4 X 4 X Ford - Driveline Windup
Driveline windup does more than just produce hop, skip and bounce. It can also cause delays in shifting out of 4WD. Let's take a closer look at the popular Ford "Touch Drive" electric shift with automatic locking hubs. The "Touch Drive" system is standard on Ranger and Explorer and optional on F-Series and Bronco. The automatic locking hubs are standard on F-Series and Bronco only. "Touch Drive" allows shift-on-the-fly into 4WD, even if the auto hubs are initially unlocked. With auto hubs, nobody has to get out of the vehicle and manually lock or unlock the hubs.
Although "Touch Drive" permits shifting from 2WD to 4WD and back on the fly, the vehicle must be STOPPED and the automatic transmission put in NEUTRAL (not PARK) or the manual transmission clutch depressed in order to shift from 4H to 4L or back. To insure this, the computer that controls the transfer case is programmed to shift only when these conditions are met. One reason for this is that in order to complete the shift, the system sometimes needs a little freedom to rotate, especially if the planetary gears are not exactly lined up. With the transmission in gear or in PARK, the components don't have this freedom to rotate.
"Touch Drive"
Figure 19:
Figure 19 shows some of the components of this system. A significant feature is the electric magnetic clutch combined with the auto hubs. One advantage of a chain drive transfer case is that the front drive components aren't moving when the vehicle is operated in 2WD. However, in order to be able to shift from 2WD to 4WD while the vehicle is in motion (shift-on-the-fly), there has to be some way to bring the 4WD components up to the same speed as the rear output shaft. If this isn't accomplished, the 4WD lockup collar doesn't engage cleanly and may grind, just like an improperly shifted manual transmission gear.
When the driver pushes the 4 x 4 button, the electric magnetic clutch is activated. It is spinning at the same speed as the rear output shaft. So when it engages the 4WD drive sprocket, it spins the 4WD components up to speed. When all components are at the same speed (synchronized), the 4WD lockup collar engages the 4WD drive sprocket to the rear output shaft without grinding the gears. At the same time, as the electric magnetic clutch spins the 4WD components (including the front axle), the front hubs automatically lock.
Built-in Delay
As you can see from Figure 19, this system has activating springs in many locations. The purpose of these springs is to provide the force to move various locking devices ... when the time is right!
A good example is the spring on the shift control motor cam shaft. When the vehicle has been in high and the driver selects 4L, the shift control motor turns the cam shaft. This rotation happens immediately. However, the range shift fork may not actuate immediately if the teeth on the planetary gears are not lined up.
During this delay, the spring winds up and stores energy. When the planetary teeth rotate slightly to allow engagement, this energy is released and the spring moves the fork that moves the range lockup collar into the engaged position.
Other springs allow for delayed activation of the 4WD lockup collar and the locking and unlocking of the automatic hubs (see Figure 19).
NOTE: WHEN THE DRIVER PUSHES THE 4 x 4 BUTTON TO SHIFT FROM 2WD TO 4WD, THE 4WD LIGHT ON THE CONTROL PANEL LIGHTS. BECAUSE OF THIS, THE DRIVER MAY THINK THAT THE VEHICLE IS IN 4WD, BUT THIS WILL NOT BE THE CASE UNTIL THE COMPONENTS ARE SYNCHRONIZED AND THE LOCKING COLLAR HAS ENGAGED. THIS DELAY OF A FEW SECONDS IS NORMAL, BUT CUSTOMERS MAY COMPLAIN ABOUT IT.
Windup and Delay - Shifting Out of 4WD
In the preceding example, the delay was not due to driveline windup in the system. However, many times a delay or a failure to shift out of 4WD is the result of driveline windup. As we explained before, in 4WD, windup can develop, especially if the vehicle is operated on hard, dry surfaces or in tight turns. The driver can select 2WD, but the shift doesn't take place even though the 4WD light goes out. Also, as long as the transfer case doesn't shift, the hubs remain locked and the vehicle remains in 4WD. If windup in the transfer case is severe, even backing up may not release it.
In order to return to 2WD under these conditions, two things must happen. First, if the transfer case doesn't shift right away, the vehicle will have to be driven in 4WD until the windup is relieved. Second, when it does make the shift, the driver will have to stop the vehicle and go straight in the reverse direction for at least 10 feet to unlock the hubs.