91 MJ auto convert 2WD to 4WD - trans?

[91Pioneer]

Now that there are 3 MJ's in the household I am considering more seriously to convert my 91 to 4WD. It's currently 4.0L Auto (AW4) RWD short box.... I have found a 1991 parts XJ and I was thinking I just need the transfer case (plus extras like driveshaft/diff) but after looking at my trans I *think* I may need to replace the transmission to convert to 4WD.

Do I need to get a 4WD AW4 in order to convert to 4WD or can I bolt the transfer case to my existing 2WD trans?

 

More unrelated questions to follow I'm sure but for now I'm assembling the parts list. I want to stick with auto since my mint condition 89 is a manual. I plan on going with the 242 transfer case in my 91 so I can just leave it in 4WD auto all winter long.

 

One other quick question, are my stock rear gears 3.55:1 ratio? The parts XJ has 3.55 gears up front.

[gogmorgo]

Getting a 4x4 aw4 is generally going to be the easiest way to do it. Hypothetically you can convert the 2wd trans if you really want to, but you’d need to change out the tailhousing and output shaft, and my understanding from a quick glance through service manual a long time ago is the output shaft comes out the front of the transmission, meaning you’re completely tearing it down. Definitely less work just to put in the 4x4 transmission.

Your stock gears are probably 3.55’s but it would be better to double check. 
I’m a fan of the 242 for winter driving (also not original to my ‘91 automatic) although I usually leave it in 2wd for the most part unless I actually need the traction. I suppose it might just be that something in my truck is wore out, but there’s some noticeable extra load on the engine when I put it in full-time, and I’m of the opinion that my mpg suffers. Not that I’ve done a proper scientific comparison there, it might be more related to the other variables that lead to the shift into full-time. But otherwise full time is significantly better for driving on the road than part-time.

 

My ‘91 doesn’t have the circuits in the instrument cluster to make the full-time light work, even if I put a bulb in. I think I still have the cluster from the 242 donor kicking around but I didn’t want to swap it out because I would have been knocking about 260,000km off the odometer, and it didn’t occur to me when I did the swap that I could just move the speedometer over to the other cluster. It took a little figuring to get even the part-time light to work, because the 231 puts the switch on the CAD, where the 242 has it on the tcase itself. But going from 2wd should be more straightforward there. 

[watchamakalit]

7 hours ago, gogmorgo said:

Getting a 4x4 aw4 is generally going to be the easiest way to do it. Hypothetically you can convert the 2wd trans if you really want to, but you’d need to change out the tailhousing and output shaft, and my understanding from a quick glance through service manual a long time ago is the output shaft comes out the front of the transmission, meaning you’re completely tearing it down. Definitely less work just to put in the 4x4 transmission.

Your stock gears are probably 3.55’s but it would be better to double check. 
I’m a fan of the 242 for winter driving (also not original to my ‘91 automatic) although I usually leave it in 2wd for the most part unless I actually need the traction. I suppose it might just be that something in my truck is wore out, but there’s some noticeable extra load on the engine when I put it in full-time, and I’m of the opinion that my mpg suffers. Not that I’ve done a proper scientific comparison there, it might be more related to the other variables that lead to the shift into full-time. But otherwise full time is significantly better for driving on the road than part-time.

 

My ‘91 doesn’t have the circuits in the instrument cluster to make the full-time light work, even if I put a bulb in. I think I still have the cluster from the 242 donor kicking around but I didn’t want to swap it out because I would have been knocking about 260,000km off the odometer, and it didn’t occur to me when I did the swap that I could just move the speedometer over to the other cluster. It took a little figuring to get even the part-time light to work, because the 231 puts the switch on the CAD, where the 242 has it on the tcase itself. But going from 2wd should be more straightforward there. 

Just an fyi mj odometers can be changed without removal from the cluster. Meaning you can make the replacement cluster match the mileage of the old.

[91Pioneer]

Hmm so it's not as easy as swapping the tail housing, that sucks because it just adds more cost and complexity. I will think about that as I start to buy parts...

[gogmorgo]

If you’re buying a donor rig it’s not going to be a ton more work to pull the trans out of it as well. If you get both trans and tcase out of the same donor then you don’t need to mess around with tcase input compatibility. Assuming the parts rig isn’t completely trashed you should be able to take everything you need apart from the rear driveshaft. 4.0/auto xj should have 3.55 gears as well to match your rear axle.

I ended up scrapping my old working 4x4 aw4 last spring. Couldn’t even give it away, but I was moving and couldn’t bring it with me. Otherwise I would have offered it to you. 
Pulling the trans might seem daunting if you’ve never done it, but it’s not that bad a job. The low-lift transmission jacks at Princess Auto are cheap enough and go on sale pretty frequently, and they’re a huge help if you’re doing it on jackstands. 

[91Pioneer]

I found a parts XJ but the junkyard thinks it's gold IMHO. $150 front driveshaft, $450 transfer case, $750 for front axle. I still need trans too. It's adding up too fast. I'm not sure I can do all the work DIY so I might wind up paying $150/hr to the mechanic for some parts? Not sure yet but it's getting expensive for me, I will probably piece all this together and shop around to find better deals.

We pulled the trans from my son's 87 MJ (manual trans 2WD) I've done it before but I prefer smaller jobs that aren't so heavy LOL - I bought a trans jack from Harbor Freight just for this job (much cheaper than P-Auto version of the same thing).

[91Pioneer]

They want $750 for the trans. Wow they really put a damper on this. $2,100 for the used junkyard parts to switch to 4WD with no labour. Ouch.

[Pete M]

that's why parts-rigs are a viable path.  

[gogmorgo]

Don’t remember how far you are out of Alberta, but if you’re on the facepage the Alberta XJ/MJ group is pretty good about coming up with stuff. 
There used to be a steady supply of sub-$1000 XJs but there’s not a heck of a lot in MB and SK anymore.

There is a couple cheap ‘93 ZJs out there, you might get lucky and find one with an AW4, I think they switched them mid-year to the Chrysler trans that you don’t want so know what you’re buying. The ZJ’s not an ideal candidate because of the low pinion d30 and some of the transfer case options, and I don’t know how well the tcase shifter would interface either. But it’s maybe an option.

[gogmorgo]

Bringing stuff across the border in bulk gets complicated. There isn’t exactly a shortage of parts up here either, it’s just figuring out where everything’s at. No one advertises anything unless they need to get rid of it.