The Mechanics of the Xterra

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    This page explains how different components of the Xterra work.  Some things are pretty obvious and won't be explained but others are a little more complex and a better understanding of how things work will help you avoid situations that can cause damage.  You may not notice it looking in the engine compartment the first time but the Xterra is actually a pretty simple vehicle.  Many systems including the fuel control, Transmission, Brakes, and Ignition are computer controlled.  But unlike many ultra modern vehicles coming out today the computers don't take a whole lot of control away from the driver and most systems will work even when the computer breaks down (just not as well.)  The only time the computer will override your commands is if you try to do something that will damage the vehicle, like shifting to 1st gear at 75 MPH.  This allows you to drive without too much thought, just push the pedal and go; or you can monitor and control everything about the drive.

    Most systems on the Xterra are cake to figure out.  If you know the basics of how a master brake cylinder works you can figure out the Xterra Master Brake without thinking, in fact many of the systems look and work just as they do in automotive texts for the last 30 years.  At the same time the Xterra is complex and modern enough that some times it seems like magic it works so well.  The ECCM controls the fuel mixture by reading over twenty different sensors mounted all over the vehicle and constantly recalculates the mixture with every piston fire.  What would take a mechanic an hour under the hood adjusting a carburetor is done in minutes as you drive to work.

    In the end the mess under the hood is not hard to understand at all, take away the wires for the ECCM sensors, and the hoses and valves for the emissions controls and your left with a basic internal combustion engine the sucks in air, mixes it with gas vapors, detonates it, and sends it out the rear.


 

How Stuff Works 4WD Pic

Pic from HowStuffWorks.com

4WD Overview

    The Xterra has a simple setup for a 4WD system.  It’s very reliable and easy to diagnose problems but it also means that you need to have an understanding of how it works to use it effectively without damaging driveline components.  Unlike some modern vehicles you need to do more than push a button to shift to 4WD; if the conditions aren’t right to shift a computer won’t keep you from shifting, and you can damage the driveline.  On the plus side you don’t have to worry about the computer malfunctioning and damaging the vehicle or disabling your 4WD.

    The driveline components are anything that is involved in the transfer of power from the engine to the wheels.  When in 4WD the power comes from the crankshaft in the engine to the Transmission (AT or MT), then to the transfer case where it splits via chain and gear to the front and rear drive shafts.  The drive shafts each go into their respective differentials, at the differentials the power is split again and sent to the left and right transaxles.  The transaxle then takes the power to its respective wheel hub and the power can be transferred to the road by friction moving the vehicle forward.  But that doesn't begin to describe all the things that come together to make it work.

Transmission

    The Transmission on the Xterra is a set of reduction gears (and clutches in the AT case) that convert the RPM of the engine into higher power at the cost of speed.  The transmission has a clutch (MT) or Torque Converter (AT) that smoothes the transition of one gear to another without stalling the engine.  The Manual Transmission (MT) has 5 forward gears and a reverse gear; the Automatic Transmission (AT) has 4 forward gears and a reverse gear.  The transmission works the same as any other vehicle, in fact you could swap it into a car (theoretically speaking) and never know any difference.

Transfer Case

Where the Magic Happens    The Transfer case is the true heart of the 4WD system, it’s similar to the Transmission but it has the additional duty of dividing the power between two output shafts.  When in 4H or 4L a gear in the transfer case is engaged that has a chain around it, the chain goes to another gear that is attached to the front drive shaft sending the power out both output shafts.

    The Xterra has 4 positions for the Transfer Case:

    The green 4WD light on the dash board indicates if the transfer case is in 2WD or 4WD regardless of the position of the selector lever; thus if the lever is in 2WD but the green 4WD light is still illuminated, you’re still in 4WD.  The auto hubs have no indicator and you must remember what condition they’re in to be able to tell when they’re locked.

