Antilock Brakes / Traction Control Systems: Description and Operation

Anti-Lock Control

ABS with AdvanceTrac(R) Stability Enhancement System

The ABS with AdvanceTrac(R) system consists of the following components:

- ABS module- attached to the Hydraulic Control Unit (HCU) , but can be serviced separately from the HCU (Hydraulic Control Unit).

- Front wheel speed sensors- one sensor is located in each front wheel knuckle. The sensors are serviced separately from the knuckle.

- Front wheel speed sensor tone rings- one tone ring is located on the outer CV joint of each halfshaft and is serviced with the halfshaft.

- HCU (Hydraulic Control Unit)- which contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. On hybrid vehicles, the HCU (Hydraulic Control Unit) is mounted in the rear, RH side of the engine compartment near the RH shock tower. On all other vehicles, the HCU (Hydraulic Control Unit) is mounted in the front, RH side of the engine compartment near the degas bottle.

- Rear wheel speed sensors- one sensor is located in each rear wheel knuckle, the sensors are serviced separately from the knuckle).

- Rear wheel speed sensor tone rings- on All-Wheel Drive (AWD) vehicles, one tone ring is located on the outer CV joint of each halfshaft and is serviced with the halfshaft. On Front Wheel Drive (FWD) vehicles, one tone ring is located on each rear wheel hub and is serviced with the wheel hub.

- Stability control sensors- located inside the Restraints Control Module (RCM) and consist of the following sensors: yaw rate, lateral accelerometer and longitudinal accelerometer. The sensors are integral to the RCM (Restraints Control Module) and cannot be serviced separately.

- Steering wheel rotation sensor- only on vehicles equipped with a 3.5L engine, the rotation sensor is located on the RH side of the clockspring and is serviced separately from the clockspring assembly. On all other vehicles, steering wheel rotation position information is sent from the Power Steering Control Module (PSCM) over the High Speed Controller Area Network (HS-CAN).

- Steering wheel rotation sensor ring- only on vehicles equipped with a 3.5L engine, the rotation sensor ring is located on the clockspring and is serviced with the clockspring assembly.

- Traction control switch- on Fusion and Milan vehicles, the switch is part of a 2 or 3 switch package (depending on vehicle options) located in the instrument panel center finish panel and is serviced separately from the finish panel. On MKZ vehicles, the switch is part of the Front Controls Interface Module (FCIM) and is serviced with the module.

- Vacuum sensor (3.0L AWD (All-Wheel Drive) , 3.5L and hybrid)- located on the front of the brake booster in the 12 o'clock position on non-hybrid vehicles, and in the 10 o'clock position on hybrid vehicles. The sensor can be serviced separately from the booster, refer to Power Brake Assist.

Anti-Lock Braking

The anti-lock braking system aids in the prevention of wheel lock-up during braking events which allows the driver to maintain steering control and stop in the shortest distance possible under most conditions. This is accomplished by the ABS module and HCU (Hydraulic Control Unit) modulating brake fluid pressure to the affected brake caliper(s).

When the ABS activates, both of the following can occur:

- A rumble or grinding sound may be heard

- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal

Electronic Brake Distribution (EBD)

The ABS module and HCU (Hydraulic Control Unit) incorporate a strategy called Electronic Brake Distribution (EBD). The EBD (Electronic Brake Distribution) strategy uses the HCU (Hydraulic Control Unit) as an electronic proportioning valve to create a balanced braking condition between the front and rear wheels and minimize the chance of rear wheel lockup during hard braking.

AdvanceTrac(R) Stability Enhancement System

The AdvanceTrac(R) system is made up of 2 distinct vehicle systems: traction control and ESC (Electronic Stability Control).

Traction Control System

The traction control system aids in the prevention of excessive wheel spin which allows the vehicle to maintain traction during acceleration. This is accomplished by the ABS module and the HCU (Hydraulic Control Unit) modulating brake fluid pressure to the affected brake caliper(s) and by the PCM modulating engine torque.

ESC (Electronic Stability Control) System

The ESC (Electronic Stability Control) system constantly monitors the vehicle's direction of travel relative to the driver's intended course, except when the vehicle is traveling in reverse, and aids in keeping the vehicle traveling along that course. This is accomplished by the ABS module and the HCU (Hydraulic Control Unit) modulating brake fluid pressure to the affected brake caliper(s) and by the PCM modulating engine torque.

