Four-Wheel Drive Systems

Welcome to four-wheel drive systems. The term four-wheel drive (4WD) is simply defined as, a vehicle that has the ability to transmit engine torque to all four wheels for improved traction. Today, there are several different types of four-wheel drive systems. This technology has evolved and will continue to do so. In the past these systems were strictly mechanical and very simple by today’s standards. Modern systems are more complex because of electronic controls but they do offer improved vehicle handling and driving convenience as compared to the past. 

There are several terms used for these systems: All-Wheel Drive (AWD), full-time four-wheel drive, part-time four-wheel drive (4WD), Auto four-wheel drive and On-demand four-wheel drive. All of these are various titles used to describe what we know as four-wheel drive.  Keep in mind that AWD – all wheel drive systems are a type of four-wheel drive. 

In the description below the AWD illustration, it states that power is sent to either the front or rear drive wheels (automatically) UNTIL the system detects a lack of traction at one wheel. When wheel slip occurs, it sends power to all four wheels. Additionally, with AWD the driver is not allowed to choose between 2WD and 4WD modes, the system is always in AWD with no option to select otherwise. In regard to 4WD the driver is able to choose between 2WD or 4WD mode. When in 4WD, equal power is sent to the front and rear axles all of the time.

Regardless of the system type or variation, all 4WD type systems have many things in common. For instance, they each have more than one driveshaft; there is typically a front and a rear shaft. In the case of front wheel drive platform vehicles there are two front axle shafts, one for each front wheel like any FWD car, and a drive line to power the rear wheels. 

Transfer Cases and Center Differentials

Something else that 4WD systems have in common is a gear box that divides power to the front and rear wheels. Four wheel drive vehicles use a gear box called a transfer- case, whereas an AWD vehicle’s gear box is known as a center differential.   

Shown below is a center differential. The difference between a transfer-case and a center differential is what happens at the CLUTCH PACK or the type of clutch that is used.  In the case of older mechanical systems, there was no clutch pack at all. They instead used a gear and synchronizer sleeve assembly to lock the front and rear drivelines together, much like shifting a manual transmission into a specific gear.  Today’s 4WD and AWD systems use some sort of clutch mechanism which has more to offer.

When it comes to wheels and tires with any type of 4WD system, all four-wheel assemblies must use the same wheel and tire size so that they will be the same in circumference. If all four wheels are not within ½” (13mm) of each other in circumference damage will occur to the transfer-cases’/center differential’s clutch/clutch mechanism.

As just stated, the difference between a 4WD and an AWD is the clutch mechanism. The clutch inside an AWD’s center differential never “locks” the front and rear drive line together like a part-time 4WD transfer case does. This allows the front and rear drive lines of an AWD system to rotate at different rotational speeds; this is necessary when encountering a turn when driven on hard surfaces. AWD vehicles are designed for and allow the vehicle to be driven on paved roads, all of the time and in most cases, there is no opportunity to select 2WD or any other mode.  The system is always in AWD.

With part-time 4WD, the transfer case’s clutch serves a different purpose. When 4WD is selected, it locks the front and rear drive shafts together so that they cannot rotate at different speeds. The reason for this is, four-wheel drive systems are for off-road use, meaning low traction surfaces: dirt, snow, sand, mud, etc. 

The image below portrays what an AWD center differential allows for during a turn; each wheel is able to rotate at a different speed. Here’s one way to look at this: the center differential allows the drive shafts to send power to the front and rear differentials at the speeds needed. Additionally, each of the differentials send power to each wheel so they can rotate at different speeds. This is necessary since AWD vehicles are driven on high traction surface; whereas 4WD systems are meant to be driven off road.  

4WD vehicles can experience driveline bind-up during a turn, when 4WD is engaged, when driven on paved surfaces because the front and rear drive-shafts are locked together rotating at the same speed. This binding conditon creates a jerking motion in the steering wheel and gets worse the sharper the wheels are turned. The reason for the jerking motion is in how U-joints on the drive lines operate.

