Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock ControlAnti-Lock Brake System (ABS) with Traction Control
The anti-lock brake system with traction control consists of the following components:
^ Hydraulic control unit (HCU)
^ Anti-lock brake system (ABS) module
^ Rear wheel speed sensor
^ Rear wheel speed sensor ring
^ Front wheel speed sensor (integral to wheel hub assembly)
^ Front wheel speed sensor ring
^ Traction control switch
^ Yellow ABS warning indicator
Anti-Lock Brake System (ABS) With Traction Control Advance Trac/IVD
The anti-lock brake system with traction control Advance Trac/IVD consists of these additional components:
^ Accelerometer (in the same component as the yaw rate sensor)
^ Yaw rate sensor (in the same component as the accelerometer)
^ Steering wheel rotation sensor
^ Active brake booster
- master cylinder pressure sensor
- pedal travel switch
Principles of Operation
The anti-lock brake control module with or without Advance Trac/IVD simultaneously manages the anti-lock braking, traction control in conjunction with the engine control systems to maintain vehicle control during deceleration and acceleration.
When the ignition switch is put in the RUN position, the 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 traction control and Advance Trac/IVD and the anti-lock brake warning indicator will illuminate. However, the power-assist braking system will function normally.
Traction Control and Traction Control with Advance Trac/IVD
With the ignition in the START or RUN positions, the traction control module functions similarly to a conventional anti-lock brake module by monitoring and comparing the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor which electrically senses each tooth of the anti-lock sensor ring as they pass through the sensor's magnetic field. When the traction control 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 traction control module triggers the hydraulic control unit (ECU) to open and close the appropriate solenoid valves. Once the affected wheel(s) return to the desired speed, the traction control module returns the solenoid valves to their normal position, and normal base brake operation is restored.
The traction control module communicates with the powertrain control module (PCM) to assist with traction control. At speeds under 40 km/h (25 mph), when drive wheels lose traction and begin to spin, the traction control 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 traction control module ends the request. The request ends 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 Advance Trac/IVD 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 Advance Trac/traction control switch allows the driver to control use of the Advance Trac/IVD system. This is independent of the ABS function, which cannot be switched off by the driver. The Advance Trac/IVD system's status is indicated by a traction control warning light in the Advance Trac/traction control switch. An illuminated Advance Trac/traction control switch indicates that the Advance Trac/IVD system is switched off. In the message center, the message "Advance Trac OFF/Traction Control OFF" is displayed. However, if another alert message is being displayed on the message center ("Door Ajar"), the "Advance Trac OFF/Traction Control OFF" message may not be displayed. If a failure is detected somewhere in the Advance Trac/IVD system which affects either the yaw control or traction control functions, the traction control indicator will illuminate. The anti-lock brake function continues to work as designed unless the yellow anti-lock brake warning indicator is also illuminated. The normal brake function always occurs, unless the red brake warning indicator is illuminated.
When Advance Track activates, any of the following can occur:
^ a rumble or grinding sound much like ABS with traction control
^ a small deceleration or a reduction in the acceleration of the vehicle
^ the traction control indicator will flash
^ if the drivers foot is on the brake pedal, a vibration will be felt in the pedal much like ABS
^ if the event is severe and the driver's foot is not on the brake, the solenoid in the active booster will be energized by the traction control module. In the case of a severe event, you may notice brake pedal movement, a "whoosh" sound and/or a loud vibration from the brake pedal.
Some drivers may notice a slight movement of the brake pedal when the Advance Trac/IVD 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 pressed. If the brake system is not bled correctly, the brake pedal movement can become more significant.
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 traction control module. The electrical brake booster actuation is necessary in severe traction control 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). When sufficient force is applied to initiate movement of the key away from the valve body, the brake booster switch changes state, signifying a driver application. When the traction control module is activating the brake booster, the brake pedal is pulled forward as the valve body moves. The release switch is held in position by the input rod spring and remains against the valve body. Although the switch changes state when the brake pedal drops. the release switch remains inactive until it encounters an external force from the driver, whether in a traction control event controlled by the traction control module, or in standard mechanical braking event.
The pedal travel switch (PTS) sends brake application inputs to the traction control module. This signal is called driver brake application. This signal only changes state when the driver has applied the brake. If the ignition switch is in the RUN position, the brake booster is not electrically actuated, and no failure has been detected, the driver brake application signal will indicate a brake application when either the PTS switch or the release switch (built into the brake booster) indicates that the driver has applied the brake pedal. If the ignition switch is in the 0FF position, the driver brake application signal will indicate a brake apply only when the PTS switch indicates that the driver is applying the brake pedal
When the ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy will attempt to keep the driver brake application signal accurate. If the Advance Trac/IVD system detects faults on the driver brake apply signal line, it will also attempt to maintain correct stoplamp operation by communicating on the standard corporate protocol (SCP) network. Control of the stoplamp operation is also required because the PTS switch can change states during a traction control event when the driver is not applying the brake pedal. The switch can also change states during the system check of the brake booster. Only the Advance Trac/IVD system can interpret these changed states and determine which are actually due to driver brake application. Similarly, the Advance Trac/IVD system can take advantage of its other sensors to detect PTS switch failures.
Electronic Brake Distribution
The electronic brake distribution (EBD) controls rear brake pressure and acts as an electronic proportioning valve. It is controlled by the ABS module. When EBD is disabled, the amber ABS warning indicator and the red brake warning indicator will illuminate.
Panic Assist Braking
Panic assist braking functions by fully applying the brakes during a panic stop, using a specially developed pressure/pedal movement mechanism. Depending on vehicle speed, if pedal stroke is more rapid than normal, the brake booster automatically applies full brake boost. Panic assist will be disabled when the driver releases the brake pedal.
DTC C1963
This diagnostic trouble code (DTC) is stored if the traction control warning indicator has been turned ON because the system could not confirm the straight-ahead position for more than 30 seconds above 18 km/h (11 mph) or if the vehicle reaches 75 kph (47 mph) before confirming the straight-ahead position. This condition will most likely occur if there is some other DTC or if driving aggressively immediately after turning the ignition on. 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.
In very rare circumstances this DTC can be stored by itself without the occurrence of any actual component failure. These conditions can cause DTC C1963 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 traction control module has been installed in the vehicle and is not calibrated.
CAUTION: When carrying out he accelerometer or yaw rate sensor calibration procedure, do not press the brake pedal.
Traction Control Module Calibration
The ABS module needs to be recalibrated whenever a component specific to the Advance Trac/IVD system is disconnected or a new component is installed. For additional information regarding the DTCs that require the recalibration procedure, refer to the Traction Control Calibration List. If a DTC is logged for any component of the Advance Trac/IVD system, the DTC must be cleared before carrying out the recalibration procedure. The need to recalibrate the traction control module is also indicated by the active/fail lamp flashing. The lamp will flash after clearing the DTCs that are indicated on the Traction Control Calibration List. If a DTC is retrieved after recalibration, refer to the Traction Control Module Diagnostic Trouble Code (DTC) Index. If installing a new traction control module, the calibration procedures as well as the configuration procedure must be carried out.
When carrying out the accelerometer or yaw rate sensor calibration using the diagnostic tool, the vehicle must be on a level surface, must be stationary and the ignition switch must be in the RUN position. When carrying out the steering wheel rotation sensor calibration and the active booster calibration using the diagnostic tool, 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.