Conventional 4WD Operating Modes


Conventional 4WD Operating Modes

The typical five-position shift pattern of a conventional 4WD gearbox is as follows.

2WD Two-wheel-drive range. In this high-range mode, the gear ratio is at 1 to 1. Both of the output shafts revolve at the same rate as the transmission output shaft.

HI LOCK Four-wheel-drive range with transfer case differential locked up (part-time, high-traction mode). A straight 1 to 1 gear ratio.

4WD Four-wheel-drive high range with transfer case differential not locked up (full time, all-surface mode). A straight 1 to 1 gear ratio.

N-NEUTRAL Both front and rear axles are disengaged from the power train and the vehicle may be towed without uncoupling the propeller shafts. However, torque is not transferred to the front axle assembly.

LO LOCK Four-wheel-drive low range with transfer case differential locked up. This mode of operation has a gear ratio generally at 2:1 (although it may go as high as 26:1), which cuts the road speed in half. Thus, the vehicle can be operated in 4WD low range over difficult obstacles and through mud, snow, and sand with the engine running up in its power band but with relatively slow vehicle speed.

The transfer case shift pattern is usually marked on the shifter knob. Manufacturers use different patterns and designations for the various gear positions. On some vehicles, an indicator light is located in the dash panel to show the driver which mode of operation the vehicle is in. when the indicator lamp is not lit, it usually indicated the vehicle is 2WD or neutral.

One system on the market contains a transfer case that does not have a low-range position. The transmission includes a low-geared sixth-speed position, which can be engaged when the transfer case is in the four-wheel-drive mode. In fact, there are single-speed transfer cases available that permit the operator to employ either 2WD or 4WD through a simple gear train. In one position, the gear train permits the power flow to the rear driveline only. An integral main drive gear in the transfer case (driven by the transmission output shaft) provides power to the rear driveline at all times. In addition, it is in mesh with an idler gear on a short auxiliary shaft. The idler gear, in turn, meshes with a front-axle drive gear to rotate the front driveline when in the 4WD mode. Or, the front-axle drive gear is moved out of mesh with idler gear when the 2WD mode is desired.

On some models, particularly more recent ones, the shift to 4WD can be accomplished while the vehicle is moving (shifting-on-the-fly). Most vehicles must be stopped before the change can be made. Also, the shift into a four-wheel-drive low mode normally requires that the vehicle is stopped first.

some of the power on-the-fly systems feature a magnetic clutch on the transfer case that speeds up the front drive shaft, differential, and axles to the same speed as the transmission. At the instant the speed is synchronized, an electric motor on the transfer case completes the shaft, smoothly and quietly. The operator cannot damage the system because it does not shift until the speeds are synchronized.

The newest of all 4WD systems is the automatic design. While there are several different systems on the market, one of the more commonly used systems has a microprocessor to control the interaxle differential, thus providing varying amounts of torque to the front and rear driving wheels. That is, in a fraction of a second, a computer device senses the difference in wheel speeds and engages a clutch-type device to begin driving the rear wheels. When the wheel speeds are equal again, the system reverts to front drive.

More Four And All Wheel Drive

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