AWD’s do not experience drive line binding because, as pointed out earlier, the center differential’s clutch allows the two drive shafts to rotate at different speeds during any degree of a turn. Power is sent to the front and rear wheels just like a Part-Time 4WD but the power gets sent in a less direct fashion since the clutch allows some slip.  The amount of slip that occurs is based on how sharp the turn is AND the traction that exists between the tires and the road surface. 

With Part-Time 4WD on the other hand, the clutch mechanism does not allow for slip when in 4WD mode. Many part time 4WD vehicles have an Auto 4WD option which basically allows the clutch mechanism to act like an AWD; allowing some slip to occur due to the as just explained .  

The distance each wheel travels in this turn is: Left Front = 27.5 ft Right Front = 21 ft Left Rear = 25 ft Right Rear = 19 ft

To understand more of how a transfer case operates watch 7 minutes of this video:  13:00 - 20:00

If this does not load, Youtube:  “Transfer Case Operation”

Transfer cases and center differentials are similar internally too. They both have gears and quite often both use a transfer chain – shown below. Some transfer cases are purely gear to gear type which is stronger but more costly to manufacture.  Car manufacturers decide what’s adequate and most feasible for their vehicle’s.

Early part time 4WD vehicles, from about the 1990’s and older, used a floor-mounted manual shift lever to engage and disengage the 4WD system. Since then most every system today has dash mounted buttons or a center consul knob that electronically controls the system. This has made changing from 2WD to 4WD mode very convenient.

When a 4WD button is selected it sends a signal to a 4WD control module which in turn operates an electric motor on the transfer case. The motor moves a shift fork in order to engage or to disengage four-wheel drive. A common phrase in the 4x4 world is, “shift on the fly” which simply means that with most every 4x4 system since the 1990’s the transfer case can be shifted from 2WD to 4WD HI mode while driving the vehicle.

WARNING: This is NOT the case when shifting into 4WD LO. When selecting LO range regardless of manufacturer you must be at a complete stop, preferably with the transmission in neutral.  The reason is you are making a significant vehicle speed change. The difference in speed ratio from HI to LO range is considerable. Typical transfer-case LO range ratios are about 2.7:1. This translates into much slower vehicle speeds as compared to the 1:1 transfer-case gear ratio used in HI. The advantage of LO is there is more torque available in order to “crawl” and maneuver up and down rough terrain. Again, the important thing to remember when selecting 4WD LO is, the vehicle must be at a complete stop or transfer case damage may occur.

The point of this video is to show that with Part time 4WD systems, when 4WD is selected TWO things need occur in order for power to reach the front and rear wheels.

1. The transfer case has to engage so that power gets sent to the front differential.
2. The differential also engages so that the power coming from the transfer case actually reaches the two front wheels.  

Youtube:  4WD Operation 2005 Tahoe

The front wheel bearing hubs on the system just viewed, 4WD Operation 2005 Tahoe, are permanently engaged hubs. These were utilized by a number of manufacturer’s for quite some time.  These wheel hubs are  splined to the axle shaft.  With these there is  no option to disconnect the shaft from the wheel hub as pictured here. Front wheel drive vehicles use permanently engaged hubs as well. This type of hub has one disadvantage when used on a 4x4 system, as you may have guessed, it adds to a wheel’s rolling resistance since the front wheel hubs are always attached the axle shaft from the differential regardless of mode: 2WD or 4WD. In the past permanent hubs were acceptable due to low fuel prices. Today, locking hubs have returned in order to decrease vehicle emissions and improve fuel economy. Think about it, by disconnecting the wheels from the front differential’s axle shafts the wheel will roll down the road with no added rolling resistance.

