Differentiating Right From Wrong

Differentiating right from wrong

This week we received a 2011 Jeep Wrangler with some grinding noise emanating from the rear of the vehicle when moving. Like most Jeep Wranglers, aftermarket upgrades are commonplace. Larger tires, polished rims, lighting are just a few items that are often found on these vehicles. This one, in particular, had all of these, along with re-gearing the front and rear differentials.

We often get asked to perform this procedure on different vehicles. But why would you want to change gear ratios? Your gear ratio has an effect on how much horsepower and torque is available to you across the RPM band.  Changing your gears shift the power band forward and backward, and can compensate for modifications like larger tires.

The axle gears have a ratio. That ratio tells you how many times the driveshaft will make a complete rotation for every one rotation of the wheel. So for example, if you have a 4:10 gears. That’s 4:10:1 ratio, which means for every full rotation of the tire, the driveshaft will rotate 4.10 times. The ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. So made easy, 41 teeth on the ring gear divided by 10 teeth on the pinion gear gives you 41/10=4.10.

So back to our dilemma. After noticing that the differentials had in fact been worked on due to the aftermarket covers installed, we proceeded in removing the covers for inspection. Once removed, we noticed that a tooth on the rear differential pinion gear had sheared off, and the carrier shims were walking out. Both of these issues usually occur when there are incorrect shims installed. So how do we correct this issue? What exactly are shims for? And how do we know if we performed the correct repair? Let’s answer that!

First, repairing the ring and pinion is simple! Replace. But since we are working on a 4×4 vehicle, one thing we must always remember. The gear ratios in the front must match the rear! Why? The transfer case is designed to work when both driveshafts are spinning at the same speed, or at least very close.  If there is a significant difference between the speeds of the front and rear driveshafts, the binding will occur causing potential damage to the transfer case and any other items in line (u-joints, axle shafts, yokes, and differential). So, gear ratios must match or be within 1% (4:10 rear—4:09 front). Knowing this information, both front and rear differentials on this vehicle have upgraded Yukon 5:13 gears installed. So, replacing with the same gear ratio and same brand would be the best logical solution. After obtaining the replacement ring and pinion set, another item that requires replacement will be both the pinion bearings and races along with the carrier bearings and races. Keep in mind that any “hard failure” such as a sheared tooth or a failed bearing contaminates all the bearings causing unevenness. Attempting to “re-run” any of the components risks reoccurring failure and incorrect shimming. Bearing and seal kits usually come with all the necessary components to rebuild your differential.

The next procedure when performing this repair is the most crucial and difficult. Setting up the differential requires special tools and a seasoned technician. Without getting too technical and too in-depth, I am going to summarize this section. Should I have further interest in this article, I will provide another article about the process involved in the complete set-up.

In a nutshell, we removed all the bearings from the carrier and pinion for inspection and disposal. We installed new carrier bearings with races. On the new pinion gear, there is a shim installed under the inner pinion bearing. The shim size was determined by performing a pinion depth reading (Requires special tool and process to be discussed later). We installed new pinion bearings and new races into the case. Before installing the pinion gear into the case, a new crush sleeve is installed (MUST BE REPLACED EVERY TIME PINION IS REMOVED). The crush sleeve is vital to achieving the correct pre-load on the bearings. Failure to replace crush sleeve or pre-load bearings properly will result in guaranteed bearing failure and most likely differential noise. Once we have the new pinion installed with new bearings, pre-load set, new pinion seal and yoke it was time for the carrier. We installed the new ring gear onto the carrier. It’s good practice to put Loctite onto the threads of the ring gear bolts to prevent from backing out and Torqueing to specification. Note: Some ring gear bolts are reverse (Left-handed) thread. We installed the carrier and races into the case. Now the fun part…shimming. Again, this is summarized but we are looking for a ring to pinion gear tooth contact. How do we achieve this? The original shims were replaced by the workshop that re-geared this previously. So, we will start there. Aftermarket shims come in a variety of thicknesses and sizes based on the differential that you are assembling since we have a Dana 44, I will be using those specifically for it. Each side of the carrier has different shim thicknesses to move the ring gear closer to the pinion gear or further to make correct tooth contact. How do we know if the teeth are contacting correctly? Simple. Apply gear contact grease to the ring gear and turn the gears. Once the teeth pass over the grease, look at the contact pattern. You want a nice oval in the center of the ring gear teeth. Adding or subtracting shims side to side to achieve this.  Easy as pie right! Well, sounds that way but it’s little harder than it sounds…but you get the idea. We installed the covers, added fluid and voila! A jeep fit for a king!