The LSD is a mechanism that limits the differential when one of the wheels begins to slip and generates an appropriate drive force in the other wheel to allow the vehicle to drive smoothly. There are various kinds of LSD.

1. Viscous coupling LSD

A viscous coupling is a kind of fluid clutch which transmits torque by means of the viscous resistance to oil. It uses this viscous resistance to limit differential slip. The viscous-coupling LSD is used as the differential limiting mechanism in the center differential of 4WD vehicles, and some viscous coupling LSDs are used in FF and FR model differentials.

2. Helical gear type torque-sensing LSD

The limited slip is accomplished primarily by the friction that is generated between the planet gear’s tooth tips and the differential case’s inner wall, and the friction that is generated between the side gear end face and the thrust washer. The principle of limited slip enables the resultant reaction force F1 (which is created by the meshing reaction of the planet gear and the side gear and the meshing reaction of the planet gears themselves) to push the planet gear in the direction of the differential case in proportion to the input torque. Due to the reaction force F1, the friction force F1 (which is generated between the tooth tip of the planet gear and inner wall of the differential case) acts in the direction to stop the planet gear’s rotation. (2/4)

3. Torque-sensing LSD

The differential limiting force is generated by both the tooth flank friction between the side gears and worm wheels and the thrust friction between the differential case, thrust washers and side gears. In this type of torque-sensing LSD, the differential limiting force changes greatly and vary rapidly in accordance with the torque applied to it. Accordingly if the accelerator pedal is released during cornering, the differential operates smoothly just like a normal differential. However, in cases where higher torque is applied, more differential limiting force is delivered.

4. Multiple-disc type

A barrel-shaped compression spring is fitted between the left and right side gears to keep the thrust washers pressed up against the clutch plate via the retainers and side gears. Therefore, the friction generated between the clutch plate and thrust washer limits the differential. HINT: The special LSD oil is used for multiple-disc type LSDs.

Center Differential and Transfer

1. Transfer

The location of the center differential is different for the viscous coupling type and the mechanical locking type. The center differential is separated from the front differential. The front differential is located inside the ring gear mounting case. It can rotate freely inside the case. Also, the hydraulic multi-plate clutch, which is used to limit the differential action of the center differential, is housed in this case. There is no switching button in newer models, and differential limitation is controlled automatically at all times. Because hydraulic pressure is controlled by the Engine & ECT ECU.

2. Power transmission

(1) During straight-ahead driving

The power is divided in two at the center differential. One part of the power is transmitted to the rear differential via the center differential right side gear. The other part of the power is transmitted to the front drive shafts via the front differential.

(2) During cornering

When a difference in the rotational speed of the front and rear wheels arises, the center differential operates, creating a difference in rotational speed between the ring gear mounting and front differential case. When this occurs, hydraulic pressure is applied to the hydraulic multi-plate clutch, with fluid, which therefore operates to reduce this difference. The friction generated to reduce this rotational difference differs depending on the vehicle driving conditions and optimum driving force always distributed to the front differential and rear differential. (2/4)

3. Center differential

(1) Construction The center differential gear is the bevel gear type and the center differential limit control mechanism is the hydraulic multi-plate type. This type of center differential uses a hydraulic multiplate clutch as the differential limiting mechanism. The hydraulic multi-plate clutch consists of the inner side clutch discs, the outer side clutch plates and pushing pistons. The clutch is located beside the front differential.

(2) Principle of operation When hydraulic pressure is applied to the pistons, the clutch discs and plates are pushed tightly together, limiting the difference in rotational speed between the ring gear mounting case on the clutch plate side and the front differential case on the clutch disc side. If the front wheels get stuck in a mudhole and spin, the differential action of the center differential is limited by the clutch, so power is transmitted to the rear wheels, making it easy for the vehicle to get out of the mudhole. During normal driving in the “D” range, since the differential limit force is weak, driving is smooth when cornering at low speeds or when parking, etc., and the tight corner braking phenomenon does not occur. In the “L” and “R” ranges, since a higher differential limiting force is obtained the power obtained at the tires is effective in freeing the vehicle when it gets stuck.

(3) Features Excellent control of the differential action limit force is achieved in accordance with the automatic shift lever position, the throttle opening angle, and the vehicle’s speed. Newer models, a computer controls the differential limiting force in accordance with vehicle speed and with differences in front and rear wheel rotational speed, to ensure smooth driving under any conditions, whether on muddy roads, when cornering or when traveling straight ahead.

HINT: Older models had a center differential control switch. When driving in the “Off” mode, the condition is the same as when the mechanical locking type is run with the differential lock switch OFF.