When the ABS, traction control or ESC (Electronic Stability Control) system activates, any of the following can occur:

- A rumble or grinding sound may be heard, much like ABS

- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal, much like ABS

- A small deceleration or a reduction in the acceleration of the vehicle

- The stability/traction control indicator (also known as the sliding car icon) flashes

Traction Control Switch

The traction control switch can be used by the driver to disable and enable the traction control portion of the AdvanceTrac(R) system. When the switch is pressed, the sliding car OFF icon in the Instrument Panel Cluster (IPC) illuminates. The conventional ABS and the ESC (Electronic Stability Control) system cannot be disabled through the use of the traction control switch. If a MyKey(TM) restricted key is in use with AdvanceTrac(R) always-on feature configured to "on", the traction control system cannot be disabled. An admin key must be used in order to enable and disable the traction control system.

Stability/Traction Control Indicators

There are 2 separate sliding car icons; the sliding car OFF icon and the sliding car icon. The sliding car OFF icon is used to alert the driver that the traction control portion of the AdvanceTrac(R) system has been disabled by the driver. This icon only illuminates continuously and does not flash. The sliding car icon is used to alert the driver that a stability/traction event is taking place (flashes twice per second) and to alert the driver of potential concerns in the AdvanceTrac(R) system (illuminates solidly).

Optimized Hydraulic Braking (3.0L All-Wheel Drive (AWD) and 3.5L only)

Optimized hydraulic braking is a system built into the ABS module to aid in bringing the vehicle to a safe, controlled stop in the event of severe vacuum loss at the brake booster. The ABS module continually monitors the vacuum in the brake booster through the use of a sensor. If a loss of vacuum is detected, the ABS module activates the pump motor in the HCU (Hydraulic Control Unit) to assist with vehicle braking.

Hybrid Braking System

The hybrid vehicle is equipped with a few additional components, including:

- Active brake booster- which includes the brake booster vacuum sensor, the active brake booster solenoid and the active brake booster travel sensor. The sensors are serviced separately from the active brake booster. The booster solenoid is not serviced separately from the active brake booster.

- Brake pedal angle sensor- located on the brake pedal bracket and can be serviced separately from the pedal bracket.

- Brake pedal feel cut-off solenoid- located on the brake pedal bracket and is serviced with the pedal bracket.

- Brake pedal feel simulator- located on the brake pedal bracket and is serviced with the pedal bracket.

- Electric vacuum pump- located in the front of the engine compartment below the intake manifold.

- Vacuum pump relays- vacuum pump operation is controlled by 1 solid state relay and 1 mechanical relay. Both relays are located in the auxiliary relay box.

Regenerative Braking

During a braking event (not an ABS or other stability control event), the regenerative braking system utilizes the electric motor portion of the Electronically Controlled Continuously Variable Transmission (eCVT) as a generator to create electrical current. This current is used to recharge the High Voltage Traction Battery (HVTB) and creates brake torque that is used in place of, or in combination with, the conventional friction brakes to slow the vehicle. The amount of brake torque provided by the electric motor is dependent upon the state-of-charge of the HVTB (High Voltage Traction Battery). When the HVTB (High Voltage Traction Battery) is almost fully charged, the amount of brake torque provided is limited to avoid overcharging the HVTB (High Voltage Traction Battery) and vehicle deceleration is produced by conventional friction braking alone.

Brake-By-Wire

Because much of the necessary brake torque required to stop the vehicle can be accomplished through regenerative braking, it is not always necessary to apply the conventional friction brakes. Under most normal brake applications, the ABS module applies the friction brakes as needed.

During a normal brake application, the brake pedal feel simulator allows the pressure applied to the brake pedal by the driver's foot to act against a spring instead of the brake booster push rod. The brake pedal angle sensor is mounted to the brake pedal assembly and provides the ABS module with an analog signal indicating the amount of deceleration the driver is demanding. If it is necessary for the conventional friction brakes to be applied, the ABS module applies a Pulse Width Modulated (PWM) voltage to a solenoid inside the active brake booster. The solenoid allows vacuum from either the combustion engine (if the engine is running) or the electric vacuum pump to move the booster push rod. This in turn pushes the piston in the master cylinder creating hydraulic brake pressure. If the brake pedal is applied quickly and forcefully enough, the driver can overcome the brake pedal feel simulator spring pressure, allowing a direct connection between the brake pedal, booster push rod and master cylinder.