Antilock Control - Traction Control and Stability Assist
DESCRIPTION AND OPERATIONAnti-Lock Control
The Four Wheel Anti-Lock Brake System (4WABS) with traction control and stability assist system consists of the following components:
^ active brake booster with integral solenoid
^ brake booster release switch
^ front anti-lock brake sensors
^ front anti-lock brake sensor indicators
^ Hydraulic Control Unit (HCU)
^ stability assist event indicator
^ stability assist module
^ lateral accelerometer
^ rear anti-lock brake sensors
^ rear anti-lock brake sensor indicators
^ steering wheel position sensor
^ two brake pressure transducers
^ Advance Trac/stability assist switch
^ yaw rate sensor
^ yellow anti-lock brake warning indicator
Principles of Operation
NOTE: The diagnostic tool refers to the stability assist module as an "ABS module". The stability assist module is an ABS module at its core,
The stability assist module simultaneously manages the anti-lock braking, traction control, and engine control systems to maintain vehicle control during deceleration and acceleration.
When the ignition switch is put in the RUN position, the stability assist module will do a preliminary electrical check, and at approximately 20 km/h (12 mph) the hydraulic pump motor is turned on for approximately one half-second. Any malfunction of the anti-lock brake control system will disable the ABS/TC/Stability Assist and the anti-lock brake-warning indicator will illuminate, however the power-assist braking system will function normally.
With the ignition in the START or RUN positions the stability assist module functions similar to a conventional anti-lock brake module by monitoring and comparing the rotational speed of each wheel. Wheel speeds are measured by the anti-lock brake sensor which electrically senses each tooth of the anti-lock sensor indicator as they pass through the sensor's magnetic field. When the stability assist module detects an impending wheel lock, wheel spin or vehicle motion that is inconsistent with the driver's commands, brake pressure is modulated to the appropriate brake caliper(s). The stability assist module triggers the Hydraulic Control Unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel(s) return to the desired speed, the stability assist module returns the solenoid valves to their normal position, and normal base brake operation is restored.
The stability assist module communicates with the Powertrain Control Module (PCM) to assist with traction control. At speeds under 40 km/h (25 mph), when drive wheels loose traction and begin to spin, the stability assist module requests the PCM to reduce engine torque, while simultaneously applying and releasing the appropriate brake caliper(s) to maintain traction. The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the stability assist module ends the request, the request is ended when the driven wheel speed returns to normal. After the vehicle speed exceeds 40 km/h (25 mph) the traction control is accomplished only through the PCM torque control.
The stability assist system constantly monitors vehicle motion relative to the driver's intended course. This is done by using sensors to compare the driver's steering input and brake application with that of the actual vehicle motion. The system does not activate when the vehicle is traveling in reverse, however, ABS and traction control will continue to function as usual.
The stability assist/Advance Tracswitch allows the driver to control use of the stability assist system. This is independent of the ABS function, which cannot be switched off by the driver. The stability assist system's status is indicated by a stability assist warning light in the stability assist/Advance Trac switch. An illuminated stability assist/Advance Trac switch indicates that the stability assist system is switched off. In vehicles with a message center, the message "ADVANCE TRAC OFF" is displayed. If a failure is detected somewhere in the stability assist system which affects either the yaw control or traction control functions, the stability assist indicator will be illuminated. The anti-lock brake function will continue to work as designed unless the yellow anti-lock brake warning indicator is also illuminated. The normal brake function will always occur, unless the red brake warning indicator is illuminated.
When stability assist activates, the following can occur:
^ a rumble or grinding sound much like ABS or traction control.
^ a small deceleration or a reduction in the acceleration of the vehicle.
^ the stability assist indicator will flash.
^ a vibration in the brake pedal much like ABS when the pedal is applied.
^ if the event is severe and the brake pedal is not applied, the pedal will move to generate higher brake forces. A whoosh sound can also be heard from under the instrument panel in an event this severe.
Some drivers may notice a slight movement of the brake pedal when the system checks itself. The brake pedal moves when an active test of the brake booster is run. During this test a small amount of pressure will be generated at the master cylinder, but no pressure will be generated in the brake calipers. This test will occur above 48 km/h (30 mph) after the vehicle has been running approximately 8 minutes. The test will only be run if the vehicle is stable, the driver is not braking and the accelerator pedal is slightly depressed. If the brake system has not been bled correctly, the brake pedal movement can become more significant.
Yaw Rate Sensor
The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.
Lateral Accelerometer
The lateral accelerometer measures the acceleration which corresponds to the force involved while the vehicle slides sideways. This acceleration has two forms. The first is the centrifugal acceleration which is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. The accelerometer only measures the acceleration along the lateral (or sideways) direction of the vehicle, on level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor will measure some lateral acceleration due to gravity even though the vehicle is not moving.