On older part-time 4WD vehicles you had to do two things in order to engage 4WD:

1. Manually shift the transfer-case into 4HI or 4LO and then get out of the vehicle and engage the front wheel hubs. This was the practice for many years and manual locking hubs were standard on most every part-time 4WD vehicle. Eliminating the need to get out and “turn the hubs” is one area that car manufacturers have practically eliminated. In the 1970s when fuel was inexpensive, some manufacturers made the first type of Full Time 4WD systems. These were convenient in that the front hubs were permanently engaged -you didn’t have to get out of the vehicle to engage the hubs.  But this early full time version proved to be unpopular since many vehicle owners had their vehicles converted to part time 4WD simply by installing locking hubs - to save fuel. As previously explained added wheel rolling resistance hurts fuel economy.

For a time in the 1980’s Ford came out with “Automatic Hubs”.  These hubs operate much like manual locking hubs except they engaged, automatically, when transfer case power was sent to the front axle shafts. These automatic hubs had a history of problems. One problem with these Automatic Hubs was they engaged only when you were driving in a forward direction. The problem was with reverse; in reverse the hubs disengaged. Once again some people responded by installing traditional manual locking hubs so that they had 4WD in reverse as well as forward.

This is an image of a 1983-1997 Ford Ranger upgrading from Automatic to  manual hubs. This was a common and simple up grade. For this application, WARN manual hubs were both an economical and a more durable solution.

Around 2004 Ford came out with another automatic hub system called Four Wheel Drive Disconnect also know as ESOF - Electronic Shift On the Fly. This video explains the benefits of these vacuum operated hubs. As with most any type of innovation these came about for convenience and fuel efficiency.

ESOF hubs are vacuum operated, and they do have some maintenance concerns. One problem that occurs with the ESOF hubs is that the vacuum hoses that operate the locking collar can leak. When such a vacuum leak occurs, the system will default to always being engaged.

These two videos explain how to service these vacuum operated hubs when they fail.

The “DIY how to test 4WD … actuators” is 5 minutes in length and contains information that is on this chapter’s quiz. The “…Clicking Humming… hub replacement” video is 22 minutes in length and is a system repair video but is more for your awareness and could be a reference if you ever needed to service these hubs. 

As discussed earlier, when in 4WD mode on a part time 4WD system two things must happen: (1) a transfer case motor (encoder motor) moves a sleeve OR a clutch is engaged so that power gets sent to the front differential and (2) the actuator motor – electric or vacuum operated - on the differential moves a fork in order to couple the axle shafts to the differential carrier so that power from the transfer case gets sent out to the wheels. If either of these motors fail 4WD mode will not occur.

These TWO videos demonstrate three types of differential actuators found on many part- time 4WD systems, enjoy.

Power Train Configuration

Lets explain more of what powertrain configuration or what vehicle platform is. Vehicle platform refers to a vehicle’s drivetrain structure. For example, most mini-vans and small economy cars are front wheel drive platforms. Even when a vehicle is an AWD, if the vehicle uses a transaxle and is a front wheel drive, 2WD type vehicle it is a FWD platform. When a FWD platform vehicle is an AWD, as pointed out earlier, the key component is the Center Differential. However, some AWD vehicles use a lighter duty type center differential known as a PTU (Power Takeoff Unit).  A PTU accomplishes the same thing a center differential does only it is simpler, lighter duty and more economical to manufacture. Additionally like most center differentials a PTU does not offer 2WD mode.

The other platform type is RWD or rear wheel drive. Most full-sized pick-ups and SUVs are RWD platform. The transmissions in these vehicles are of longitudinal design where the transfer case mounts at the aft end of the transmission. Transfer cases are larger, offer more torque capacity and are a bit complex in that they usually offer 2WD, Auto 4WD, 4WD HI and 4WD LO modes. 

This image is of a PTU - power transfer unit. This particular unit bolts onto a transaxle with 5 bolts. The rear driveline attaches to the companion flange. PTU’s usually contain their own lubricating oil, separate from the transmission just like many center differentials and transfer cases. However, on some AWD vehicles the transmission and center differential are combined into one housing.  Subaru is one manufacturer that does this.     One more thing to state about vehicle platforms is two other types do exist but they are not nearly as common. These platforms are rear-engine RWD and mid-engine RWD. The 2020 Corvette is a mid-engine platform vehicle. This is because the engine sits directly behind the passenger compartment.

