YJ Wrangler Has a Failed Rear Axle Differential

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My son's 1991 YJ Wrangler developed a clunking noise under load in all gears.  We did a little more looking and found the differential gears worn badly. It's been a long time since I worked on a rear end, think I remember the steps. This is a 1991 Wrangler YJ 4-cylinder. I believe it has a Dana 44 rear in it. The person that had it before my son kinda mixed a lot of different parts in the Jeep. The rear seems to be original but not sure. Can't find any markings. Thanks for any comments before I begin...

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You should have a Dana 35 rear axle with 7.625” ring gear diameter, adequate for the four-cylinder, also used with the 4.0L six in YJ models.  The 35 axle will work to 33” diameter tires.


The 35 has an oval diff cover, the 44 cover has flat left and right sides.  I've placed diff cover photos below of a Jeep Dana 30 high pinion front, a Dana 35 and a Dana 44 rear: 


post-1-0-98586400-1372637781_thumb.jpg post-1-0-34298900-1372637783_thumb.jpg post-1-0-30038000-1372637784_thumb.jpg

At left is a Dana 30 front axle, high pinion like a stock YJ Wrangler. Center is a Dana 35 rear in a YJ Wrangler. Right is a Dana 44 rear in a later TJ Wrangler. Note cover shapes. (Click to enlarge.)

Cannot open these photos? Consider joining the forums—for free—to get full access!


I did a step-by-step article at the magazine on rebuilding a Dana 30, similar in basic ways to both the 44 and 35.  For understanding concepts like the carrier bearing preload, ring-and-pinion gear backlash and pinion depth measurement, you'll find the article useful.  Read the article before you take the plunge.  If you have a factory-level workshop manual, use it and follow the step-by-step procedure.  My article is at http://www.4wdmechanix.com/How-to-Dana-30-Axle-Rebuild.html.


Primary difference between the 30, earlier 44 and the Dana 35 is the Dana 35’s use of a crush sleeve for pinion bearing preload.  The 30 and earlier 44 typically use a shim stack...Use care when adjusting the pinion bearing preload, as over-tightening requires removal of the new crush sleeve and installing another one—the bearings need their slight preload, and over-tightening the pinion load will crush the sleeve too much. 


Simply backing off the pinion bearing nut to reduce load (a common mistake) will loosen pressure from the crushed sleeve.  In this case, the pinion relies totally on the pinion nut for holding preload…The nut is unsafe, as the bearings are not loaded from the backside.


If you’re on a budget or time constraints, we can discuss servicing the differential without removing the pinion nut, leaving the pinion shaft in place.  If the axle is otherwise in good condition, this can be done.


Also use care when removing the differential “gear shaft lock screw” and the shaft.  The screw and shaft removal is necessary to access the axle shaft C-clips and remove the axle shafts (necessary step for differential removal).  Factory level manuals like to trivialize this step, showing a simple box end wrench for loosening this screw.  The screw is high tensile and susceptible to snapping while being removed, especially after a lengthy time in service.


I actually prefer using an air impact, set to very light pressure, for “jolting” this screw loose.  The stretching force of a socket and hand ratchet, or fiddling with a box-ended wrench that can round off screw head corners, is not a good idea.  “Twisting” this stubborn screw out with hand tools often results in the screw suddenly shearing.


A sheared gear shaft lock screw is an absolute nightmare.  The screw acts as hard as a carbide drill bit if you attempt to drill for a screw extractor.  Suffice to say, if you find yourself in this dilemma, I can first offer sympathy and then a practical solution or two!  You cannot disassemble the axle without removing this differential gear shaft.


Trust this helps...Will add info as needed!




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Unfortunately, with work and all going on we have only been able to pinpoint the problem and get the parts. The differential gears have a missing tooth and others are worn.


We actually found a whole other rear end and plan to replace it this weekend. At this point I only plan to pull differential cover, inspect, clean, and change gear oil. Of course replace brakes or any other components as needed. Aside from alignment of axles for straight traveling and re-tightening our work, are there any special items we should look at?


Thanks in advance...

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Since you have a four-cylinder, the rear axle ratio should be 4.11.  Count the ring-and-pinion gear teeth and do the math.  4.10 is 41-10 tooth count.  4.11 should be a 9-tooth pinion and 37-tooth ring gear.  The Dana 35 should be 4.11 for a four-cylinder application YJ model.
There's a lot of misinformation floating around about 4.10 versus 4.11 ratios.  Some think these are each the same gear tooth count of 41 (ring) and 10 (pinion).  They are not.  The true 4.11 is 37 teeth on the ring gear and 9 teeth on the pinion.  Please do a count of your old gear set's teeth versus the replacement axle to confirm a match.  Let us know what you find!
So, why is the Dana 30 front axle on a YJ Wrangler 2.5L a 4.10 ratio?  A couple of possibilities:  1) the axle housing can accept the 41/10 gear sizes more readily or 2) because Dana prefers this tooth count/ratio for gear fit into the Dana 30 axle housing, using a particular differential case design.  41-10 ring-and-pinion gear sets work with a common differential case flange offset.  Sometimes the axle housing itself will only accept a certain ring gear thickness or differential mounting flange offset.  The differential case ring gear mounting flange has a specific offset.  A particular offset may, for example, work with the popular 3.55, 3.73 and 4.10 gears. 


