Staying On Track – UnderhoodService

Staying On Track

You’ve seen the “Advance Trac” badges on the backs of Explorers and Expeditions, but what is it? Have you ever been asked by your Ford dealer parts department if a vehicle you are working on has “IVD”? What’s IVD? Not all vehicles that are equipped with Advance Trac carry the Advance Trac insignia. Advance Trac is available on many unsuspecting vehicles like the FreeStar, Taurus and even Focus. After 2006, all Explorers and Mountaineers come standard with Advance trac with RSC. 2008 and up Taurus, Taurus X, Edge and these vehicle’s Mercury counterparts come standard with Advance Trac with RSC. A large number of Expeditions and a growing number of E-series and F-series have it as well, and are sporting the RSC versions.

The terms Advance Trac and Integrated Vehicle Dynamics (IVD) are different names Ford has given to the same system. Two interchangeable names, one system. IVD is a function of the ABS module. It uses ABS and traction control functions together to help steer the body of the vehicle to match the driver’s demand. IVD systems with only that ability are also referred to as stability assist. Roll Stability Control (RSC) is an IVD system that also aims to limit body roll. If your shop is already capable of performing ABS system repairs, then you are already mostly prepared for Advance Trac faults since it is really just an addition to ABS. Let’s look at how it works and what parts have been added. We’ll look at the individual pieces of the Ford Advance Trac system, then we’ll look at what the ABS module does with these pieces.

To make this discussion more clear, as well as other technical information you may come across, I want to start off with defining some terms involved with the system. Even though this is all done by the ABS module, there are four separate functions being performed.

ABS Function: This is the function we’re all aware of that has become commonplace over the last couple of decades. That is, during braking if a wheel stops spinning, the ABS module will release the brake at that wheel by venting hydraulic pressure to it.

Traction Control Function: Again, a common feature nowadays. During acceleration, if the ABS module sees a wheel spin faster than desired, the ABS module will apply the brake to that wheel and, if needed, also command the PCM to retard ignition timing, induce a misfire, or even shift the trans up a gear to reduce torque at the wheels.

Stability Assist: The ABS module is monitoring the vehicle’s intended course and comparing it to the measured vehicle’s motion. If the measured vehicle’s motion does not match that of the driver’s intended course, ABS and traction control functions are used to attempt to bring the vehicle’s direction back to what the driver requests. This is a basic IVD system, usually found on passenger cars like Lincoln LS and Focus.

Roll Stability Control: This is Stability Assist with an added sensor. With RSC, the ABS module is also monitoring a roll rate sensor to determine if the vehicle is entering a roll over situation, and will now use ABS and traction control functions to help prevent that. This is usually on SUVs, trucks and vans. However, Taurus sedan has this in 2008.

Brake Pressure Transducer
The brake pressure transducer is a potentiometer-type pressure sensor that usually mounts on the master cylinder and is threaded into place (See BPT photo 1 and BPT photo 2).

As the fluid pressure in the hydraulic brake system increases, so does the resistance value inside the sensor. As the resistance value changes, so does the calculated psi value PID in the ABS module (See Brake Pressure Transducer).

Steering Angle Sensor
The steering angle sensor, also referred to as a steering rate sensor, mounts on the steering shaft behind the dash. It is a dual vane laser optic sensor that sends pulses to the ABS module which represent steering wheel movement (See photo Steering Angle Sensor).

The steering angle sensor provides information about the direction of steering wheel movement as well as the rate of the movement. The steering angle sensor does not directly provide information about the steering wheel’s position in relation to center. Instead, steering wheel center is a learned value stored in the ABS module’s KAM. Steering wheel center, to the ABS module, is equal to the total number of sensor pulses from lock to lock then divided in half. Once the steering wheel center is learned, then each pulse from the sensor in either direction is an addition or subtraction from the learned center value. As you can see in the two steering wheel sensor data PID shots, anything right of center is a positive degree value and anything left of center is a negative degree value (See Steering turning right and steering turning left).

The steering wheel center is learned during what is called an “IVD initialization sequence” which will be discussed in more detail later.

On a model that is optionally equipped with IVD, but may not be equipped on the vehicle you are inspecting, you may find the reluctor for the sensor present, but no sensor mounted there (See Reluctor Without Sensor photo).

Lateral and Longitudinal Accelerometers and Yaw Rate Sensor
The lateral accelerometer, longitudinal accelerometer, roll rate and yaw rate sensors all have a similar purpose for different aspects of the vehicle body’s movement. The yaw rate sensor and lateral accelerometer are both present on all models with IVD. They are both three-wire sensors that work by varying their voltage values to the ABS module. The lateral accelerometer measures the lateral G-force (side to side) force on the body of the vehicle in turns. The sensor’s center voltage is idealistically 2.5 volts. Turns to the right increase voltage over 2.5 volts, while turns to the left decrease voltage under 2.5 volts. The yaw rate sensor measures the rotational G-force placed on the body of the vehicle in relation to its center of gravity. This sensor’s center voltage is idealistically 2.5 volts. Turns to the right increase voltage over 2.5 volts, and turns to the left decrease voltage under 2.5 volts. The longitudinal accelerometer measures G-forces in forward and reverse directions. The longitudinal accelerometer is only equipped on four-wheel-drive vehicles. Its purpose is to measure the vehicle response to the automatic engagement of 4×4 during an IVD event.