Power Splits

Something to point out with AWD is how engine power is split between the front and rear axle.  Many are biased at a  80 / 20 split, meaning 80% to the front wheels and 20% to the rear wheels, UNTIL one of the wheels slips due to a loss in traction. This is just one example since these split percentages vary by system and manufacturer. 

It is when wheel slip occurs that an electric motor applies pressure to the AWD clutch mechanism until a 50 / 50 split occurs. This is a common split design which ensures that equal traction is sent to each axle. However, some systems are more sophisticated and can send up to 60% to which ever axle the system detects has the best traction.

In regard to part-time 4WD systems, quite often a multi-disc type clutch is used. When in 4WD mode high force is applied to the clutch plates as compared to Auto 4WD mode. In Auto 4WD, a smaller amount of force is applied to the clutch plates until there a certain amount of wheel slips takes place. It is at this point in time that additional clutch pressure is applied in order to eliminate wheel slip.  Auto 4WD  works “on demand” like an AWD system; which is to say it operates and varies clutch pressure as the system detects wheel slip conditions. This is a great option when driving on patchy/ice, paved roads.

Below is a cut-away view of a multi-disc clutch. This one has a magnetic coil right next to the clutch plates. This is the kind of clutch mechanism that is at the heart of what some refer to as an “intelligent AWD” system. It has the capability to apply & release various amounts of pressure on the plates quickly in order to divide power more precisely as compared to older type clutch mechanisms. 

Below is a description by Toyota about such a system. This phrase is worth studying in order to get an idea of the kind of refining that is occurring in AWD and 4WD systems today. Pay attention to some key words in order to understand how this is actually a combination of AWD and 4WD. Toyota calls this system Full-Time Multi-Mode. 

To quote Toyota:

Another common power split on a number of FWD platform vehicles is something close to an 88/12,  88% to the front wheels and 12% to the rear. As pointed out earlier these are not the only split values used among manufacturers but this should help you understand that an AWD system is BIASED which means a certain amount of torque is sent to each axle during normal or none slip conditions.  But when a slip event occurs, the AWD clutch begins the process of dividing torque.

AWD power split during wheel slip is a dynamic process, meaning a lot of things work together to maintain vehicle stability and traction control. When an AWD system goes into power split mode signals from many sensors such as the wheel speed sensors, the MAF mass air flow and Yaw rate sensors all come into play which enables the system to act, which often results in the transmission upshifting to a higher gear, the brakes being applied to a wheel with little traction, engine throttle opening is reduced and ignition timing is retarded, etc. This way power is sent to the wheels with best traction plus engine torque to the drive wheels is reduced in order to prevent wheel slip. 

When a system “acts” during a wheel slip event, power is divided only to the point that is needed. As you know by now, the multi-disc clutch will only send what is needed based upon programmed calculations and readings from input speed sensors for the particular event. It truly is an active, dynamic event.

Note that this rear differential is more sophisticated than what has been explained thus far. This power divider has a multi-disc clutch for each axle shaft/wheel.

This video demonstrates a 4WD system that BMW has developed. It explains in detail some differences between AWD and 4WD.  Something else to consider with modern AWD systems is, they are part of a vehicle’s stability control system which includes the anti-lock brakes and traction control systems as well. These systems are continually being improved and refined each year; all of which improves the driving experience.    

Youtube: “AWD vs 4WD - What's The Difference?” …If this video doesn’t load

Here is an example of an older style floor mounted 4x4 shift selector. It is old school but today’s 4WD systems accomplish basically the same thing when 4WD mode is selected, which is engine power is split to each of the differentials. 

Here are just TWO of the many variants of 4x4 systems that have been developed by Jeep, study these.

