Small sports utility vehicles and cars with all-wheel drive capability are becoming more commonplace. Every major auto manufacturer has its version. High-speed CAN bus networks, powerful computers, and lightweight drive components make adding all-wheel drive to existing front-wheel-drive models in their lineup more practical. Besides providing added traction and control on adverse road conditions, all-wheel drive creates a unique, visceral driving experience. These vehicles are known for enhanced handling characteristics and improved control. In short, the vehicle feels like it is in control even in extreme conditions.
Ford added light-duty, all-wheel-drive vehicles to their lineup in 2005 in the Freestyle, Taurus X, and Five- Hundred models. 2007 and later model applications have more common parts and control systems (figure 1). Besides an All-Wheel Drive badge, these vehicles can easily be mistaken for their 2-wheel drive counterparts. As these vehicles age, we are seeing driveability issues arising from problems with the all-wheel-drive components. We’ll look at these vehicles and explore the systems, problems, diagnostics, and solutions.
How the System Works
Ford’s All-Wheel-Drive system, also referred to as Active Torque Coupling (ATC), consists of a Power Transfer Unit (PTU), rear differential, Rear Drive Unit (RDU), rear driveshaft, and an All-Wheel Drive relay module.
Torque from the engine is transferred through the transaxle to the PTU. The torque is transferred from the driveshaft to the rear axle, which drives the rear half shafts through a differential assembly. The Active Torque Coupling system is always active and requires no driver input. The system continuously monitors vehicle conditions and automatically adjusts the torque distribution between the front and rear axles. During normal operation, most of the torque is delivered to the front wheels. If wheel slip between the front and rear wheels is detected, or if the vehicle is under heavy acceleration, the system increases torque to the rear wheels to prevent or control wheel slip. When it is functioning properly, there should be no perceived speed difference between the front and rear axles when launching or driving the vehicle on uniform surfaces. Traction should be similar to a selectable Four-Wheel Drive (4WD) system in 4H (4WD HIGH) except without binding in turns.
Power Transfer Unit (PTU)
The PTU is basically a one-speed transfer case. It receives input torque from the transmission differential carrier. The torque is transferred through gears to an output flange. This is a full-time transfer that cannot be uncoupled inside the PTU. It consists of a series of gears that mesh to turn the direction of torque output 90-degrees to drive the output shaft for the rear differential. It is a sealed unit and does not have a common sump with the transmission. It requires 75W-140 Synthetic Rear Axle gear lube (capacity = 18 ounces/ 0.53 liters). There is no drain plug, only a fill plug. It is designed to be a non-serviceable unit.
Rear Drive Unit (RDU)
The RDU bolts onto the rear differential and houses the clutch used to control torque transfer to the rear wheels. The coupling clutch housing receives a constant torque input from the rear driveshaft. An electromagnet is used to control the clutch apply. The AWD Relay Module uses a 12 volt, pulse-width modulated signal to energize a stationary electromagnetic coil inside the RDU. A plate inside the rotating clutch pack is positioned to apply the clutch when the magnet is energized to control the amount of torque transferred to the rear axles (figure 2). The longer the ‘on’ time for commanded pulse-width modulation, the more torque is transferred to the rear axles. As electricity is supplied to the electromagnet, the clutch element begins to transfer torque to the rear axle. The electric signal is modulated based on PCM inputs (figure 3).
All-Wheel Drive Relay Module
The PCM controls the output commands for the AWD system. It sends signals to the All-Wheel-Drive Relay Module to control the RDU. It is a small module located in the passenger’s side kick panel. The strategy for controlling torque to the rear axles is determined by driving conditions as well as demand from driver input. There are no direct driver inputs to control the AWD system.
What Goes Wrong?
Most complaints related to issues with AWD operation will seem vague. The customer may describe a loose feeling driving on a curvy road or crow-hopping or shuddering sensation making tight turns. In most cases, diagnosing driveability issues related to the AWD system function is as easy as unplugging the RDU connector or removing the AWD fuse. If the complaint goes away by disabling the AWD system, then the problem is most likely related to its operation and/or components.
An ‘AWD DISABLED’ indicator in the dash may be illuminated when a fault is detected, but not in all cases. If a fault is detected, the PCM cuts power to the RDU. No torque will be sent to the rear axles.
The system is fundamentally simple for controlling the RDU. The computer responds to wheel speed sensor and steering angle sensor data in the ABS module as feedback for how much torque to apply to the rear axle. The end game is to keep all speed sensors reading the same while driving down the road. If there are issues with the ABS system, the RDU computer may revert to default operation by cutting power to the RDU. Other data used for torque and engine management is also used as input for fine-tuning AWD management.
Fixing the Problems
Repairing issues with these vehicles is very straightforward. All the components mentioned are available through Motorcraft parts. If you are tempted to use recycled parts, beware! The failed parts that you are looking for are most likely going to be bad used. Even though certain parts will cross over to other makes and models, always order parts by VIN! There can be running changes that may apply to your application.
PTU failures are the most common issue with these vehicles. Oil leaks from the PTU usually are not caught in time to save the bearings and gears from irreversible damage. The seals are available, however, the bearings are not sourced through the OEM. Ford sells the entire assembly for about $750US.
The Rear Drive Unit is sold as a repair kit. The coupling clutch assembly is the part that fails the most. The kit comes with a new coupling clutch assembly, new bolts, drive hub, and magnetic coil that must ALL be installed as an assembly. These are matched components that are calibrated to control torque delivered to the rear-drive assembly.
There is a 4-digit calibration code that must be programmed into the PCM. It is included in the paperwork in the kit. If the paperwork is not in the kit or lost, the code also appears on the coupling clutch assembly (figure 4). Before driving the vehicle after repairs are complete, enter the 4-digit code using a scan tool or a J2534 pass-through device. Note, most aftermarket scan tools will provide access to the PCM for entering this number. You do not need an OEM website subscription to access this function!
Inspect the rear driveshaft universal joints, carrier bearing, and flanges for issues. This would be the same routine for heavy-duty 4-wheel drive vehicles. Parts are available through the aftermarket as well as OEM.
Interconnected powertrain management systems are here to stay. As the complexity of these systems increases, diagnosis becomes more difficult. When we understand individual systems that are introduced into vehicles, we can merge current diagnostic skills with new information to efficiently and effectively diagnose driveability concerns. This helps get vehicles out of the shop and back on the road sooner!