    In 2H the engine power is sent only to the rear axle, the same as a typical rear wheel drive vehicle.  The front wheels turn independently of the front axle because the auto lock hubs are disengaged, so the front drive train is stationary reducing drag.  The ratio in the transfer case 1:1, the power coming into the front is sent out the back with no reduction.  In 2WD the Xterra is the most efficient with gas due to the only resistance being from the rear wheels, this also means only the rear wheels generate traction.  Imagine a man pushing a heavy box from behind, that's 2WD.

    In 4H the transfer case divides the engine power equally between the front and rear axles at the same ratio that it sent the power to the rear in 2H (1:1).  The front wheels receive engine power as soon as the auto locking hubs engage (about 1/3rd of the front axle revolution) and now all wheels have access to engine power (but isn’t in true 4WD because the differentials are open or limited slip, more on that later).  Now you have a guy pushing a heavy box from the rear and another guy pulling from the front.  Speeds should stay below 55MPH so no damage to the drive line is sustained now that it has greater resistance coming from the front (you shouldn’t be going that fast in a situation you need 4WD anyway).  Since all four wheels are receiving power the system requires a little more power from the engine and thus Gas mileage is a little bit less than 2WD.  But on the trail you can actually save gas by having the Xterra in 4WD because less power is wasted from wheel slippage because you have more traction with all 4 wheels working.

    4L is the same setup as 4H except that the power runs through a planetary gear in the transfer case that increases power but decreases speed (the ratio is 2.02:1, poor by 4WD standards).  It’s the same as comparing 1st gear in the transmission to 4th; you get a lot more power but your top speed is reduced and the engine will red line at a lower speed.  The Xterra should never exceed 30 MPH while in 4L (You’ll be hard pressed to anyway, the engine will be screaming at 30 MPH).

    The procedure for shifting the transfer case from 2H to 4H is to pull the lever back quickly and firmly, if you do not shift firmly the transfer case may not engage properly.  To shift from 4H to 4L come to a stop, put the transmission in Neutral (or Park with AT), then push the transfer case lever down to the floor and pull back through Neutral to 4L, then let the lever pop back out.  If grinding occurs during any shift or if the transfer case won’t shift come to a stop or be more firm with the shifting.  Never leave the transfer case in Neutral; if the T-Case is in neutral the transmission will be disconnected from the wheels and neither the engine nor the transmission will keep the vehicle from moving.

Speeds:

    Sometimes the transfer case won't want to come out of 4WD, this is caused by a buildup of stress between the front and rear drivelines, reversing a short distance usually relieves the stress.  And it helps if the wheels are on a slippery surface rather than asphalt.

Drive Axles

    When the power leaves the Transfer case it goes to the front or rear drive axles; both are pretty much the same except for size and length.  There are two universal joints on the drive shaft, one at the transfer case and one at the differential; these are needed so that the components on either end can move without bending the shaft.  In the front this isn’t as important because the front differential and transfer case are both attached to the frame (but the u-joints are still needed to counteract frame flexibility and vibration).  In the rear the u-joints are needed because the rear axle is separated from the frame by the suspension, so the axle needs to be able to be able to drop and rise as the rear axle bounces and twists over bumps.      

Front Driveline and Differential

Rear Drive Line and Differential

Differentials and Transaxles

Transaxle between the Tierod and Sway bar

The Transaxle is in the center of the photo
The CV Boot is to the right.

    After the drive axles the front and back start to act differently.  In front the power enters the front differential and is divided to the two transaxles but in front the transaxles aren’t solid like in back.  Since the suspension is independent each side needs to flex independent of the differential and opposite transaxle; so each transaxle has two Constant Velocity Joints to allow the axle to bend (they’re covered with rubber boots).  A CV Joint works like two U-joints in one; it allows the power to be transferred smoothly without as much stress on the joint.  After passing through the CV joints and the axle the power goes into the hub and turns the wheel.  In the back the power enters the rear differential and is divided between the two transaxles (both are housed inside the rear axle tube), the power then goes into the hub of the wheels and turns the wheel (the rear hubs are always locked).

