4-wheel Drive System

4-wheel Drive System Overview

A four-wheel drive (4WD) system distributes engine torque to all four wheels of a vehicle. Since it can meet increased demands on traction, it is advantageous for driving on slippery or unpaved roads, and lessens the risks of getting stuck in mud, sand, snow, etc. Therefore, this technology has been widely used in off-road vehicles.

In recent years, however, 4WD systems are becoming popular to be used in all kinds of passenger vehicles. They are considered as being instrumental in delivering a safe ride.

4WD Power Distribution

In two-wheel drive (2WD) vehicles, the total engine power is transferred to the road surface by two of the vehicle wheels, whereas in 4WD vehicles, the power is transferred by all four wheels. Consequently, each driving wheel in a 2WD vehicle handles 50% of the overall output while each driving wheel in a 4WD vehicle handles 25%.

These values are only valid, however, when the tires of the driving wheels are in firm contact with the road surface. When driving on slippery roads, the amount of power that can be transferred to the road surface may be smaller.

If, for example, we consider that the maximum amount of power that can be transferred to a slippery surface (per tire) is 30% of the engine output, a 2WD vehicle can transfer no more than 60% (30% x 2 tires) of the engine output. However, a 4WD vehicle can still transfer 100% of the engine power since each tire is only required to transfer 25% of the output.

For this reason, it is generally considered that a 4WD system can effectively transfer the engine power to the road surface. However, it cannot avoid all risks of wheel spin or skidding, and will not eliminate the need of snow tires, tire chains, etc.

Honda 4WD Systems

4WD systems employed by Honda can be divided into the following three types:

  • 4WD system with Dual Pump System (DPS)
  • Viscous coupling type 4WD system
  • Viscous coupling type 4WD system with ABS
A 4WD system provides the aforementioned benefits without doubt, however, there are drawbacks concerned with its implementation. Since its mechanism is more complicated, a 4WD vehicle will usually be heavier and have more mechanical loss, resulting in lower fuel efficiency. Also, there is a phenomenon known as tight-corner braking* which is unique to 4WD systems.

Honda 4WD systems avoid these shortcomings of the regular 4WD system to a good extent, first of all by operating in front-wheel drive (FWD) mode for most of the time. They automatically switch to 4WD mode only when the front wheels begin to spin. Engine power will be distributed to all four wheels only when more traction is needed.

*Tight-corner braking This phenomenon often occurs when a 4WD vehicle is attempting to negotiate a tight corner on a dry, paved road surface. Although the brakes are not being applied at this time, the vehicle speed drops, and in some cases, the engine may even stall. This happens when a wheel speed differential is not permitted to exist between front and rear wheels. 

4WD System with Dual Pump System (DPS)

Automatic switching to 4WD mode is accomplished by engaging a wet-type, multi-plate clutch located inside the vehicle’s rear differential. Two (front and rear) pumps that are also assembled inside the differential case control the clutch hydraulically.
DPS Schematic During 4WD Operation
  1. Multiple plate clutch
  2. Rear pump
  3. Relief valve
  4. Oil case
  5. Front pump
When front and rear wheel speeds are the same, the differential clutch assembly is not engaged and drive torque is not distributed to the rear wheels. The vehicle acts as a 2WD vehicle.

When the front wheels spin faster than the rear wheels by more than 2.5%, the front oil pump turns faster than the rear pump and a pressure will build up. This pressure forces the differential clutch assembly to engage. The engaged clutch then passes the drive torque to the rear wheels, producing 4WD.

It should be noted that the clutch will not engage when the rear wheels turn faster than the front wheels. Therefore, 4WD may only be engaged when starting or accelerating in forward or reverse direction, or decelerating in reverse direction. The vehicle will return to 2WD when coasting in forward or reverse direction, or decelerating in forward direction.

4WD will also be canceled by a thermo switch when the differential gets too hot. In the DPS, automatic transmission fluid (ATF) is used instead of differential fluid. To protect the system, the differential clutch assembly is lubricated by hydraulic pressure generated by the oil pumps in both 4WD and 2WD modes.

Viscous Coupling Type 4WD System

Construction

In the viscous coupling type 4WD System, a viscous coupling (known as the No. 2 propeller shaft) is fitted between the front (or No. 1) propeller shaft and the rear (No. 3) propeller shaft in the capacity of a center differential. The front propeller shaft is connected to the viscous coupling’s case. The rear propeller shaft is connected to the viscous coupling’s shaft (hub). The rear propeller shaft is also connected to the rear differential, which is in turn connected to the rear wheels via the left and right drive shafts.

Viscous Coupling  

The viscous coupling encapsulates two groups of thin metal plates known as outer plates (or case plates) and inner plates (or hub plates). The outer plates are fixed to and rotate together with the case. The inner plates are splined to the shaft. Both groups are sandwiched together and a high-viscosity, silicone oil fills the space between the plates.

  1. Case plate
  2. Case
  3. Silicone oil
  4. Hub
  5. Hub plate
The viscous coupling permits a certain amount of speed difference to exist between the front and rear propeller shafts, however, the viscosity of the silicone oil will offer resistance to limit the differential speed. One propeller shaft cannot rotate completely freely from the other. Thus, the viscous coupling functions as a limited-slip differential.