Lets share a bit more about transfer cases. The images below are of a New Process transfer-case or T-case. New Process was a company that made T-cases for Chevrolet, GMC, Jeep, and Dodge. These typically have an identification tag located on the back side of the case. When replacing a transfer-case knowing the model is critical. The numbers on the left tag indicate that this is a Model 242, the ‘D’ means its for a Dodge. If this letter is a ‘J’ the application would be for a Jeep, if it happens to be a ‘C’ it indicates Chevrolet, etc.

The image on the right is of a MP 3023 or Magna Powertrain.  Magna Power purchased New Process/New Venture Gear some years ago.   

Both ID tags indicate that the gear ratio is 2.72:1 when in LO range – see the bottom numbers. There are no numbers to indicate HI range because in HI range the ratio is 1:1.

Some New Process history may be of interest. In 2011 due to newer innovations with 4x4 technology New Process was bought out by New Venture who was later purchased by Magna Powertrain. If you want to know all the various models that were made by New Process go to Wikipedia, search ‘New Venture Gear.’ - LINK  Here you will find a list of all the transfer cases they  made and the type of coupling device inside each model.  This is helpful information for off-road enthusiasts.

The next few slides explain transfer-case operation for units that have a sleeve and synchronizer assembly, not a clutch assembly. This slide and the next two slides demonstrate how the RANGE and MODE forks work. Two terms that you need to understand are RANGE and MODE. Range is simply high speed vs. low speed - when HI or LO range are selected.  Mode however refers to 2WD vs. 4WD - when 2WD or 4WD are  selected. The transfer case in this diagram is in 2WD mode, high range. The top RED ARROW is pointing to a mode synchronizer assembly shift sleeve. This sleeve is moved by a shifting fork. Depending on the vintage of the vehicle this fork is moved manually (floor mount shift lever) or it is moved by an electric motor – when you push a button. Click to the next slide and then click back to this slide. Watch the position change that occurs at the Mode Synchronizer Assembly.

When the sleeve is moved towards the rear (right) it engages the drive sprocket to the rear output shaft. That’s really all it does. When this sprocket is engaged or locked to the rear output shaft, the chain will now send power to the Front Output Shaft, which is connected to the front differential’s driveshaft.   Click to the previous slide if you wish to observe how the mode change occurs. In HI range, power from the transmission passes straight through to the rear output shaft at a 1:1 ratio. The chain will send the same speed and power to the front output shaft since both drive sprockets (the top drive sprocket and the lower drive sprocket) are the same in tooth count.  This is 4WD mode, high range.

This diagram portrays 4WD mode in LO range. Click back to the last slide and then back to this slide watching the Range Clutch’s position change. The Range Clutch is located next to the Mode Synchronizer Assembly. The Range Clutch slides in order to either couple or uncouple the blue input gear and the (yellow) rear output shaft.  When the Range Clutch is uncoupled, the Helical Planetary Assembly changes the ratio of the output to 2.72:1. It may be hard to visualize what these planetary gears do but this is what changes the output ratio. The planet carrier gears rotate with the rear output shaft in low range. In high range the input and output shafts rotate at 1:1 ratio because they are coupled together.

As far as a multi-plate disc clutch vs. a gear system that uses a synchronizer assembly the results are the same. The advantage of a multi-disc clutch system is it allows for 2WD,  Auto 4WD or 4WD modes with fewer parts. This is because the clutch plates can be free (no application), applied partially or applied with full force. When the system is in Auto 4WD a certain amount of slip is allowed to occur just like an AWD system while cornering.

In 4WD mode the clutch plates are applied at full force which locks the front and rear drive lines together, this is a no slip clutch mode. An electric motor, encoder motor, is what controls the amount of pressure  applied to the multi-disc clutch plates.

