Four Wheel Drive (4 X 4) - General Information: Overview
9209ford01
Article No.
92-1-8
01/06/92
FOUR WHEEL DRIVE (4 X 4) - GENERAL INFORMATION, OPERATING CHARACTERISTICS AND SERVICE TIPS
LIGHT TRUCK: 1990 and prior BRONCO II
1992 and prior AEROSTAR, BRONCO, F-150, F-250, F-350, RANGER
1991-92 EXPLORER
ISSUE: Ford 4-wheel drive vehicles were discussed at length in the 1991 April, May and June issues of Shop Tips. Topics included locking hubs, transfer cases, vehicle operating characteristics, driveline windup and service tips. This information is being reprinted here to better assist the technician with questions concerning 4WD operation and service.
ACTION: Refer to the following pages for Ford 4WD information.
CHAIN DRIVEN TRANSFER CASE
Figure 1:
In the 1980 model year, Ford introduced chain type transfer cases. Chain drives provide significant advantages (Figure 1). Most noticeable of all, the transfer case is easier to shift.
Equally important, in 2-wheel drive, all the 4-wheel drive components are disengaged, reducing noise and component wear and maintaining fuel economy. This is accomplished by placing the high and low range planetary gears ahead of, and separate from, the drive shifting mechanisms. When the 4-wheel drive locking collar and the front locking hubs are disengaged, the whole front-wheel drive system is isolated and does not affect 2-wheel drive operation.
Incorporated in chain-driven transfer cases is a positive pressure oil pump driven by the output shaft to the rear wheels. This provides lubrication to the transfer case whenever the rear drive shaft is turning - either during vehicle operation or when the vehicle is towed.
NOTE: IT IS IMPORTANT TO CONSULT THE VEHICLE'S OWNER GUIDE BEFORE TOWING ANY 4 x 4. THERE ARE SPEED AND DISTANCE RESTRICTIONS THAT VARY FROM VEHICLE TO VEHICLE.
Chain drive transfer cases may be manually or electrically shifted. Let's look at a widely used example of a manually operated transfer case, the New Process Model 208. Although not currently used in production, the operating principles are the same as Warner 1354 and 1356 models. This lightweight, chain-driven transfer case was introduced in 1980 for use in Bronco, F-150, F-250 and F-350 4 x 4 's. It was in continuous use until the 1987 model year and is a typical example of how chain drive works. Speed selection is controlled by a single lever shift mechanism that provides a choice of Neutral, 2-High, 4-High and 4-Low.
Neutral
Figure 2:
Figure 2 shows the power flow condition with the shift selector in neutral. No power is transmitted to either front or rear in this position. All the planetary gears turn freely with the input shaft, and the chain sprocket floats freely on highly efficient needle bearings.
Figure 3:
Figure 3 shows the planetary gear set in cross section. At the center, attached to the input shaft, is the sun gear. This gear meshes with four gears that surround it like planets (which is why this component is called a planetary gear). In turn, the four gears mesh with an outer gear called the annulus. Annulus is another word for ring.
As the sun gear turns, the four planetary gears attempt to rotate. If the annulus ring gear is locked (as it is in low gear), the planetary gears "walk" around the inside of the annulus. If the planetary gears are locked to their gear cage (as in high gear), the whole planetary assembly (planetary gears, planetary cage and annulus) rotate as a single unit.
2-Wheel Drive - High Range
Figure 4:
Figure 5:
When the driver selects 2H, the hub of the planetary gear assembly slides rearward, putting the transfer case into the high speed range (direct drive). The input shaft and the rear output shaft are locked together (Location # 1, Figure 4). This results in direct drive, straight through to the rear drive shaft. The whole planetary gear set rotates, forcing the output shaft to turn at the same speed as the input shaft. In addition, the 4-wheel drive locking collar is disengaged so none of the 4WD components turn.
4-Wheel Drive - High Range
In 4-wheel drive - high range (4H), the planetary gear set stays where it was in Figure 4. The action of the shift lever causes the 4-wheel drive clutch to move forward, locking the chain sprocket to the rear output shaft (Location # 2, Figure 5). Now, both the rear and the front wheels are being driven in the high range.
4-Wheel Drive - Low Range
Figure 6:
To get maximum pulling power, the driver selects 4L. When this happens, the planetary gear set moves forward, and the annulus gear engages a locking plate (Location # 3, Figure 6). With the annulus unable to turn, the planetary gears "walk" around the inside. The result is that the planetary gear cage now turns more slowly than the input shaft. Because the planetary cage is attached directly to the output shaft, the output shaft now rotates at a slower speed than the input shaft. This action increases the pulling power available to the wheels. It's like having an additional set of lower gears for extra power.
RECENT TRANSFER CASE DEVELOPMENTS
During the 1980's, Ford introduced other transfer cases that eventually replaced the New Process 208 we have already described. Let's look at one of these, the Warner 13-56, and then compare it to the Aerostar electronic 4-wheel drive system. The Warner 13-56 is used on Bronco and F-Series and comes in two versions: Mechanical shift and the electrically controlled "Touch Drive." Let's look at the "Touch Drive" version.
Figure 7:
The Warner 13-56 with "Touch Drive" is a chain-driven transfer case in which the shifts are activated electrically (not mechanically). The basic functions of this system are much the same as any other 4 x 4 transfer case. By touching the control switch on the instrument panel, the driver can select either 2- or 4-wheel high plus 4-wheel low range, if conditions require. In Figure 7, you can see the planetary gears and the chain. Attached to the housing in the lower right, you can also see the electric motor that actuates the shift mechanisms.
NOTE: IT IS IMPORTANT TO REMEMBER THAT ELECTRICALLY SHIFTING TRANSFER CASES DO NOT HAVE A SELECTABLE NEUTRAL POSITION. BECAUSE OF THIS, IF VEHICLES EQUIPPED WITH ELECTRIC SHIFT ARE TO BE TOWED FOR LONG DISTANCES, THE REAR DRIVE SHAFT SHOULD BE DISCONNECTED. IF THIS IS NOT DONE, THE TRANSFER CASE (WHICH IS PROTECTED BY POSITIVE LUBRICATION) WILL DRIVE THE TRANSMISSION (WHICH IS NOT WELL LUBRICATED UNDER THESE CONDITIONS), AND THE TRANSMISSION MAY "BURN UP."