Longitudinal Accelerometer (U4 vehicles)
The longitudinal accelerometer measures the acceleration corresponding to the force involved when the vehicle moves forward and rearward in the horizontal plane, along the centerline of the vehicle's front and rear wheels.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The steering wheel rotation sensor uses two signal lines to transmit information to the stability assist module, whether the steering wheel is being turned left or right and how far it is being turned. The steering wheel rotation sensor does not indicate the position of the steering wheel relative to straight ahead. The stability assist system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The stability assist system confirms this position and modifies it as necessary during every new driving cycle.
Active Brake Booster
The active brake booster functions like a conventional brake booster with the added feature that it can be actuated electrically by the stability assist module. The electrical brake booster actuation is necessary in severe stability assist events and makes sure that the hydraulic control unit can generate enough brake pressure to maintain vehicle stability. The brake assist from the brake booster is especially useful in cold weather when the hydraulic control unit cannot draw the brake fluid from the reservoir quickly at cold temperatures.
A solenoid within the active brake booster has a release switch to indicate when the driver is stepping on the brake pedal. The solenoid provides electrical actuation of the brake booster. Without the force of the input rod, the air valve is directly opened due to the movement of the energized solenoid. With increasing current applied to the solenoid, the air valve opens and output force is created. With decreasing current applied to the solenoid, the air valve is closed and the vacuum valve opens reducing output force.
The release switch indicates when the brake pedal has been depressed. It is integrated into the booster key that normally sits against the rear shell of the boot in its rest position and it adjoins the valve body when in its balance position. The switch itself provides indication from two positions. In its rest position, it supplies two signals, Normally Open (NO) and Normally Closed (NC). Upon application of sufficient force to initiate movement of the key away from the valve body, the brake booster switch changes state, signifying a driver application.
When the stability assist module is activating the brake booster, the brake pedal is pulled forward a,; the valve body moves. The release switch is held in position by the input rod spring and remains against the valve body. Although the Pedal Travel Sensor (PTS) switch changes state when the brake pedal drops, the release switch remains inactive until such time as it encounters an external force from the driver, whether in a stability assist event controlled by the stability assist module, or in a standard mechanical braking event.
Electronic Brake Distribution
The Electronic Brake Distribution (EBD) controls rear brake pressure and acts as an electronic proportioning valve. It is controlled by the stability assist module. When EBD is disabled, the amber ABS warning indicator and the red brake warning indicator will illuminate.
Brake Assist Braking
Brake assist braking functions by fully applying the brake booster during a panic stop. Depending on vehicle speed, if pedal stroke is more rapid than normal, it manipulates a valve in the brake booster that automatically applies full brake boost. Brake assist will be disabled when the driver releases the brake pedal.
DTC C1963
This Diagnostic Trouble Code (DTC) is stored if the stability assist warning indicator has been turned ON because the system could not confirm the straight-ahead steering wheel position for more than 30 seconds above 18 km/h (11 mph) or if driving aggressively immediately after turning the ignition on. This condition will most likely occur if there is some other DTC or if the vehicle reaches 75 km/h (47 mph) before confirming the straight-ahead position. If there is another DTC logged, the repair procedure is to follow the procedure for the first DTC. This DTC can be stored if:
^ the PRNDL or Reverse switch has failed (check operation of PRNDL display and back-up lights).
^ chassis modifications have been made that were not authorized by Ford.
^ the vehicle is driven aggressively immediately after start-up.
In very rare circumstances this DTC can be stored by itself without the occurrence of any actual component failure. These conditions can cause DTC C 1963 to set with no other DTC:
^ The steering wheel has been turned while the ignition is off and then the vehicle is started, and either the vehicle is driven continuously in a circle or the vehicle is driven with continuous swerving, sliding or tire spinning.
^ A new stability assist module can have been installed in the vehicle and was not calibrated.
Stability Assist Module Calibration
NOTE: When carrying out a calibration on a 4x2 vehicle, if the diagnostic too] instructs to calibrate the longitudinal accelerometer, ignore this request - 4x2 vehicles are not equipped with a longitudinal accelerometer.
The stability assist module needs to be recalibrated whenever a component specific to the stability assist system is disconnected or a new component is installed. For additional information regarding the DTCs that require the recalibration procedure, refer to the Stability Assist Calibration List. If a DTC is logged for any component of the stability assist system, the DTC must be cleared before carrying out the recalibration procedure. The need to recalibrate the stability assist module is also indicated by the active/fail lamp flashing and the presence of DTC C1998. The lamp will flash after clearing the DTCs that are indicated on the stability assist calibration list. If a DTC is retrieved after recalibration; refer to the Stability Assist Module Diagnostic Trouble Code (DTC) Index. If installing a new stability assist module, the calibration procedures as well as the configuration procedure must be carried out. For additional information regarding the module configuration procedure, refer to Powertrain Management. When carrying out the lateral or longitudinal accelerometer or yaw rate calibration on the scan tester, the vehicle must be on a level surface, must not be moving and the ignition switch must be in the RUN position. When carrying out the steering wheel rotation sensor calibration on the scan tester, the ignition switch must be in the RUN position with the engine running and the steering wheel must be rotated back and forth from lock position to lock position.