The Avocado (1988 Cherokee) with about 152K miles on its NP231 transfer case began making some not so friendly noises.Ê There was a definite GRrrrr . . GRrrrr sound when ever throttle was applied or removed.Ê I thought there was something wrong in the rear axle and took it to the experts at Four Wheel Parts Wholesalers in Oklahoma City, OK.ÊÊ Jerry Cantrell, Store Manager, and his technicians deserve a special thanks.Ê They put the Avocado on jack stands and had me recreate the problem while they listened to the different components with a stethoscope.Ê It is this level of professionalism required to prevent expensive misdiagnosis.Ê They told me the noise was originating from the transfer case.Ê Since it was obviously time to go through the case and replace worn components, I figured it was the right time to upgrade.Ê Cherokees (XJs) come with either NP231 (Command Trac) or NP242 (Selec-Trac) transfer cases (some early models have NP207). Today, there are many different ways to upgrade the XJâs transfer case and they differ in price and complexity.Ê You can replace the transfer case with a bulletproof Atlas II case from Advanced Adapters like we are planning for Project Colorado Red; or you could swap in a 4 to 1 gear ratio conversion in your case.ÊÊ Whatever upgrade, you should include eliminating the rear output slip yoke found on the original model.ÊÊ Why is this feature considered the weak link of an otherwise dependable transfer case? First, it has little tolerance to allow lifting the vehicle.Ê As the vehicle is lifted and articulation is increased, the rear drive shaft must ãgrowä to allow the rear axle to drop.Ê Likewise, it must ãshrinkä as the rear axle moves upward.Ê Since the factory design uses a fixed length rear drive shaft, a splined slip yoke moves in and out of the rear of the transfer case to allow the growing and shrinking.Ê Lifting the vehicle, say 3 inches, pulls the slip yoke outward leaving less contact area with the splines.Ê Additionally, the factory design uses a single Cardian u-joint at each end of the driveshaft.ÊÊ This means careful shimming of the rear axle and possibly lowering the transfer case to eliminate drive line vibrations.ÊÊ Another consideration for eliminating the slip yoke is trail worthiness.ÊÊ Several times over the past ten years, I have helped fellow fourwheelers who damaged the rear driveshaft get home.ÊÊ Removing the rear driveshaft with the factory design and attempting to continue in front wheel drive only, is not possible unless you have some way to plug the rear output shaft opening.ÊÊ I have cut liter soda bottles and jammed them over the rear housing affixing them with duct tape to prevent fluid loss.Ê Why worry with all this?Ê Convert your transfer case to a fixed yoke and drive away with confidence.Ê Follow along as I describe how the M.I.T. kit installation is performed.Ê I had a master Jeep mechanic perform the operation to document the tricks of the trade for you to reference.

Here you can see the transfer case before disassembly and the components of the M.I.T. conversion kit.Ê M.I.T provides great instructions.

After removing the extension housing, you must remove the snap ring (a.k.a. locking ring).Ê This is one of two snap rings along the main shaft.Ê It was the one on the front of the shaft that broke and caused my unknown noises. The same noises would be present if only the rear one broke.Ê If you suspect you have a broken snap ring and only want to fix this one item, it would be prudent to remove the extension housing and check this rear one before removing the transfer case from the vehicle.Ê Save this snap ring as it will be reused.

Remove the front output yoke and carefully pry the case open at the designated pry points.Ê This will separate the halves of the case.

Remove the internals and inspect all components for wear.Ê I could not find a repair or tune-up kit available.Ê All parts from the dealer are al-a-carte so make notes of what you need and get to the parts department. At a minimum on a high mileage case, replace the chain, oil pump, and seals.ÊÊ Thoroughly clean all surfaces before reassembly.

Here is a place where you can really mess up.ÊÊ Make sure the oil passages on the front of the case line up with the seal adapter.Ê This has nothing to do with the MIT conversion but is critical to any transfer case reassembly.

Lubricate all seals to facilitate assembly and extend seal life.

The new oil pump should be primed prior to installation.Ê Just pour in the fluid and work the pump by hand.

This Jeep expert will not use anything other than Mopar gasket maker for this procedure.Ê He said he has never had a ãbring backä due to fluid leaks.

Once the case halves are reassembled, proper positioning of the oil pump and pick-up tube is a bit tricky.Ê Also, make sure the bolts holding the case halves together go back in the proper place.Ê Some have washers and others do not.

Once the speedometer gear housing is in place lubricate the rear bearing and partially fill the case.Ê Turn the shaft by hand several rotations to prime all components.ÊÊ Here you can see the modified main shaft which has been threaded on the end.Ê M.I.T. requires your old shaft back as a core or expect a $50 charge.

Here is a stock case in the foreground and the modified case in the background.Ê The M.I.T. kit shortens the case length by 5.5 to 6 inches which is more than competitors.

Once back in the vehicle we are ready to drive away with confidence in front wheel drive.Ê A new rear drive shaft will be required to compensate for the additional length and incorporate a slip spline like the front drive shaft.Ê M.I.T. uses a standard Spicier yoke which is easily replaced in the event it is damaged.Ê Some of the competitors use custom set-ups which force you to call them for spare parts.Ê Hmmmm . . . wonder what happens if the other guy goes out of business?

M.I.T. has slip yoke conversion kits available for the NP231, NP242, and NP207 transfer cases.Ê Suggested retail price is $465.00 and requires your main shaft as a core.Ê Without a core, add $50 for an XJ core and $100 for a TJ core.