    The way differentials work is pretty complicated, go to HowStuffWorks.com for a comprehensive explanation of a differential.

    On stock Xterra’s the front differential is always open and the rear is either open or a Limited Slip Differential (LSD) depending the package you got.  This is the reason that you can’t have a true 4WD on the Xterra; since the diffs are open the power takes the path of least resistance.  If one wheel is stuck or has too much more traction than the other wheel the free wheel will spin taking up all the power.  The LSD limits this by sharing the power regardless of how little traction the free wheel has but an LSD can only send a percentage of power to the spinning wheel, and the ratio with the Nissan LSD in the Xterra is very low.  There's usually a stick that says LSD on the differential but if you want to be sure lift the rear end off the ground and turn one wheel by hand; if the other wheel turns the same direction you have and LSD, if it turns the opposite direction you have an open differential.

    To remedy this situation you can add aftermarket Lockers to the differential.  When the axles are locked they act as if the gears in the differential were welded; both wheels get an equal amount of power no matter how much traction they have.  A vehicle with a locked axle will have both wheels turn at the same rate, if both axles are locked and the transfer case doesn’t have a differential then all 4 wheels will turn at the same rate whether one is in the air or bogged down in 4ft of mud.  But when the wheels are locked the driveline has to cope with much more stress.  The Xterra driveline is quite sturdy but even the strongest driveline is susceptible to abuse.  If you have lockers you have to be much more conscious of your driving methods offroad.

    The two front transaxles in the Xterra are identical regardless of which side there on and can be repaired on the trail.  The wheel needs to be at full droop, and the hub components need to be removed but on the differential side the axle unbolts and can be removed.  So carrying an extra CV Shaft for extreme trails can be good insurance.

Hubs

    The front wheels have auto-locking hubs so that they can turn the wheel when power is applied to them (4WD) and spin freely to minimize drag when they’re not needed (2WD).  The auto hub is disconnected from the axle, until the axle turns about 1/3rd of a rotation for the hub to engage; after that it acts as if it was fully locked.  For the hub to unlock the wheel has to turn the opposite direction it did to lock for 1/3rd a turn.     The auto hubs make a "click" noise when they engage, this is normal; if they make a clattering noise they are having trouble engaging and damage may result.  Stop, reverse 6ft, pull forward 6ft and then drive normally to see if the clattering continues.  If it does stop take the transfer case out of 4WD and alternate reversing and pulling forward till the clattering stops.  Then try to go to 4WD again.

    If you shift from 2H to 4H and accelerate too quick the hubs may not have a chance to engage or worse the hubs can be permanently damaged and begin clattering, come to a stop and then proceed slowly until the hubs engage.

Independent Suspension

    The Xterra has IFS (Independent Front Suspension) and a Solid Rear Axle, most SUV’s built after 90’s were built like this but the new trend is fully independent front and rear to improve road handling, but in almost all situations they lose offroading capability.  Most of the fully Solid Axle group likes to call these “Grocery Getters” or “Soccer Mom Vehicles” because they aren’t as capable, but accusing them of only being able to tackle supermarket parking lots is a huge underestimation.  Plus driving an air conditioned, dust free, Xterra offroad in the summer is much more comfortable than an open top Jeep Wrangler; the Jeep can go further but if you’re miserable from sweating and having sand blown in your face you’re not going to want to go further.

TheIndependant System

The Independent Suspension

    The reason independent suspensions are such a problem is they don’t flex as well as solid axle vehicles.  Flex (also called articulation) is the ability to have one of your wheels up on a ledge but have all three of your other tires firmly on the ground; the higher that one wheel can go the better your flex.  This manifests itself on the trail when you get in a situation that the axles are “crossed up”; that’s when wheels on opposite corners of the vehicle are lifted.  Imagine crossing a deep gully at an angle: the front drivers side wheel is on the far side of the gully, the front passenger tire is down in the gully, so is the rear driver side wheel, and the rear passenger side wheel is on the near side of the gully.  If your suspension doesn’t flex enough the two wheels in the gully won’t be able to touch ground; for a few seconds you’ll “teeter-totter” till your weight shifts and one of the wheels in the air comes back onto the ground.  But even after that wheel touches the ground it will only do so enough to balance the vehicle; it still isn’t getting as much traction as the two wheels on top of the gully.  Now the problem with open differentials becomes apparent; the two wheels on the top of the gully have a lot of traction and the two in the gully have little or no traction so all the power will go to the wheels with little traction.  One will throw dirt in the air and the other will spin in mid air; you’re stuck!

