NV3550 Swap

If you've been paying attention (to the rest of the site). You know that I own a 1992 Jeep YJ. What you probably dont know is that I've been nursing a failing transmission since somewhere around December of 2004. Jeep sold the 4.0L YJ from 1989 until the end of its production in 1995 with an AX-15 (they also sold the AX-15 with the TJ from 1997 until 2000). The AX-15 also comes in a few other vehicles, 1/2 ton pickups, and other light duty applications. Even some sedans if I recall correctly. The problem with MY AX-15 (and this is a common problem with the AX-15) is that the synchro's are failing. Whenever i try to shift between gears, I am met with some resistance, and usually a bit of grinding. This can usually be avoided by double-clutching, which has helped me to keep this transmission operational for almost a year now. However recently its gotten much worse, to the point where i have trouble just shifting from one gear to the next. I fear that my AX-15's final days are upon it.

For months now, I've been checking boneyards, local jeep shops, and internet forums for a replacement for my AX-15. I've decided to go with an NV3550, for reasons which I will elaborate on in just a bit. It seems that these are a rare commodity! My local guy says he's never even seen one!

Most write-up's like this one go into details on what transmissions are available, and what the author decided on, and why, and all sorts of pro's an con's. Well I dont claim to be a transmission expert so I will NOT explore every option. I will however explain the options I had available to me, and why I chose what I did.

First a bit of history. When I bought this YJ, in May of 2001, it was in very nice condition. Low mileage (for a 1992 anyway, 54K!) and it ran pretty nice. Not to mention the condition of the body and paint. Anyway, in December of that year the AX-15 went from working well, to shot, in the course of 2 weeks or so. So I thought about selling it and getting a NEW Jeep, but that wasnt financially possible at the time. I ended up borrowing the cash for a rebuilt AX-15, and labor to install it at the company garage from my parents. In a week or so I picked my baby up at the shop with its freshly rebuilt AX-15, and a new clutch. I got in, started it up, and i couldnt get it into 1st gear. Something's not right there. So I walked right back into the shop and told them, I cant shift this into 1st, FIX IT! Well hey didn't, they told me that the internal slave cyl on my trans is hard to bleed, and that there was probably just a little air in the lines, and that it would settle and work well. If not bring it back in and theyll re-bleed it (wrong) again.

So in the course of the next few weeks it did seem to get better, and I was semi-pleased with it. I still had problems every now and then getting into 1st gear from a standstill though, but it was so uncommon by then that I didnt bother to take it back to the shop. So fast forward to December (or so) of 2004. About 3 years from when I had the re-built AX-15 installed. To this point I'd never been exceptionally pleased with the performance of my AX-15, but it was too late to get any kind of warantee out of it. One day I'm driving my jeep and I feel the slightest little grind as the gears mesh shifting from 4th down to 3rd. I knew exactly what it was, but I didn't want to admit it. Over the next few days I continue feeling the same little grind when shifting from 4th down to 3rd. So i accept the fact that this trans is failing, and start to baby it. Double clutching when i downshift from 4th to 3rd. Slowly but surely it gets worse, next it begins to grind when downshifting into 2nd, then when upshifting into 4th at high rpm's. Finally what convinced me that "it was time" was when at idle, out of gear, I began to hear a rattling/grinding noise from the trans. That started around the beginning of August in 2005.

So, if you havent guessed, I dont put a lot of faith in the AX-15, and I've vowed never to install one in any vehicle again. Thus leaving me a few options for a replacement this time around. Which are as follows:

AX-15 (Not a chance!)
NV4500
NV3550
NV3500

NV4500



The NV4500 is a heavy duty 5-speed 3/4 ton transmission. Its physically larger than the AX-15, and longer. Depending on what donor you get this trans out of, it varys in gearing. Both flavors however feature a very low 1st gear, and a very sturdy construction. This transmission is VERY desireable for a Jeep swap. However, it is a more involved, and much more expensive project. The end result is great, but I have a limited budget to work with.



The NV3550 is manufactured by the same company that makes the 4500, it is a lighter duty transmission, with dimensions almost identical to the AX-15, which is'nt surprising, because this transmission was designed as a replacement for the AX-15 when Jeep stopped using them in 2001. Jeep TJ's with the 4.0 and a 5-speed manual from 2001 to 2005 came with the NV3550 stock. This makes it an excellent swap for a YJ with a 4.0! You get a sturdier transmission, at an affordable price, and its almost a direct bolt in replacement. It's got the right input shaft to accept the 4.0, the correct bolt pattern for a YJ or TJ 4.0 bellhousing, and the correct output shaft and bolt pattern for the NP231J. Because the dimensions are so similar to the AX-15 it will fit between the existing skid plate and body, and you wont need to change driveshafts. If you have an older YJ (1992 or older) with a 4.0 you'll also need a bellhousing that supports an external slave cyl. You should be able to scavange one off of any 1993+ wrangler that came with a manual trans and a 4.0.

NV3500



The NV3500 is a variant of the 3550, from what I can tell the 3550 is a modified 3500. I could be wrong however. It has the same gear ratios as the 3550, and the same dimensions, the only difference is that the bellhousing is integrated, and I dont know if this will bolt up to a Jeep engine. There are probably adapters available if it wont. These were sold in 1/2 ton dodge pickups.


What I chose and Why:
Well, the AX-15 rebuild was out of the question. The 4500 was too expensive of a swap, and had a possibility of too many complications. The 3500 may have worked, but I wasnt sure about the bellhousing. Plus I couldnt find one in my pricerange. So, I was left with the 3550, which I was leaning tward from the beginning.

Next Page