Center Differential and Transfer

1. Transfer

The layout of the center differential, front differential, transfer drive gear and transfer driven gear is almost the same as with the center differential mechanical locking type, but viscous coupling is built into the transfer block to limit the differential action of the center differential. The center differential assembly consists mainly of the center differential right case, center differential intermediate case, center differential left case, center differential pinion gears, and center differential right side gear.

2. Power transmission

(1) During straight-ahead driving The transmission of engine power from the center differential drive gear to the center differential right and left side gears is the same as that in the center differential mechanical locking type. The power from the center differential right side gear is transmitted via the transaxle mode select sleeve to the transfer drive gear, transfer driven gear and rear propeller shaft. It is also transmitted to the viscous coupling outer plate. Since the No.2 intermediate shaft is inserted into the center differential left side gear, power is also transmitted to the viscous coupling inner plate.

(2) During cornering Since the viscous coupling acts to reduce the difference in the number of revolutions between the center differential right and left side gears, the differential action of the center differential is limited. The differential limiting function is not so strong as to obstruct smooth turning. (2/4)

3. Center differential (viscous coupling)

(1) Construction The center differential gear is the bevel gear type and the center differential limit control mechanism is the viscous coupling type. The viscous coupling is built into the transfer ring gear mounting. The inner plates of the viscous coupling are joined to the center differential left side gear and the front differential case, while the outer plates of the viscous coupling are joined to the ring gear mount case and the center differential right side gear. Since the viscous coupling is built into the transfer ring gear mounting case the construction has been changed from the three-axis center differential shaft, based on the mechanical locking transaxle, to the four-axis type. A viscous coupling is a kind of fluid clutch which transmits torque by the viscous resistance of oil. It uses this viscous resistance to control the operation of the center differential, creating an effect in the center differential like that created by an LSD (Limited Slip Differential).

(2) Operation The center differential with viscous coupling operates when rotational speed differences between the inner plates (front) and outer plates (rear) of the viscous coupling occur, transmitting power and limiting the differential action of the center differential. Torque is transmitted by the viscous resistance of the silicone oil.

<1>When plates rotate at identical speeds No viscous resistance is generated.

<2>When plates rotate at different speeds When the plates begin to turn at different speeds, the silicone oil particles are pulled away from each other and a resistance is generated. This resistance reduces the difference in speed between the two plates.

<3>Transmitted torque characteristics

During normal operation The amount of viscous resistance due to differences in the rotational speeds of the outer and inner plates increases or decreases in proportion to the extent of the difference in speed.

During humping In the viscous coupling continues to operate with the outer and inner plates rotating at large different speeds, the temperature and internal pressure inside the viscous coupling increases. The inner plate, which can move in an axial direction, is pulled toward the side where pressure is lower. As a result, the inner plate contacts the outer plate directly to create a large resistance. This condition is called humping. Since there exists no speed difference between the inner and outer plates during humping, the temperature and internal pressure inside the viscous coupling decreases. The compressed bubbles expand again and separate the inner plate from the outer plate.

Center Differential and Transfer

1. Transfer

This transfer is a two-speed (Low- High) type. This transfer is a compact and lightweight. Its main feature is the adoption of the torque sensing type center differential with mechanical locking type. In this transfer, a planetary gear train is used in the reduction mechanism and a silent chain is used to reduce noise for the front drive. The method for switching the modes of this transfer involves the operation of a 4WD control switch to actuate the two shift motors. Thus, the transfer shift lever has been discontinued and the ease of operation has been improved.

Transfer shift actuator

The transfer shift actuator, which contains “H”-”L” shift motor limit switch, center differential lock shift motor, “H”-”L” shift motor limit switch, center differential lock shift motor limit switch and center differential lock detection switch, cannot be disassembled. (1/3)

2. Power transmission

(1) “H” position The splines at the rear of the transfer input shaft mesh with the internal gear teeth of the high and low clutch sleeve. Also, the high and low clutch sleeve is meshed to the differential case. Thus, the rotation of the input shaft is transmitted to the high and low clutch sleeve, the differential case.

(2) “L” position The external teeth of the high and low clutch sleeve are meshed with the planetary spline piece, and the planetary ring gear is fixed. Thus, the rotation of the input shaft is transmitted in a reduce from to the planetary sun gear, planetary pinion gear, planetary pinion gear shaft, planetary carrier, planetary spline piece, high and low clutch sleeve, high and low clutch hub, and differential case.

3. Center differential

(1) Construction Torque sensing type center differential with mechanical locking type consists of a differential case, coupling, ring gear, 8 pinion gears, sun gear, and carrier.