Solid Axle

Solid Axle Suspension

    In a solid axle vehicle the two wheels on an axle have a solid connection to each other, the pivot point is at the far wheel.  Depending upon your suspension set up you can get a lot of wheel travel from the highest point the wheel can go, to the lowest.  With an Independent Suspension your wheel travel is limited by the length of your upper and lower control arms and the pivot point is only a foot or two from the wheel  No matter what modifications or lifts you make to the suspension your wheel travel will be about the same if the control arms are the same length as before.  Another drawback with an Independent Suspension is when one wheel is pushed up and the other doesn’t react at all, in fact if the sway bar is connected the other side is pulled up too; this is ok when speeding around a curve on the streets but offroad it’s a problem.

    There are two ways to fix the problems of too little flex.  The first and best is to improve the suspension so that it is more flexible but as I just said this isn’t always possible on an Independent vehicle.  The second is to replace your open differential with a locker, with a locker the problem of lifting a wheel on the trail isn’t such a problem because the wheel on the ground will share the power being sent to the axle with the wheel in the air.  If the vehicle stuck in the gully had lockers (and the transfer case doesn’t have a differential) all 4 wheels will turn at the same rate and the two wheels with traction will be able to pull the vehicle out of the gully.

ABS

Pump powering lines from the Master Cylinder    The ABS is a complex system but the interaction that that the driver has with it is pretty simple.  When you need to brake just push the pedal, even in snow and ice, press firmly on the pedal and DON'T pump the brakes.  The ABS system will pump the brakes in and out much faster than you can.  There will be a vibration from the pedal and a rhythmic sound from under the hood but that's normal, just keep pressure on the pedal.

    The ABS system has sensors at each wheel that monitor to make sure none of the wheels lock up when braking, if one does the ABS will release the pressure to that wheel enough for it to start turning again then it tries to keep as much pressure on the brakes without locking up the wheel.  A pump under the hood applies the pulsing action and your foot provides the pressure, so if the ABS were disabled the brakes would act the same as any non-ABS system.

    Some 4-wheelers prefer non ABS systems because when the brakes lockup and slide on dirt they build up little piles of dirt in front of the tires that aid slowing the vehicle.  I think that I'd rather have the wheels on the verge of lockup but still turning getting the same benefit as on ice and snow.  Plus one of the big benefits with ABS is that even while locking up the brakes you maintain a bit of control, when normal brakes lockup turning the wheel won't change the direction of travel.  With the ABS when you start sliding while going around a curve you will keep turning although not as well.

Springs and Shock Dampeners

    The Xterra uses Leaf Springs in the Rear and Splined Torsion Bars in the Front to absorb shocks from the road.  It has four Shocks to dampen the bouncing motion caused by the springs.  The Shocks are actually Shock Dampeners although most people call them Shock Absorbers.  The springs are absorbing the shock, the dampeners just keep the motion from the springs from getting out of hand.  Without the Dampeners the Xterra would bob up and down like a boat on the ocean each time it goes over a bump.  In the rear the Dampeners are mounted to the axle so that one is angled forward and the other is angled back.  This prevents "Axle Wrap"; Axle Wrap is when so much power is sent to the axle that the axle rotates opposite the direction the wheels turn, a problem with all leaf spring vehicles, and more pronounced with "spring over" setups.  Luckily the Xterra has a "spring under" setup to lessen severe axle wrap and the dampeners absorb what little is left.


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