To rephrase, a multi-disc clutch can have no application, be partially or fully applied. This is a great design in that the transfer case can offer 2WD, Auto 4WD and 4WD modes of operation.  A popular transfer case model in the past, used by Dodge, General Motors and Jeep in the1990’s until 2011, is the New Process 246 as shown in the movie on the next slide.    

A multi-disc clutch is a great design in that the transfer case can offer 2WD, Auto 4WD and 4WD modes of operation with one component. The New Process 246 as shown in this movie below.      

Youtube: Auto 4WD vs 4WD

Haldex Unit

Another type of 4WD system is by Haldex. The Haldex unit is part of the rear differential or drive axle assembly. It is a multi-clutch disc system that mounts to the front of a differential where an electric motor varies the pressure applied to clutch plates depending on the mode that is selected. These are found on Ford Explorers and other SUV’s.

Based upon sensor input, such as wheel speed and engine torque load (MAP sensor), the system can deliver various amounts of torque to the rear wheels by applying the clutches as needed in order to control torque to the wheels. This way a correct torque amount can be applied as needed under current driving conditions. During a wheel slip condition high pressure is delivered to the clutch pack(s) to deliver more of an equal torque to both wheels. When driving straight at high vehicle speeds or when performing tight maneuvers less pressure is needed at the clutch plates so that the wheels are for the most part free in order to allow the wheels to rotate at their own independent speed during such conditions.

Youtube: Haldex Gen V all-wheel drive system

Diagnosis Steps

Repairing a 4WD system malfunction begins by following a systematic approach.  You should recognize this 7 Step approach.  It will lead you to success IF you follow it. 

1. Verify the problem
   1. Try to duplicate and visit with the customer in order get a complete idea of the problem.
2. Visual inspection
   1. Check fluid levels, inspect connectors, linkages, etc.
   2. Check for DTC’s -  diagnostic trouble codes
3. Gather Data
   1. Look for TSB’s – technical service bulletins
   2. Research all available diagnostic data
4. Develop a diagnostic plan
   1. Create a list of tests that will help you determine the problem
5. Testing
   1. Perform tests to determine the cause
6. Repair and determine root cause
7. Verify the repair – test drive under the conditions that the problem occurred

Diagnostic Story

When diagnosing a 4WD system fault, break the system down into sub sections. There are two: the transfer case and the front differential, which includes the wheel hubs. For example the first thing to ask is does the transfer case engage when 4WD is selected?  If it does, the next question should be: is the problem with the differential or the wheel hubs?  Lets examine the a complaint:  “No 4WD operation”.  Lets apply the 7 Steps.

1. Verify the complaint: The technician talked with the customer, test drove the vehicle and verified that the 4WD system did not work (no jerky steering wheel while turning on paved surface).  To further understand the fault, the service technician put the vehicle on a lift, with all four wheels in the air and with the engine running and tried to engage the 4WD system. In doing it was determined that the drive line from the transfer-case to the front differential was not rotating; the transfer case failed to engage. In other words it wasn’t sending power to the front differential.

2. Visual inspection: The technician decided to perform a visual inspection:

1. He looked to see if there was a flashing 4x4 light or LED on the dash, there were no lights illuminated. *Look for clues like these early in the diagnostic process.
2. He then operated the 2WD and 4WD buttons, ‘listening’ to see if any actions could be heard in the transfer case. By listing we can often get an idea as to whether or not a transfer case motor works. In this instance it did work or was trying to.
3. A check for DTC’s was done, and a C0327 was found. He then looked up the criteria that sets this code on a professional information resource – Mitchell’s ProDemand. This is a resource that many repair facilities subscribe to. The point here is, the criteria that sets this code could be studied. In this case a DTC sets when the encoder motor sensor cannot determine mode position. Whenever this occurs it prevents the motor from engaging the transfer case. The motor will operate and will try a to engage a few times but then quite and go to 2WD because the system can’t determine which mode it is in.

* The first TWO diagnostic steps have now been performed. 

