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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. The first question is which manual 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 if necessary since I need tires, and retain my stock axles (3.07 ratio *I believe*), 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 case 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?
 

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The AX15 is a good choice behind the OM617. Mercedes 4x4 offers the parts you need to bolt the OM617 directly to the AX15 for just under $400.

The NP231 is a good transfer case for mild off roading and the 2.72 low range works very well with the OM617. You could also go to a 4:1 low range if you need more gear multiplication depending on what type of trails you like.

3.08 gears suck. Sorry there is no other way to put it. The OM617 performes more like a gas engine and really like to run 2200+ rpms. You will probaly be ok with a 28 inch tire but your axle gearing is really what's limiting you. 31s with 3.08s are not going to be a good combination with the OM617. You may be able to live with it but it's going to be a dog.

For 26 inch tires I would run 3.55s

For 28-30 inch tires I recommend 3.73s

For 31s I would recommend 4.10 gears with the OM617

For 33s I would recommend 4.56s or even 4.88s

For 35s I would recommend 5.29s

This is based off my calculations using grim jeeps ratio calculator for best OM617 performance based off the MB 4 speed autos with a the following gear ratios: (1st 3.99, 2nd 2..39, 3rd 1.45, 4th 1.00)

also consider most of the 300D turbo OM617s came with a 2.88 or 3.08 and only had 24-25 inch tires in a 3500lb car.

The AX15 of course has a huge advantage of OD and it's unlikely you will ever drive it over 80mph any way you really need the more aggressive gearing to compensate for wind resistance, large tires, and extra weight.

My recommendation to people when looking at a OM617 swap is the gear the vehicle like you would a 4 cylinder Toyota. The engines best operating range is similar to a 22RE.

I hope this helps.
 

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OP I'm in western Mass, near Strubridge. I have a 1984 300SD, the car or the motor are not for sale but if you wanted to drive it to see what it feels like power wise ( or lack there of Lol ) then let me know. My odometer stopped at 206.000 mikes so I cant tell ya how many miles are really on it as I got the car with it not working and seeing how they old owner drove it to my friends junkyard I don't know any history on it.

But what I can tell you is it runs great, even on old cooking oil that I ran threw it last fall, smelled like French fries Lol.

Jason.
 

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Devil,

I second everything CBR said. I have a '94 YJ that I converted into a pickup with an '83 OM617. It originally was the 4 banger with an AX-5 transmission. Currently has a '97 AX15 bolted to the original '94 NP231. I had to swap the T-Case input spline, wasn't very difficult. Stock axles were 4.10 and I kept them. Tires are 28" ish.

The old Mercs do indeed run like gassers. Mine is pretty happy in the 2300rpm-3000rpm range, turbo doesn't even spool till around 2200rpm. For a daily driver it handles just fine. Can really be a dog getting into traffic but once it's moving and the turbo is spinning it moves along just fine. My engine has 235,000 miles on it and I know isn't performing as well as it could. Compression is healthy but the injectors are in much need of a rebuild (parts sitting on my bench, have been for almost a year...)

You will need to modify your oil pan and put a lift in. I can dig up some pictures of what I did for the oil pan if you'd like. If suspension travel isn't a high priority then extended bump stops are a very good idea.

It's currently snowed in but once I dig the Jeep out I will post some pictures of some of the pertinent modifications.

Welcome to the club!
IMG_0191.jpg
 

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Discussion Starter #5
The AX15 is a good choice behind the OM617. Mercedes 4x4 offers the parts you need to bolt the OM617 directly to the AX15 for just under $400.

The NP231 is a good transfer case for mild off roading and the 2.72 low range works very well with the OM617. You could also go to a 4:1 low range if you need more gear multiplication depending on what type of trails you like.

3.08 gears suck. Sorry there is no other way to put it. The OM617 performes more like a gas engine and really like to run 2200+ rpms. You will probaly be ok with a 28 inch tire but your axle gearing is really what's limiting you. 31s with 3.08s are not going to be a good combination with the OM617. You may be able to live with it but it's going to be a dog.

For 26 inch tires I would run 3.55s

For 28-30 inch tires I recommend 3.73s

For 31s I would recommend 4.10 gears with the OM617

For 33s I would recommend 4.56s or even 4.88s

For 35s I would recommend 5.29s

This is based off my calculations using grim jeeps ratio calculator for best OM617 performance based off the MB 4 speed autos with a the following gear ratios: (1st 3.99, 2nd 2..39, 3rd 1.45, 4th 1.00)

also consider most of the 300D turbo OM617s came with a 2.88 or 3.08 and only had 24-25 inch tires in a 3500lb car.

The AX15 of course has a huge advantage of OD and it's unlikely you will ever drive it over 80mph any way you really need the more aggressive gearing to compensate for wind resistance, large tires, and extra weight.

My recommendation to people when looking at a OM617 swap is the gear the vehicle like you would a 4 cylinder Toyota. The engines best operating range is similar to a 22RE.

I hope this helps.
That was extremely helpful, thank you!

What do you guys recommend for replacement axles? 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?
 

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Are you sure your YJ came with 3.08 gears?

And yes if your trying to find something on a budget go to your local pick and pull and look for a (4 speed automatic) 4 cylinder YJ. Some the 4 speed automatic jeeps came with 4.10 and even the super rare 4.56 stock gears. Like I said you will need to verify the axle ratio because jeep did some crazy things during the late 80s and early 90s including gear ratios ranging from 2.88 to 4.56. If your going to find 4.10s or higher in a stock YJ then that's where to look. Stay away from the 3 speed automatics they did not have over drive and often came with 3.08 or even 2.88 gears.

If your set on running 31 inch tires I would recommend at least 4.10s.
 
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