FourWheelDrive Why FourWheelDrive z Advantages y Improved traction

Four-Wheel-Drive

Why Four-Wheel-Drive? z Advantages y. Improved traction and handling y. Able to transmit torque to all four wheels z Disadvantages y. Higher initial cost and weight y. Requires special service and maintenance

Definitions z 4 X 4 = Four wheeled vehicle, four wheels can receive torque. z 4 X 2 = Four wheeled vehicle, two wheels can receive torque.

4 WD Designs z. RWD vehicles equipped with an added transfer case, a front drive shaft, a front differential, and front drive axles

4 WD Designs z FWD vehicles with an added transfer case, a rear drive shaft, and a rear axle

4 WD Definitions z. Transfer case - An auxiliary transmission mounted to the side or rear of the main transmission. y. No differential action is provided.

4 WD Components z Transfer case y Transmits power from the transmission to the front and rear drive shafts

4 WD Components z Transfer case y Will typically be a gear or chain type

Types of Transfer Cases z Drive chain design y. Has less weight to improve fuel economy y. Is usually used with planetary gearsets y. Is the most common design

Types of Transfer Cases z Gear-type y. Uses only gear-sets to transfer power y. Stronger & more durable

4 WD Definitions z. Center differential serves the same purpose as a transfer case but allows for differential operation.

4 WD Definitions z. Limited-slip center differential. Controls differential operation through a disc or cone-type clutch. y. Later designs are computer operated.

4 WD Components z. Drive shafts y Connect to the front and rear differentials

4 WD Components z Universal joints or CV-joints y Are used to connect front axles to the wheel hubs

4 WD Definitions z. Part-time 4 WD - Typically systems that operate in two-wheel drive until the driver commands four-wheel drive operation. Torque is then divided (50/50 fixed) between the front and rear drive-shaft. y. They do not corner well on dry pavement. y. Selection of two- or four-wheel-drive is done by a shifter, electric switch and/or locking hubs.

Part-Time 4 WD Shift Controls z Electric switch or shift lever y. Allows the driver to select which axles receive power y. Power can be directed to: xall four wheels xtwo wheels xnone (neutral) y. Some vehicles have a low-speed range that can be selected

Part-Time 4 WD z. Older designs may have different driveaxle gear ratios. y. This will result in a push-pull type action if the vehicle is driven on dry pavement. y. This phenomenon is called drive-line windup.

Part-Time 4 WD Shift Controls y. Manual Shift lever y. Some vehicles have a low-speed range that can be selected

Part-Time 4 WD z. Locking Hubs y. Manual or automatic operation

Part-Time 4 WD z. Locking Hubs ymanual operation

Part-Time 4 WD z. Locking Hubs yautomatic operation - vacuum or electromagnetically operated.

Part-Time 4 WD z Locking Axles - final differential will be of an “open” design. When one axle is disconnected from the wheel the spider gears rotate freely, neither axle receives torque. z Used with an open differential

Part-Time 4 WD z. Locking Axles - Toyota’s Automatic Disconnecting Differential (ADD)

Transfer Case Modes of Operation z Ranges available with a part-time transfer case: y Neutral

Transfer Case Modes of Operation z Ranges available with a part-time transfer case: y Two-wheel-drive—high

Transfer Case Modes of Operation z Ranges available with a part-time transfer case: y Four-wheel-drive—high

Transfer Case Modes of Operation z Ranges available with a part-time transfer case: y Four-wheel-drive—low

4 WD Definitions z. Full-time four-wheel drive (4 wd) Constantly provides power to all four wheels. These systems typically do NOT have a provision for “low” operating range. y. Axles and differentials are constantly locked. y. Usually equipped with a center “differential” instead of a transfer-case. z. All-wheel drive - see above

4 WD Systems z. Full-time systems y. Cannot be selected out of four-wheeldrive y. May have an open or limited-slip center differential.

Full-time 4 WD z. Open Center Differential - If the wheels on either final differential loose traction, maximum torque is delivered to the axle with the LEAST traction. z. For this reason, limited-slip center differentials are far more common today.

Full-time 4 WD z. Newer computer-controlled designs allow for different operational modes. z. Ford Automatic Four-wheel-drive (A 4 WD) y. Navigator and Expedition y 4 WD AUTO - full power to the rear wheels. The front wheels receive power as required. y 4 WD High - Full power to both wheels, no center gear-reduction. y 4 WD Low - Low gear ratio in center differential for (2: 1 ratio) for slow, high torque driving

4 WD Definitions z. On-demand four-wheel drive Automatically controlled 4 WD. One axle is driven until slip is detected or anticipated. y. Computer controlled yuses wheel speed sensors and/or axle speed sensors to detect slippage. y. When slippage is detected, torque is sent to the opposite axle. The amount sent depends on the amount of slippage.

4 WD Definitions z. On-demand four-wheel drive

4 WD Definitions z. Viscous coupling in a center differential

4 WD Definitions z. Viscous coupling in a center differential

On-demand four-wheel drive z. Ford’s control-trac system ynormal operation - 96% of torque is sent to the rear wheels. y. Upon slippage a computer energizes an electro-magnetic clutch in the transfer case. Up to 96% of torque can be sent to the front wheels.

4 WD Systems z. Four-wheel drive systems without a center differential. x. Tire sizes must all be the same (this holds true for any 4 WD application) x. Tread wear must be similar x. Tire changes on an all-wheel drive must be done in sets of four x. Gear ratios should be the same in the front and rear differentials.

Front Axle and Hub Service Guidelines z Most front axles are serviced like those in 2 WD systems z Follow the recommended procedure for servicing 4 WD hubs z Some hubs are not serviceable and must be replaced

Effects of Vehicle Modifications z Installing larger tires and lift kits y. Alters vehicle handling by changing unsprung weight y. Increases steering effort y. Causes a rough ride y. May cause vehicle to wander and vibrate y. Possible driveshaft working angle issues y. Possible Vehicle Speed sensor issues x. May necessitate computer programming

Maintenance Tips z Check the fluid level often z Check for fluid contamination any time the vehicle has been driven through deep water z Use only the recommended lubricant z Always follow the proper towing procedures