3. Gather data & study it. What has been discovered thus far? The DTC that was found sets whenever the encoder motor’s position sensor fails. A YouTube video was found about this DTC. You only need to view the first 5 minutes to get an idea as to how to fix the problem.

4. Develop a plan: At this point the technician determined since the transfer case would not engage and that a DTC was found it was time to change this sensor.

5. Testing: with this story a determination had already been made. No further testing is required.

6. Repair and determine root cause: The sensor was replaced. The video shows how to replace this.

7. Verify the repair.  The transfer case was able to engage into 4WD mode once the position sensor was replaced.

Some other diagnostic insights that are common to 4WD systems are the following:

1. Noise and vibrations: these are often drive-line or U-joint related issues. Inspect the U-joints and the driveline splines. You may wish to check driveline run-out as shown in the video below.
2. Other noises: To find a 4WD problem you may have to drive the vehicle in both 2WD and 4WD modes to see if this has any impact on the noise. At times, technicians have been known to remove one of the drive lines in order to see if this will impact the noise.

TIP: After installing a new transfer case on some GM units, like the 246GM, drive the vehicle 15 miles in Auto 4WD before trying to engage 4LO. The system needs to ‘learn’ before 4LO is ever engaged. 

Don’t forget the basics when diagnosing a 4WD or an AWD issue. Be certain to inspect tire inflation as well as tread condition.  Unequal tire inflation can cause 4WD engagement issues.

Youtube: “Drive line Run Out - bad U-joint”

4x4 hubs

Something to share about vehicles with manual locking hubs 4x4 wheel hubs is, first, it only takes one of the two manual locking hubs to fail in order to render the 4WD system inoperative. There is no such thing as 3WD meaning, both hubs need to engage in order for power to reach either of the two wheels. When dealing with a problem where the transfer case does send power to the front differential but neither of the front wheels operate, inspect BOTH locking hubs. 

These two manual hubs from 2002 Ford F-250. During inspection it was discovered that one of the hubs (left) was broken.  A new set of hubs was installed and the problem was fixed.

To verify whether a wheel hub can engage, (engine off) raise the front wheels and try both positions “free” and “lock” while spinning each wheel individually by hand in order to verify that the axle shaft from the differential can lock to the wheel hub assembly. The video below shows how this is done.

Whenever working on a vehicle with manual locking hubs performing this test is a great place to start when diagnosing a “NO 4x4 operation” system fault.  

Youtube: Ford Ranger 4x4 hub engagement test

1. A weird problem that occurs a times with 4WD systems is when a service technician checks transfer-case fluid level only to find that it is TOO FULL of oil.  Too full means that when the level plug is removed a lot of oil pours out, not just a small stream or a few drops. This is a fairly common scenario. The real cause is the oil seal located between the transfer-case and the transmission.  When this seal fails, itallows transmission fluid to be pulled into the transfer-case from the transmission; thus the transmission becomes low and the T-case comes to be over-filled with fluid. The fix is to replace the front seal on the transfer-case.
2. Here’s a tip about RTV – room temperature vulcanizing – silicone. They are NOT all the same. Specific RTV silicones are made and should be used. Otherwise leaks can occur in the future. For example, please use silicone that is designed for automatic transmission fluid -ATF -when silicone is used to seal a transfer-case housing that uses ATF lubricant. (RED package) If gear oil is used then use a silicone that is compatible with gear oils such as 75W-90 or 80W-90. (GREEN)
    

What are viscous couplers? In the past they were quite common in AWD center differentials. They are comprised of two different sets of thin – alternating - steel disks inside a housing that is filled with a silicone based oil.  Viscous couplers are pretty amazing in how they operate; one set of alternating discs connect to the front drive line and the other set of plates connect to the rear drive line. The advantage of a viscous coupler is that it operates mechanically and automatically. They were developed for AWD systems and do not need an electrical ON / OFF switch. A viscous coupler is what divides engine torque to both the front and rear differentials. They  allow the two drive lines to rotate at different speeds, as needed, when the vehicle encounters a turn.  