Operation of the Viscous Coupling Type 4WD System

In normal driving conditions, no difference in speed will exist between the front and rear wheels. The front and rear propeller shafts and the viscous coupling will all rotate together like a single rigid shaft, passively. No drive torque will be distributed to the rear wheels. The vehicle will operate in 2WD mode.

When the front wheels start to spin faster than the rear wheels, this will force the rear wheels to turn since the viscous coupling will limit the speed differential between front and rear wheels. In response to the speed difference, the viscous coupling begins to distribute drive torque to the rear wheels. As the differential speed rises, the silicone oil stiffens and the locking torque between the inner and outer plates increases.

If this large difference in speed were to continue, the accompanying rise in temperature would cause the silicone oil to expand. This would make the inner plates and outer plates adhere to each other (the hump phenomenon). As a result, the coupling becomes stiff as far as a rigid connection. However, this also makes the speed difference to zero so that the temperature of the oil drops and the plates move apart little by little.

In this way, the viscous coupling will switch the vehicle to 4WD mode automatically.

Viscous Coupling Type 4WD System with ABS

The viscous coupling type 4WD system for an ABS-equipped vehicle will always switch to or stay in 2WD mode when the ABS operates. Since the 4WD system tries to distribute equal torque to all four wheels while the ABS individually controls the deceleration of each wheel, vibration may occur if both systems operate together.

This type of 4WD system does not contain an independent center differential. Rather, the viscous coupling is incorporated in the rear differential case and operates both as a center differential and as a rear differential.

The viscous coupling unit has one group of outer plates (case plates) which is combined with two groups of inner plates (hub plates). One group of the inner plates is connected to the left drive shaft while the other group is connected to the right drive shaft.

  1. Case (carrier)
  2. Hub plate
  3. Hypoid ring gear
  4. Hub shaft (inner shaft)
  5. Dog clutch
  6. Hypoid drive pinion
  7. Case plate

This differential firstly operates as a limited-slip center differential. When there is a difference in the rotation speed between front and rear wheels, the viscous coupling will stiffen to limit the speed difference to increase. Drive torque will be transferred to the rear wheels through the viscous coupling. The vehicle will switch to 4WD mode.

In 4WD mode, the differential also operates as a limited-slip rear differential. Since each of the left and right drive shafts is fitted with its own independent set of inner plates, speed differences between left and right wheels can be accounted for and drive torque can be transferred smoothly.

The rear differential case is fitted with a dog clutch and a solenoid. The ABS control unit activates the solenoid to disengage the dog clutch when the ABS operates.

  1. Dog clutch
  2. Solenoid
  3. Shift rod
  4. Shift fork
  5. Sleeve

 4WD Temporary Disengagement

A 4WD system may present difficulties in some situations when towing the vehicle or performing inspection or maintenance. For this reason, vehicles equipped with a viscous coupling type 4WD system (with or without ABS) are also provided with a device which can be used to disengage the power transfer link to the rear wheels. In this condition, the vehicle will be similar to a front wheel drive vehicle. This temporary disengagement function will be useful in the following situations:
  • When a speedometer tester, a brake tester, or a chassis dynamo is being used.
  • When drive torque is directed to raised-up front wheels while the rear wheels are still in contact with the ground (ex. when an on-the-car wheel balancer is being used).
  • When the vehicle is being towed with one set of wheels (front or rear) mounted on a dolly and the other set of wheels in contact with the ground.
In any of these or other similar situations, failure to disengage the link to the rear wheels will result in possible operation of the 4WD system. Should this happen, the vehicle may suddenly move from its stable condition, resulting in a highly dangerous situation.

CAUTION: The 4WD system with Dual Pump System (DPS) is not provided with a disengagement function.

Viscous Coupling Type 4WD System

Use the 2WD/4WD select sleeve which is located inside the transfer to disengage the transfer driven gear and the transfer distance collar. Doing so will ensure that drive torque cannot be transferred beyond the propeller shaft.

CAUTION:

This procedure is to be performed with the engine off.

Loosen the lock bolt on the top of the transfer case and slide the 2WD/4WD select rod to the 2WD position. Then retighten the lock bolt to complete the switching procedure.

NOTE:
The vehicle must always be returned to the 4WD condition when service work has been completed. Also, the lock bolt must be securely tightened.

Viscous Coupling Type 4WD System with ABS

By securing the differential case’s dog clutch in the 2WD position, you will ensure that drive torque cannot be transferred beyond the rear differential.

CAUTION:
This procedure is to be performed with the engine off.

Loosen the lock bolt A located on the side of the differential case and move the plate C. Turn the (orange) selection bolt B so that its alignment marker is moved from position A4" to position A2". Then replace the plate C in its original position and re-tighten the lock bolt A.

Fixing torque: 1.4 - 1.9 kg-m

NOTE:
  • If the engine is started after the above disengagement procedures have been performed, the rear differential clutch warning lamp will turn on. This is normal, however, and does not indicate that a failure has occurred
  • The vehicle must always be returned to the 4WD condition when service work has been completed, and the lock bolt must be securely tightened. If the engine has been started in this condition, turn it off and remove the engine compartment’s ABS B2 fuse for a period of at least 3 seconds so that the flashing of the ABS control unit’s self-diagnosis lamp can be halted.

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