OEM Transfer Cases

[IowaEagle]

Thanks to Jim Bolton who did the research and originally posted the info on the web in 1999.

AMC used three different transfer cases; The Eagles all used the 23 spline transfer cases, regardless of the transmission they were used behind. This is different from Jeeps, which also used a 21 spline case, typically found behind 4 or 5 speed standard transmissions.

The cases used were:

NP 119 - '80 thru early '81 -- Full Time 4 Wheel Drive; Viscous Coupling

NP 128 - 1986 only; Open Differntial

NP 129 - mid '81 thru '88  -- Select Drive Models; Viscous Coupling

These cases were manufactured by New Process and were designated models 119, 129 and 128. The 119 was 4 wheel drive all the time, whereas the 128 and 129 were switchable to 2 wheel drive. The 119 and 129 used a viscous coupling differential unit in the case, which allows some slippage between the front and rear output shafts and allows use of 4 wheel drive in any road conditions. The 128 case used an open differential to allow slippage. The differences in these methods of differentiation can be likened to the differences between a limited slip rear-end and an open rear-end. A worn out viscous coupling will behave much like an open differential.

This information was obtained at http://www.n0kfb.org/homepage/amc/tech/transfer_case/swap/index.htm and they also provide swap info for members who want to install a hi-lo transfer case.

[Mavericke]

So how do I tell the difference between transfer cases?

I have about 4 of them laying around and I would like to install the one that appears to be rebuilt. It has also been sand blasted and painted! Woot... but I want to make sure that I it as "on the fly 4x4". The 80 model is obviously all-time... my 82 has a switch but I never paid attention to it because the vacum lines were never hooked up and the only one I ever owned and drove with the switch was my 81... and I had to stop to switch it to 4x4 and back.

Well, when I got my 85.... Brandon was telling me how cool it was that mine was "on the fly". Upon further inspection... he was right! That's just the coolest! So... how do I make sure that I am able to retain this feature when swapping transfer cases? I believe the pretty one is from an 83.

[IowaEagle]

There should be a red aluminum tag screwed to the case.  All 129 and 128 cases are switch on the fly capable.  It was the axle disconnect that made them stop and shift.

[Pa sx/4]

I would love to have a 128 open Differential. I could lock the diff and be less likely to get stuck. I like the idea of both drive shafts spinning the same .( Never had may eagle stuck anyway)

[Mavericke]

Thanks for the info IE... I'll make sure I install one or the other. So as long as I use the original axles from the 85, then I'll be good? Sounds easy enough.

I have never noticed a red aluminum tag though. Probably because most of them have been black from old grease... the rebuilt one has been painted, but I'll find it. Thanks again!

[IowaEagle]

Yeah, the '85 is a shift on the fly version.

[Mavericke]

Sweet! That's good to know! Thaknks!

[RumbleBird]

I Just bought an '86 Eagle wagon and after reading this, I'm starting to think maybe the 128 transfer case is less desirable than the 129 in my '84 parts car. Would there be any advantage to switching to the 129? I'd definitely like the '86 to be as capable off-road as the '84 I used to drive was. I'm also curious about the locking front axle. I've read that the shift-on-the-fly Eagles don't have this, and would like to know how this affects the 4-wheel drive. It would be a big help if anyone knows a bit more about this than I do...

[IowaEagle]

The 129 will swap right in and provide you with the safety and security Roy Lunn designed into the Eagle.   The non locking axle is felt by many to be a good thing as that means there is one less thing to go wrong when you need 4WD.

[projectscreamingeagle]

so if the front end in my 84 has the axle disconnect in it but i permantly locked the hubs together does this mean that i can "shift on the fly"?

[IowaEagle]

Yes.

[projectscreamingeagle]

sweeeet!

[txjeeptx]

To answer Mav's question about ID of cases without the build tags(a 2" round tag with a screw in the middle of it and lettering stamped in it) -

The 119 case will have no linkage attachment where the shift fork comes out of the front half of the case, it will only have a locking nut to hold the non-moveable shift fork in place, or it is a bolt plug, if there is no shift fork(haven't had a 119 apart, haven't looked at an exploded diagram to see if theres a shift fork er not).

The 128 and 129 will look the same from the outside. Both will have a shift-able linkage attachment. On the 128, you will be able to relatively easily turn the front and rear outputs against each other(turn each the opposite direction of the other) when it is in 4wd mode, since it has an open differential. I'm not sure if its a lock-able differential like the NP 242, where the differential is locked when the case is in 4wd Low mode. Its probably not lockable. Anyway, the 129 has the viscous coupler, and it should take some considerable effort or even a wrench to turn the front and rear outputs against each other when the transfer case is in 4wd mode.

So - 119, no shifter linkage attachment
     - 128, has shifter linkage attachment, open diff, can turn outputs against each other in 4wd
     - 129, has shifter linkage attachment, viscous coupler, difficult to turn outputs against each other in 4wd
Even a worn out viscous coupler will have much more resistance than the open diff.

[Kenny]

I have an 88 wagon with 85,000 miles, i thought low mileage would be a good thing but bad things can happen when they sit for long periods of time. but im hoping the year/type transfer case thats in mine will still get me through snow, mud, and occasionally a mild jeep trail. 

[IowaEagle]

You have the NP 129 in your '88.  It is a good transfer case and can be driven on any type of road surface.  The NP 128, found only in 1986 models, is the one that has some minor issues but it is pretty bullet proof.