Viscous couplers work on friction, heat and expansion principles. What happens is the fluid inside the sealed viscous coupler, when cool, it is low in viscosity which allows the plates slide past each other with little resistance. When one of the four wheels experiences too much of a wheel slip condition it causes either the front or rear drive-line to spin much faster than the other until the clutch engages. What happens is, the oil inside the viscous clutch gets hot, quickly which thickens and expands causing the plates to press tighter against each other. This causes an increase in resistance between the plates. The result is nearly equal power gets sent to each of the drive axles. Nearly means this clutch can still allow the front and rear drive lines rotate at different speeds during a turn. This is due to the traction that exists between the tires and the road.  This traction impacts the viscous clutch in order to force the clutch to allow some slip.

As with many things on a vehicle, when too much slip occurs excessive heat is created which, after some time, can destroy the viscous coupler. One result is where there is so much heat that the alternating plates weld themselves together causing the front and rear drive shafts to lock up together – forcing them to rotate at the same speed like 4WD mode.

To protect an AWD’s viscous coupler from this problem, the rule mentioned earlier in this chapter that tires / wheels assemblies must be much the same in diameter and circumference.  This way the front and rear drivelines will operate more similar in speed which will keep the viscous clutch operating at acceptable temperatures.  

The most common reason that a viscous coupler gets destroyed is when at least one tire is different in circumference as compared to the other wheels.    

Keep the following in mind when using a temporary spare tire. Here is a typical vehicle manufacturer recommendation when using a temporary spare tire: Vehicle speeds should not  exceed 50 or 60 MPH and this wheel should not to be used for more than 200 miles. Using a temporary type spare tire beyond 200 miles can cause viscous clutch damage. The same holds true for today’s AWD multi-disc clutches.

When the viscous clutch plates weld together due to excessive heat damage it will cause the big transfer chain in the center differential to “jump” or skip past drive sprocket teeth during a turn. The symptom is: the vehicle will drive just fine in a straight line but during a turn a very noticeable thumping noise is heard coming from the center differential area.

Another way viscous couplers get destroyed is when the vehicle gets stuck in snow or mud and the driver spins and spins the wheels for several minutes trying to get out of the situation. Making the wheels spin and spin excessively creates excessive heat causing the viscous clutch plates to weld to each other.

Here’s some insight as to how sophisticated some AWD systems have become.  Lets describe  Ford’s Intelligent AWD. This AWD system is an innovation that many vehicle manufacturers have been implementing for some time. Ford also refers to this as their Terrain Management System. This system gives the consumer more options based on traction conditions, something that has not been offered in the past. View the image below since it will be referred in the video on the next slide – enjoy. 

Youtube: Terrain Management System /Ford How-to / Ford

Some concluding statements about 4WD and AWD systems. Four wheel drive system components do add weight and purchase cost to a vehicle as compared to 2WD vehicles.  That being said, 4WD and AWD vehicles have gained a lot of popularity due to the fact that consumers want driving stability and safety.  One example of this trend is that in the past Toyota’s number one selling vehicle was the Camry. Today, Toyota sells far more Rav 4’s than it does Camrys. Consumers want AWD vehicles due to the benefits of improved vehicle traction and stability, especially during winter months.   

These systems are being refined more every year. Four wheel drive systems enable consumers to travel with more confidence and with more safety than ever before. Though these systems do require additional maintenance many feel that these added costs are very worth while.

 Review Questions for Practice:

AWD on many vehicles cannot be disengaged, no buttons or levers to select.

True

False

What is the difference between a transfer case and a center differential?

A transfer case locks the front and rear driveshafts together. A center differential allows the front and rear driveshafts to slip when rounding a corner.

Why do all four wheels on an AWD or 4WD need to be at or near the same in circumference?

To protect the transfer case/center differential's clutch mechanism.

What type of vehicle platform is a full-size Toyota pickup?

Option 1

Option 2

Option 3