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The Go Anywhere Jeep Cherokee

How Lockers and Transfercases Work - Quick and Dirty Install

  


Frequently the new Jeep Cherokee owner has confusion over the topic of lockers and transfer-cases and although I am not an automotive expert, I'll try to clear up some of the misunderstanding regarding these two topics.


First let's understand the flow of power through the drive-train of a 4WD vehicle.  The power starts in the engine and flows into the transmission just as with virtually every automobile whether 2WD or 4WD.  Where it differs in a 4WD vehicle is that the power then outputs from the transmission into the transfer-case.
  

The function of the transfer-case is to selectively feed the rotational force being generated by the engine/transmission to the front and rear axles via their respective drive-shafts.  How the transfer-case splits the power depends upon its type and what mode been engaged.

When the transfer-case is in Part Time mode whether high or low, the front and rear driveshaft’s are more or less physically engaged/locked together within the transfer-case. Because of this positive engagement power is sent to both driveshafts, simultaneously.


The majority of Jeep Cherokees came with either a part-time (NV231) or full-time (NV242) transfer-case.  The manufacturer of these transfer-cases was known as either New Venture Gear or New Process Gear now owned by Magna International.  Both transfer-cases are tried and true platforms that offer very good durability/reliability to all but the most extreme off-road vehicles. 


When the transfer-case is in Part Time mode whether high or low, the front and rear driveshaft’s are more or less physically engaged/locked together within the transfer-case. Because of this positive engagement power is sent to both driveshafts, simultaneously.


A negative aspect of this engagement is when trying to turn right and left.  The front tires will want to travel in a different track/path than the rear wheels. Because of this different track the front driveshaft will want to turn at a different speed than the rear driveshaft. When this occurs and you are on a surface that provides a high level of traction such as dry asphalt or concrete, you will feel driveline bind that is occurring inside the transfer-case.


Due to this transfer-case driveline bind part-time mode is only used in wet/snow/off-road conditions where a tire can slip a little to dissipate the binding from within the transfer-case. If this transfer-case binding isn’t relieved what typically occurs is the stretching of the transfer-case chain.


Most transfer-cases use a thick chain and a set of gears to provide power to both the front and rear driveshafts.  When the chain has been stretched too much it will start slipping on the gear teeth and you will hear a popping noise when accelerating in 4wd. At that point the transfer-case needs to be rebuilt.


When in 2wd only the rear driveshaft is engaged and because of this no transfer-case bind will be observed. Although the transfer-case can be internally engaged or locked together as demonstrated above, no one refers to that process as a “locker”.


So what is the differential? On both the front and rear axles there is a round area often referred to as the pumpkin but is technically known as the center-section.  Contained within there resides components that comprise the differential.  Often in conversation the term "differential" may be erroneously used to refer to the entire axle, although it isn't technically correct, people generally know what is meant when it is used this way. 


Typically stock Jeeps, such as our XJ’s, come with what is termed as “open” differentials.  Axles typically have two (much like the transfer-case has a front and rear driveshaft attached to it), one for each wheel. Without an open differential each axle would act in a very similar fashion as a transfer-case in part-time 4wd. 


If these axle-shafts were “locked” together and the vehicle turned right and left, one wheel will want to spin faster than the other. As with the transfer-case, if this on a surface that provides a high level of traction such as dry asphalt or concrete, you will feel driveline bind emanating from inside the differential.  The negative aspect in diff-binding is instead of stretching a chain like you would in a transfer-case, you run the risk of chipping a gear tooth or some other component within the differential depending on what happen to be the weakest link.   If none of the above occur at the very least premature wear of the drive-train parts will occur.








To prevent diff-binding from occurring differentials that come from the
  factory are most commonly referred to as open. An open differential will
  provide power to the wheel with the least amount of traction. When only one
  wheel is getting power the other wheel is spinning free of engagement and
  hence no diff-binding will occur. So when utilizing an open diff even though
  we call it 4wd, it really is only 2wd seen as only one tire from both the
  front axle and rear axle will be receiving power. Because of open
  differentials 2WD mode technically is 1WD. Once again the primary benefit of
  an open differential is the better on-road manners it has in perfect driving
  conditions, which tend to be more common.

  

  So you want to enhance traction? There are a few ways of accomplishing this
  and the most common terms for traction-aiding devices are LSD (Limited Slip
  Differential, commonly known as a limited slip) lockers and a spool. 

  

  First off, LSD’s do differ from a true locker and shouldn’t be referred to as
  one. Incorrect usage of the “locker” moniker in reference to a LSD can be insulting to some owners that have paid the
  high dollars to buy their locker and you will unnecessarily incur their
  wrath. LSD’s are typically biased towards providing a good on-road ride, although
  they can be built differently. 

