locks! Haven't finished those yet (I put this into a 86 2,5, which doesn't quite fit the 4,0...)

Amund2 said:

Come on! All Toyotas and all the japanese 4x4's (at least in this country) come with free wheel hubs from the factory, they probably don't all have warns, but I've seen warn hubs on stock vehicles. Don't tell me they are there for towing or for breaking shafts!

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I didn't say that. I simply said someone who installs a hub kit on a vehicle that doesn't have it already isn't doing the conversion for "gas mileage" (Cost vs Gains). At least I haven't heard of it before.

MJR said:

I didn't say that. I simply said someone who installs a hub kit on a vehicle that doesn't have it already isn't doing the conversion for "gas mileage" (Cost vs Gains). At least I haven't heard of it before.

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Actually, way back in the dreamtime, when such vehicles as a Jeep CJ-5 came without locking hubs, that's one of the main reasons people did install Warn hubs - that and drivetrain wear. But of course, the hub conversion truly disconnects all of the drivetrain, whereas the axle disconnect leaves 1 and a half axle shafts turning, plus a part of the differential innards, a much lesser saving.

I was under the impression that flat-towing an XJ requires that the front axle be connected. The procedure for the 87 was pretty specific, requiring that the TC be shifted into N while the engine ran, so that the disco would engage.