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OEM Transfer Cases

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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 and they also provide swap info for members who want to install a hi-lo transfer case.

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.

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)

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!


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