So, why is the Dana 35 rear axle a 4.11:1?  Likely because the rear axle housing and differential case are better suited for a 37-tooth ring gear size with a 9-tooth pinion.  The ring gear diameter is constant for a given axle type.  (As such, the Dana 30, 35, 44, 60, etc., each have their own ring gear diameter.)  The ring gear thickness and differential case flange location/offset must line up the ring gear with the pinion gear's teeth.  Again, from a manufacturer's perspective, it is easier and cost effective to use ratios that will enable use of a particular differential case over a wider range of gear ratios.


Generally speaking, the 10-tooth pinion is stronger by design than a 9-tooth.  This takes into account that the ring gear diameter remains constant.  (Dana 35 is 7.625" diameter.)  Housing design plays a role in this choice, too, determining the position of the ring gear laterally


Consider that a numerically higher (lower gearing) gear set is under less load due to the lower gearing, and this helps offset the  pinion tooth count and the smaller pinion gear head diameter.  This means that a 9-tooth pinion gear can survive in a numerically higher (lower gearing) axle because the smaller pinion gear head gets less load due to the reduction ratio—a trade-off.  This chart, found at the Quadratec website helps illustrate the relationship between ring gear tooth count, pinion gear tooth count and axle ratio.


Will a 4.10 front axle work with a 4.11 rear axle ratio?  Absolutely, and this is very common due to manufacturers mixing axle designs front to rear, like a Dana 44 at the front of a Ford Bronco or F-pickup and a Ford 9-inch axle at the rear.  Sometimes this is just the available options offered for a given manufacturer's axle type.  Over the years, you can find mixes like 3.70 with 3.73, 3.54 with 3.55, 4.10 with 4.11, 3.07 with 3.08, 4.86 with 4.88, the list goes on, there are many examples of these "near ratios" throughout the 4x4 make and model applications.  Some have to do with outsourcing of axles from two manufacturers, some have to do with given ratios and axle housing or differential case designs.
According to the official Mopar parts listing, your rear axle should be 4.11 (OEM ring-and-pinion replacement gear set kit #83504938).  Your front axle should be 4.10:1 if it has the vacuum axle shaft disconnect:  Mopar complete axle assembly #04637925, 4.10 Ratio, tagged #53009329 or #52068203. 


Note: Mopar also lists a non-disconnect front axle at 4.11:1 ratio for the 1991-93 YJ Wrangler.  The complete axle is part #04741047 at 4.11 ratio, tagged #52068206.  This must be a one-off item (listed as "KDX").  Frankly, I've never seen a YJ without the disconnect mechanism...If anyone has seen a "KDX" model, please share your comments!


Compare axle tag numbers if still in place.  We can verify ratios from tag numbers;  you can count gear teeth as easily if the diff covers are removed.


Do you have OEM or aftermarket limited slip (locking differentials) in either of these rear axles?  Note the axle shaft bearing condition by rotating each axle shaft and shaking the shaft or brake drum sideways.  Note whether you have C-lock type axle shafts (OEM for the '91 application) or pressed-on wheel bearing type axle shafts.  Compare the brake shoe sizes, brake drum sizing, cable hook-ups and pinion flange/U-joint design and size.  That should do it! 


Torque all fasteners to spec, and make sure you bleed the brakes after hooking up hoses and lines.  (Keep debris from entering the open brake lines or hoses!)  If the replacement rear axle's wheel cylinders look questionable and you plan to use them, rebuild or replace the cylinders.  (I would at least rebuild them, who knows their condition, right?)  Replace worn or thin brake lining, and resurface the brake drums if necessary.  Install new shoes and spring/hardware kits. 


This is a good time to flush and purge the brake system, using fresh brake fluid (type DOT 3).  I've shared that my preferred method for flushing and purging the hydraulic system of contaminants is with a vacuum bleeder, applying vacuum at the calipers and wheel cylinders.  This draws old fluid and contaminants from the cylinders, doing a much better job than pedal or pressure bleeding from the master cylinder end.


Warning:  Keep the brake system clean...Do not use petroleum distillate or chlorinated solvents (found in most aerosol brake parts cleaners) around any of the brake system hydraulic seals and other rubber parts.  Read the labels:  These solvents are for metal parts, not for use around rubber!  Use denatured alcohol or isopropyl alcohol to clean brake hydraulic system parts;  dry parts thoroughly with filtered, dry compressed air before installing new rubber seals and cups.  Use only recommended (DOT 3 or equivalent) brake fluid.


Be sure to test drive the Jeep carefully, and let us know how the job turns out!

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