Stability Assist
Special Tools:
The stability assist module executes control of the anti-lock brakes, full speed traction control and stability assist functions to enhance the drivers control of the vehicle. The stability assist manages the interactions between the anti-lock, traction control, and engine control systems to optimize the vehicle traction during deceleration and acceleration. In addition, stability assist function modulates brake pressure during braking and non-braking conditions as required to counteract excessive vehicle rotation while cornering.
The stability assist module is operational with the ignition switch in the RUN or START positions.
The stability assist module continuously monitors and compares the rotational speed of each wheel. The wheel speeds arc measured by the anti-lock brake sensors which electrically sense each tooth of the anti-lock sensor indicator as they pass through the sensors magnetic field. When stability assist detects an impending wheel lock, wheel spin or vehicle motion that is inconsistent with the drivers commands, it modulates brake pressure to the appropriate brake caliper(s). This is accomplished by triggering the hydraulic control unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel returns to the desired speed, the stability assist module returns the solenoid valves to their normal position, and normal base brake operation resumes.
The stability assist module is self-monitoring. When the ignition switch is turned to the RUN position, the stability assist module will do a preliminary electrical check, and at approximately 20 km/h (12 mph) the pump motor is turned on for approximately one half-second. Any malfunction of the anti-lock brake control system will cause the ABS/TC/Stability Assist to shut off and anti-lock brake warning indicator to illuminate, however the power assist braking system will function normally.
The stability assist module communicates with the powertrain control module (PCM) to assist with traction control. At speeds under 40 km/h (25 mph) the stability assist module requests the PCM to reduce engine torque, while simultaneously applying and releasing the appropriate brake to restore traction when one or both drive wheels lose traction and begin to spin, The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the stability assist module ends the request (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.
An all new stability assist system helps maintain vehicle stability at the limits of tire adhesion. System effectiveness varies with vehicle speed, road conditions, and steering inputs.
The stability assist system constantly monitors vehicle motion relative to the driver's intended course. This is done by using added sensors to compare the steering inputs from the driver with the actual motion of the vehicle.
The driver's steering input is measured from a steering wheel rotation sensor. The motion of the vehicle is determined from a yaw rate sensor (measures rotation about the vehicles vertical axis, caused by steering left/right), a lateral accelerometer (measures the acceleration generated from the vehicle sliding sideways), and the wheel speeds used for anti-lock brake system (ABS) and traction control function. When there is a discrepancy between the driver's inputs and the vehicle motion, the stability assist changes the force at each tire to help control the vehicle. If the vehicle is beginning to rotate excessively left or right, spin out, or slide sideways, the system will attempt to correct the excessive motion. If the vehicle is not responding to steering inputs, the system will attempt to increase the turning response of the vehicle.
The stability assist system defaults to ON when the engine is started. The system does not activate when the vehicle is traveling in reverse. In reverse, ABS and traction control will continue to function as usual. The system uses ABS and traction control (including control of the engine) as well as its more advanced controls to enhance vehicle stability. The stability assist/traction control switch (ATS) 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 status is indicated by a stability assist warning light in the stability assist/traction control switch. The illuminated stability assist/traction control switch indicates stability assist system is switched off. In vehicles with a message center, the message " ADVANCE TRAC OFF" is displayed.
During a stability assist event you may experience any of the following behaviors, which are normal:
- 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
- if your foot is on the brake pedal, you will feet a vibration in the pedal much like ABS. As with any vehicle equipped with four-wheel anti-lock brakes, do not pump the brakes, but instead, press firmly on the pedal.
- if the event is severe and your foot is not on the brake, the brake pedal will move to apply higher brake forces. A whoosh sound may also be heard from under the instrument panel in an event this severe. All of these behaviors are normal during a stability assist event.
The stability assist system continually monitors and checks all of the sensors and actuators used to improve the stability of the vehicle. Some drivers may notice a slight movement of the brake pedal when the system checks itself If the brake system has not been bled correctly, the brake pedal movement may become more significant. 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 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 depressed at least a small amount. 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.
Yaw Rate Sensor
The yaw rate sensor measures the rotation rate of the vehicle as it turns left and right.
Lateral Accelerometer
The lateral accelerometer measures the acceleration which corresponds to the force required to slide the vehicle sideways. This acceleration comes in two forms. The first is the centrifugal acceleration which is generated when an object 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, so 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 stationary.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor scans the steering wheel rotation sensor ring in the steering column which has holes punched in it at regular intervals. The steering wheel rotation sensor uses two signal lines to tell 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 tell the stability assist module the position of the steering wheel relative to straight ahead. The stability assist system learns this by comparing the steering wheel position with other signals and remembering the position it has learned. The stability assist system confirms this position and modifies it as necessary during every new driving cycle of the vehicle.
Brake Booster
The brake booster is a normal brake booster which has the added function that it can be actuated electrically by the stability assist module. This is sometimes necessary in severe stability assist events, to make sure that the hydraulic control unit can generate enough brake pressure to improve vehicle stability. The help from the brake booster is especially needed in cold weather when the hydraulic control unit cannot draw the brake fluid from the reservoir when it becomes very thick and viscous at cold temperatures. Within the booster is a solenoid for electrical actuation and a release switch to indicate when the driver is stepping on the brake. 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 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 brake pedal position (BPP) 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.
The master cylinder attached to the booster functions the same as on vehicles not equipped with the stability assist system. It differs in that there are two pressure transducers that are installed in spare outlet ports. The values measured by each pressure transducer should be similar. The pressure transducer is a diaphragm type unit. When a pressure is seen the diaphragm is stretched, causing a voltage to be output. As the pressure increases or decreases, the voltage also increases or decreases with it. The configuration of the pressure transducers, which transducer is connected to the primary port and which is connected to the secondary port, is not known by the stability assist module. The harness connectors can be connected to either transducer, therefore, diagnosis of pressure faults requires some care to identify exactly which transducer is failed.
Modules which are normally connected to the BPP switch are connected to a signal from the stability assist 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 applied when either the BPP switch or the release switch built into the brake booster indicate that the driver has applied the brake pedal. If the ignition switch is in the OFF position, the driver brake application signal will indicate a brake apply only when the BPP 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 try to keep the driver brake application signal accurate. If the stability assist system detects faults on the driver brake application 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 BPP switch can change states during a stability assist 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 stability assist system can interpret these state changes and determine which ones are actually due to the driver applying the brake. Similarly, the stability assist system can take advantage of its other sensors to detect failures in the BPP switch.
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 position for more than 30 seconds above 18 km/h (11 mph). This condition will most likely occur if there is some other DTC. If there is another DTC logged with this DTC, the repair procedure is to follow the procedure for the other DTC. This DTC can be stored if:
- 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 may be stored by itself without the occurrence of any actual component failure. Two conditions can cause the 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 stability assist module may have been installed in the vehicle and was not calibrated.
Stability Assist Module Calibration
The stability assist module needs to be recalibrated whenever a component specific to the stability assist system is disconnected, moved, 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 re-calibrate the stability assist module is also indicated by the active/fail lamp flashing. The lamp will flash after the clearing of DTC's 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.
When carrying out the lateral accelerometer or yaw rate calibration on the NGS Tester, the vehicle must be on a level surface, not moving, with ignition switch in the RUN position. When carrying out the booster learn cycle on the NGS Tester, the ignition switch must be in the RUN position with the engine running and the brake pedal must not be applied. When carrying out the steering wheel rotation sensor calibration on the NGS 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.