(2) Principle of operation 1. Normal driving (front wheel speed = rear wheel speed) The driving force that is input by the differential case is transmitted by front: 40, rear: 60. When a rotational difference exists between front wheel and rear wheel, the torque distribution of the driving force (torque) that is input by differential case changes instantly by the pinion gear and clutch plate operation before the torque is transmitted. 2. Front wheel skid driving (front wheel speed > rear wheel speed) The center differential distributes more torque to the rear wheels.

4. Rear wheel skid driving (front wheel speed = rear wheel speed)

The center differential distributes more torque to the rear wheels.

Center differential lock mechanism

When the center differential lock switch is ON, the center differential lock shift motor within the transfer shift actuator operates. This slides the center differential lock shift fork and locks the center differential.

Center Differential and Transfer

1. Transfer

This transfer is a two-speed (Low- High) type. The center differential is located inside the low speed output gear of the output shaft. Power is transmitted to the center differential from the transmission via the input shaft, the high or low speed idler gear and the high or low speed output gear.

2. Power transmission

Low-speed Mode

3. Center differential

The center differential gear is the bevel gear type and the center differential limit control mechanism is the mechanical locking type. The center differential gear unit consists of two pinion gears. During straight-ahead driving, when there is no speed differential between front and rear wheels, the center differential pinion gears do not revolve. When a speed difference between the front and rear wheels occurs due to cornering, etc., the center differential pinion gears revolve, absorbing the speed difference.

4. Shift fork slide system

A shift fork slide system which uses a shift fork shaft to switch between the “L4″ range and the “H4″ range, and to carry out locking of the center differential, is used. Switching between low and high speeds is accomplished by the transfer lever, while locking of the center differential is accomplished electrically by a motor driven transfer shift actuator.

5.Transfer shift actuator

Drives motor and changes driving mode (“Free” or “Locked”) in accordance with signals from 4WD control relay.

<1>Motor control limit switch When the contact plate slides under the contact springs while turning with the rotation of the driven gear, the electrical connection between it and the contact springs changes, causing the motor to stop in the optimal position at all times.

<2>Spiral springs If the operating resistance of the front drive shift fork shaft is large, the rotational force of the motor is partially “stored” in these springs. Afterward, when the operating resistance is reduced, the spring force causes the front drive fork shaft to slide.

Center Differential Locking System

1. General

The center differential locking system mechanically locks the center differential, forcing power to be transmitted evenly to front and rear differentials. If any one of the wheels is stuck, the other three wheels also spin. Therefore, locking the center differential distributes power it equally to all four wheels and makes it possible to get out.

2. Components

The system consists of the following parts.

(1)Center differential lock indicator light Indicates condition (free or locked) of center differential to driver

(2)Center differential lock switch Switches center differential control mode between “Lock” and “Free”.

(3)Center differential lock indicator switch Detects that center differential is locked.

(4)”L4″ position switch Detects transfer shift lever position (“H” or “L”)

(5)Center differential lock control relay Locks or unlocks center differential depending on signals from various sensors.

(6)Transfer shift actuator (1/2) 1. Features of range switching mechanism The chart on the left shows in which conditions each position of the transfer shift lever and differential lock switch are used.

HINT:

When the transfer shift lever is in the “L” range position, the L4 position switch is on, so center differential lock control relay operates regardless of whether the differential lock switch is on or off, and the motor turns, causing the center differential to be locked.

Types of Transfer

1.FR ear differential.

The center differential and front differential are contained within the transfer.

2. FR based full-time 4WD

In this type, the power is transmitted from the transmission to the center differential and to the front differential and rear differential.

3. FR based part-time 4WD

In this type, when the transfer is not connected, the power is transmitted from the transmission to the rear differential. When the transfer is connected, the power is transmitted to the front and rear differential.

Types of Center Differential Gear

The center differential serves the role of absorbing any rotational difference generated between the front and rear wheels during cornering. The following types of gears are used for absorbing the rotational difference.

1. Bevel gear type

The bevel gear type center differential distributes the torque to the front and rear in the ratio 1:1.

2. Planetary gear type

The bevel gear type center differential distributes the torque to the front and rear in the ratio 1:1, but the planetary gear type can vary the distribution of ratio.

Types of Center Differential Limit Control Mechanism

When attempting to get out of deep snow or escape from a mud-hole, it is necessary to limit the differential action of the center differential. There are the following types of center differential limit control mechanisms.

1. Mechanical locking type

Switching between “FREE” and “LOCK” is accomplished manually.

2. Torque sensing type

It generates a limited-differential torque in proportion to the drive torque, and instantly changes the front and rear torque distribution in order to restrain the slippage of the front and rear wheels.

3. Viscous coupling type

Limitation of differential operation depends on difference in rotational speeds between front and rear wheels.

4. Hydraulic multi-plate clutch type

Differential is limited by hydraulic pressure being applied to multiple disc clutch.