Seamless Pressure Other options for separating front and rear drive are clutch packs like those found in automatic transmissions that are hydraulically powered (often externally) and add pressure as slip is determined, and gerotor-pump systems like that used in the Grand Cherokee's optional Quadra-Drive system.
AWD systems in cars rarely use a center differential, preferring some sort of clutch pack, hydraulic, electromagnetic, or electromechanical device for operation.
The Acura MDX (and Honda Pilot) uses a series of electronically controlled wet clutches to transfer power front to back. A similar setup is used for the rear wheels.
Regardless of how power is applied to the front driveshaft in the above systems, the front axle itself must also be engaged. In the early Jeep days, the front axle was always connected, even if not being driven from the transfer case, as in 2WD. Then manual locking hubs were put on the end of the axles to connect the wheels.
Manual-locking hubs require you to physically rotate the hub to lock it and engage front drive. This is inconvenient, but it's inexpensive and means you can still drive if anything in the front drive breaks and allows use of any aftermarket front differential. Some enthusiasts have converted their vehicles to have locking hubs on the rear wheels as well. Whatever breaks in the driveline, as long as the wheel bearings are still good, can be pushed or towed home.
What's In a Name?
The companion chart lists most of the SUVs and pickups sold in North America available as four- or all-wheel drive. The column headings are defined as follows (information in parentheses indicates optional systems or engines).
Name: The oft-trademarked name given to the drive system by the manufacturer.
Type: All- or four-wheel drive.
Default: Which wheels are driven at vehicle startup prior to any driver intervention.
4 HWY: Can the vehicle be operated in 4WD on pavement?
Low-range: The low-range gear ratio in the transfer box expressed as "x.xx:1," with high-range noted only if it's not the standard 1:1.
Center diff: The mechanism used to split power among the front and rear axles, even if not a differential. Note that "lock" means the unit can be locked for off-highway use, and (rr) indicates the unit is located at or in the rear axle
CP=hydraulic clutch pack;
EM=electromagnetic clutch pack;
GR=gerotor;
VCU=viscous coupling unit.
TQ split: The front/rear torque split designed into the system at startup. This value may or may not be variable.
Trac Aid Diff: Lists the limited-slip (ls) or locking (lk) differentials available by front or rear axle and whether standard or optional.
ETC: Electronic traction control on front, rear, or all four wheels; standard unless listed as "opt." Does not apply to 2WD versions.
2WD?:Shows if a two-wheel-drive version is offered. Is it front or rear drive?
Engine: Standard and (optional) engines for the base 4WD or AWD model.
Hp/Tq: Horsepower/torque for the standard and (optional) engines.
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Automatic hubs replaced manual hubs as a question of convenience (you don't manually turn anything). Auto hubs are more complex than manual hubs, but less so than a disconnect system. The '03 Expedition uses a vacuum-powered hub disconnect that gives the same effect as auto hubs, but doesn't require slow speed or reversing direction to unlock front drive.
Feeling Disconnected The next advance is disconnect systems, like Toyota's ADD and GM's Insta-Trac. With this type of system there are no locking hubs and one front wheel is always linked to the front differential. The other wheel has a disconnect system on its axle shaft, which engages by a sliding sleeve or dog clutch--vacuum, heat, electric motor, or hydraulically powered--to link both front wheels to the differential. These systems are convenient, but don't always engage instantly, and aftermarket front differential options are limited because one side of the differential is always driven.
Some manufacturers, like Jeep, have come full circle and don't use any front-drive disengagement system on various models. These systems offer ultimate simplicity, but if anything breaks in the front drive, the vehicle may be undriveable, and aftermarket front-differential options are limited as with a disconnect system.
For those vehicles derived from front-drive cars or minivans, the AWD tends to use either a VCU for the front/rear connection (Vibe/Matrix, RAV4), or run primarily as front drivers adding rear drive only as required (MDX, CR-V, Rendezvous). In many of the latter cases, the driveshaft going from front to rear is always engaged, with the rear-drive engagement handled at the rear axle.
The VCU systems work the same way as 4WD transfer cases, with a limited amount of front/rear slip and drive to both ends all the time. The Aztek/Rendezvous and CR-V use a gerotor pump just ahead of the rear differential, which adds rear drive automatically and only when front slip is detected. The Acura MDX uses just one differential, between the front wheels, and is always driving its rear driveshaft. Instead of a conventional differential in back, it has a ring-and-pinion gearset with an electromagnetic clutch pack attached to each axle shaft. Wheel-speed sensors determine what is slipping where and engage the clutch packs individually as required, and since it's electronically controlled it can think ahead. If the vehicle is in first gear and the throttle is hit hard, the system knows the front tires will spin and add rear drive before tire spin actually happens.