Aerostar Electronic 4-Wheel Drive (E-4WD)

Figure 8:

The Dana Model TC-28 transfer case (Figure 8) used in the Aerostar E-4WD is a full-time, chain-driven unit. Full-time means that the system is always in 4-wheel drive. Under normal driving conditions, when traction is good, 2/3 of the torque entering the transfer case is directed to the rear wheels, 1/3 to the front wheels. This split in the torque ratio is accomplished by a planetary gear set that functions a little differently than the planetary gears in the 4 x 4 transfer cases. In the unlocked mode, the planetary gears act as a differential producing the 2/3 - 1/3 torque split and preventing driveline windup.

When road conditions become slippery or otherwise result in loss of traction, this results in a variation in the front and rear output shaft speeds. At this point, the microprocessor reacts to this difference in speed by activating the electromagnetic clutch and locking up the differential action of the planetary gear set. In this drive mode, the front and rear drive shafts are locked up and turn at the same speed.


Lockup is provided for 3-3/10 of a second. Then the microprocessor unlocks the clutch. If there is no loss of traction indicated, the torque split returns to 2/3 to the rear and 1/3 to the front. If the data from the sensors continues to indicate loss of traction, after four tenths of a second, the microprocessor will signal lockup again.

NOTE: WHEN A LOSS OF TRACTION OCCURS, THE CYCLING CLUTCH MAY PRODUCE A SLIGHT BUMPING OR PUSHING SENSATION. THIS CONDITION IS NORMAL AND WILL CONTINUE UNTIL VEHICLE TRACTION INCREASES.

Aerostar 4-wheel drive is not designed for off-the-road service. However, it has some important advantages for on-the-road conditions. It allows the Aerostar to handle like a rear-wheel drive vehicle on dry pavement, while offering the control and responsiveness of a 4-wheel drive vehicle when traction is poor. In addition, it also allows the front and rear axles to rotate at different speeds, preventing driveline windup during normal operation.