Hello! I recently purchased a 1988 YJ and almost immediately began thinking about doing a diesel swap. I absolutely love the older Mercedes diesels so I decided to go with an OM617

Unfortunately I have never attempted anything like this before. Not only this will be my first diesel conversion, but this is also my very first Jeep. I know that I have a very steep learning curve ahead of me, but I am looking at this as a great learning opportunity.

I am sort of stuck on step 1 of this project and I am looking for some guidance. Basically I am trying to put together a parts list for all the drive-train components - engine, transmission, transfer case, axles, etc.

Unfortunately my 1988 came with the plagued Peugeot BA10 transmission, which has to go. This will set me back additional few hundred dollars, but it is what it is. The question is what to replace it with? Normally people swap these with an AX15 or NV3550, both of which are 5-speed manual. I have been doing some reading and it looks like even though the AX15 is a great transmission for a gasser, it is not the ideal choice for a Diesel due to its gearing; basically 5th gear will be useless. So the question is which transmission do you recommend for the OM617?

Second on the list is the transfer case, which is dependent on the transmission. My Jeep came with a NP231, but it is Peugeot specific (21 splines vs 23). So it would have to be replaced or modified.
I also know that tire sizes and axle ratios matter as well. I currently have 31x10.5x15 tires and stock axles. I would prefer to change tire sizes (go down to 30's for example) and retain my stock axles to keep the cost at a minimum, but I am not sure if this is possible or even advisable. Again, Diesels are a different animal. The question is can I retain my stock axles? And if yes, which transfer care will be best suited?

I am not planning on building an off-road monster for rock climbing, etc. I would like to use my Jeep as a fun weekend vehicle so it will see mostly highway/city driving as well as some moderate off-road trail use.
I currently only have a shackle lift and perhaps a body lift, no crazy suspension modifications. I can take a few pictures or take measurements if that helps.

Once I get my "shopping list" I can start gathering all the components and I will document and post my build.

Can you please help me get started?

 

The Ax15 is a fantastic transmission and good for this conversion, cheaper, simpler and easier to find than the NV. The NP231 out of the Jeep Cherokee Xj is perfect, strong and affordable. If keeping stock gears tires need to be as small as possible. Search for my thread for more info.

 

I have nothing against the AX-15, it is actually on top of my list, but I was just wondering if there is something better I should consider. I believe my stock axles have a ratio of 3.07 and I plan on keeping them, unless you guys tell me it is a bad idea.

When you say "keep the tires as small as possible", what maximum size would you recommend? I will find and read your thread as well.

 

with 4.10 ratio 31" 5th gear is 2400rpm at 65mph, with 4.56 ratio and 33" tires 5th gear is also about 2400rpm at 65mph. With your current setup 65mph in 5th gear is 1840rpm which is to low. The sweep spot is 2400rpm. I'd personally change gears.

 

I am not very knowledgeable about the MB diesel motors, except that they are generally set up for higher RPM than the average diesel motor.

First, do some research on the motor of your choice. Find out what the torque curve and HP, and at what RPM's are for that motor. This will help you know the best operating RPM for that motor. Then, go to this website and start playing with different tire size, transmissions, and transfer cases. Gear Ratio Calculator
This gear ratio calculator has many of the common gear boxes and tire sizes already in it. You just have to choose what you want. The nice thing is that you can play with lots of different combinations without spending any money. And in doing so you can learn a lot and choose what you want for a final vehicle.

Don

Don

 

100% accurate. Peak usable HP and torque on these with factory parts is 2400rpm.

 

Fair enough, sounds like I need new axles. What do you guys recommend I swap them with? I am looking for something cheap and common... I believe the YJ 4-cylinder axles will fit the bill, but should I be looking at something else?

 

Definitely go for the YJ 4 cylinder axles. They can be had fairly cheap and everything bolts right up. The only bad part is the Dana 35, but if you don't beat on it and maybe do a few upgrades, it will survive. You said you wanted to stay around a 30" or 31" tire, so it should be okay. Or you could go with an 8.8 out of an Explorer, which will require some modifications (spring perches, emergency brake cables possibly). As for the transfer case, I would stick with the 231 and just change out the input gear. I just had my case apart to reseal it and they're super simple. Get a good set of snap ring pliers for the snap rings without the holes in them (sear clip?) and go easy with the RTV. Also, make sure you get the right input gear. They changed from a flat tooth gear to a pointed gear at some point. A good guide for doing this is Bleepin Jeep Youtube. Matt completely pulled a case apart multiple times to do an SYE install as well as swapping input gears and then installing a TeraLow 4:1 kit. Good luck.

Joe

 

Awesome, thank you. I will try to find a set of YJ 4-cylinder axles to keep things simple. I am not building an offroad monster so I am sure the Dana 35 will be just fine for me. Are there any other gotchas with the axle swap? Anything else I need to make sure I grab from the parts vehicle?

 

Nope, straight swap. The only thing is they switched to a larger U joint in 94 or 95, so that might be something to look out for. You can either swap your pinion yoke onto the new axle, use the driveshaft from the donor truck, or run an aftermarket driveshaft, if you plan on going SYE. I have a 93 4 cylinder YJ, so I'm in the same boat as you with my conversion of swapping everything out, except I have to gear down to a 3.07 or 3.53 for my low revving Kubota.