  

  Most LSD’s contain a pack of friction clutches that get pressed together via
  some springs in an effort to somewhat make the axleshafts appear as one. Depending how strong the springs/clutches are will dictate the
  biasing of the LSD. Weaker springs will engage less aggressively and will
  perform better on the street. Strong springs and clutch packs will be biased
  towards engaging harsher and limiting the slip in the differential while
  off-road. 

  

  The primary negative aspect of a clutch-based LSD is
  the biasing doesn’t engage positively enough to be effective in many off-road
  situations such as rock crawling. The other thing is that the
  springs/clutches don’t last forever, they have a finite life. Fortunately
  they are typically rebuildable for a fraction of
  their original purchase cost.

  

  The other type of LSD is gear driven and is commonly known by the brand name 
  Torsen. A Torsen LSD uses gears
  to bias the torque control within the differential. Because of the gear
  driven nature of a Torsen-based LSD, wear and tear
  issues do not occur and can make them a more attractive than their
  clutch-based counterparts. The most common Torsen-based
  LSD in the off-road industry is Detroit’s Truetrac. The Truetrac
  comes the closest to providing locker level of
  performance, without actually being one. In addition to the above LSDs are some factory offerings. Some higher end Jeeps
  may come with a factory option that is typically some sort of LSD.


Lockers come in two varieties, lunch-box and full-case lockers. Lunch-box
  lockers are cheaper to purchase, easier to install but weaker than their
  full-case brethren. The lunch-box lockers typically only replace a special
  gear set within an open diff, known as spider gears and utilize a stock differential
  component known as the carrier. Because the lunch-box locker doesn’t
  necessarily replace the carrier it can be installed by an off-road
  enthusiast. Some of the most popular lunch-box lockers are the Aussie Locker
  and PowerTrax Lockrite.

  

  A full-case locker replaces all of the stock differential components, both
  spider gears and the carrier. Because it replaces the factory carrier, the
  resulting locker is considerably more expensive and stronger than a lunch-box
  unit. Also the installation of a full-case locker is considerably more complicated
  and may need a mechanic to accomplish correctly.





 

  How does a locker function? It is the spider gears that normally allow one
  wheel to travel faster than another. When a locker (lunch-box or full-case)
  replaces the spider gears and power is applied it will positively engage both
  axleshafts without the use of clutches. The only times when the locker should disengage is when it senses the vehicle making a
  turn, in which case the locker is supposed to disengage. The most popular
  full-case Locker is the Detroit Locker, ARB Air Locker and the OX Locker.

  

  Beyond the above is a device that makes both axleshafts
  become one 100% of the time. This device is known as either a spool or
  mini-spool and can occasionally be a Lincoln-locker. A spool is similar to a
  full-case locker and replaces both the carrier and spider gears with the
  spool. A mini-spool is similar to a lunch-box locker, which retains the stock
  carrier and replaces the spider gears. A Lincoln-locker is when the spider
  gears on a stock differential are permanently welded together. The full spool
  is strongest followed by the mini-spool and lastly the Lincoln-locker. Spools
  are best on vehicles were off-road is their primary use. Otherwise wear and
  tear on tires and other driveline components will occur when used on the
  street.

  
Lockers and LSD’s can be either automatic or user selectable. Automatic
  devices such as the Detroit Locker engage and disengage entirely on there own
  whimsy. When driven on-road an automatic device can cause a vehicle to yaw
  when gas is applied and lifted. Plus automatic lockers can create a somewhat
  undesirable symphony of noises, but this is purely user subjective. 

  
User selectable lockers are usually air, electric or cable activated. The
  benefits of a selectable locker can be numerous. Most selectable lockers
  function as an open diff when disengaged and because of this can be
  completely transparent in their use on-road. Because of this selective nature
  they can be used when only necessary on-road/off-road, such as when
  encountering deep snow or when on an off-camber mud slope. Normally an
  automatic locker will engage and cause the vehicle to somewhat fish tale and
  lose control. Where a selectable locker can be left disengaged and allows the
  vehicle the best chance to keep a straight path. Selectable lockers also make
  tight turns off-road somewhat easier when disengaged. The downside to them is
  the added cost and complexity which tends to cause a greater potential for a
  component failure.




 



                       ***Disclaimer***

The above information has been created by someone that is an enthusiast and not a trained
automotive professional.  My intentions are not to have any physical harm come to anyone, 
but to highlight what I have done to my own personal vehicle or share my knowledge or
experiences.  If you decide to use any of my information, you bear the responsibility to
verify its safety and accuracy.


***Please do not link to this article or republish it without my permission***