On CAN vehicles, the longitudinal accelerometer will be mounted together with the lateral, roll rate and yaw sensor in what is called a sensor cluster. A 2×4 vehicle may still have a sensor cluster, as shown in the picture, but will not have a longitudinal sensor inside of it. A sensor cluster communicates on the CAN network and will have four wires — power, ground, CAN low and CAN high (See Yaw Rate Sensor). On Explorers, you’ll find the yaw/accelerometer sensor mounted on top the drive tunnel buried beneath the center console and some HVAC duct work as seen in the photo.

Notice the screen captures labeled “Body Roll 1” and “Body Roll 2.” In Body Roll 1, I swerved to the right. In Body Roll 2, I swerved to the left.

As the body rocked one way then the other, the data PIDs indicated the G-forces created in each direction.

Roll Rate (shown as ROLLRAT in scanner screen shot) is the measured amount of body rotation in relation to the vehicle’s front-to-rear horizontal axis.

As for the longitudinal accelerometer (Seen as LONG_ACCEL on this scanner), notice how it behaves in the screen shots labeled “Acceleration 1”, “Acceleration 2” and “Deceleration.”

As I accelerated, the G-forces applied to the body were reflected as a positive value on the data PID (See Acceleration 1).

When I let up on the gas pedal, and allowed the vehicle to coast briefly, you can see that the measured G-forces dropped back down to zero as I coasted for a split second while switching from the gas pedal to the brake pedal (See Acceleration 2).

As I applied the brake to bring the vehicle to a stop, the measured G-forces then moved into a negative value on the scan tool (See Deceleration).

Active Brake Booster
The active brake booster is similar to the conventional vacuum brake booster. The difference is, the addition of a two-position brake pedal switch, a brake booster actuator, and a release solenoid. The actuator and release solenoids are so that the ABS module can apply and release vacuum in the brake booster to apply the brakes as it needs to. Doing so in this manner means that during an IVD event, the brake lights may light up to warn the driver behind them. This also means that the brake pedal can move by itself if the module commands it to do so. The module routinely does this as part of a self check at start up. The movement at self check is very slight and may not be noticed unless someone happens to have their foot on the pedal at just the right moment. The brake pedal switch in the booster is present so that the ABS module knows to compensate for the driver’s input on the brake pedal as well. The switch is a two-throw switch — one throw is normally open and the other is normally closed when the brake pedal is released. When the brake pedal is applied, the throws trade off so that the normally open throw closes while the normally closed one opens.

One of the easiest ways to inspect a vehicle to determine if it is equipped with IVD or not is to inspect the vehicle for the presence of an active brake booster. The easiest way to identify an active brake booster is the rectangular electrical connector that mounts directly in the case of the booster (See Active Brake Booster Photo).

Some of these boosters will also have an electric actuator, often visible from the outside, like on Expeditions but not on the Sport Trac this photo was taken of, that is controlled by the ABS module for the purpose of applying the brake pedal.

Putting It All Together
The ABS module watches the steering angle sensor, yaw rate sensor, wheel speed sensors and the brake input. The module is comparing the driver’s requested steering against the body’s response to that request. If the body does not respond to the driver’s input, or the driver does not respond far enough to the body‘s movement, then the module will apply and release the brakes as needed to use brake-steer to correct the body’s movement. Imagine what would happen if the rear of the vehicle were to begin to slide in a turn and begin to spin the vehicle around. The module would see the driver’s request for steering direction. It would see that the body of the vehicle has suddenly taken an exaggerated course, and it may also notice a sudden speed value change in the rear wheels. It would then “know” that the driver has entered a slide. At that time, it would likely apply the brakes to the wheels on the outside of the turn to help prevent the vehicle from entering a 360 degree spin-around.

Now, image a similar case, only this time it is the front wheels that break traction and the body of the vehicle is not responding to the driver’s demand to turn. The module would then “know” that the body is moving in a straight line when the driver doesn’t want to go straight. It would then likely apply the brakes to the wheels that are in the inside of the turn to brake-steer the body into the driver’s desired course. Now, imagine a system that also has RSC ability. If the driver enters a turn that is too sharp for the speed, then the ABS module will “know” by watching the roll sensor that the vehicle may be entering a rollover situation. In which case it may take measures to quickly slow the vehicle down.

To be aware of and compensate for the driver’s brake pedal efforts, the ABS module watches the brake pedal switches in the active brake booster. The module is aware of the combined brake hydraulic pressure created by the driver’s pedal efforts as well as the module’s own brake efforts by watching the brake pressure transducer. If needed, the module can command ignition timing retard and induce misfires as needed to lower engine torque during an IVD or traction control event.

The module does not just sit there and wait for something to go wrong before it reacts. If it did, then it would likely respond too late. It is constantly measuring the body’s roll-rate in turns. It decides when the vehicle is in a situation where rollover is possible. It will then take needed actions to reduce the engine torque and change brake torque as needed before it is too late. So if you get one of these vehicles in, and the complaint from the customer is that the vehicle bogs, skips and slows down going around curved interstate exits ramps, you really need to know how fast they are moving. It could be a problem with the roll-rate senor or it could just be that the ABS module is trying to save the driver’s life.

The module does not command IVD events while the vehicle is traveling in reverse, so the MLP sensor is technically a member of the Advance Trac system as well.

There will be an Advance Trac disable switch located in the vehicle for the driver to deactivate IVD functions. Pressing this switch to deactivate the Advance Trac will not disable the normal ABS functions of the module, only the stability assist and traction control abilities will be disarmed.

Let’s Fix Something
This 2002 Lincoln LS had the message “Check Advance Trac” displayed on the odometer as well as a flashing traction control light. The fault code retrieved from the ABS module was a C1277 — Steering Wheel Angle Offset Failure (See LS1).

The ABS module knows that while driving (VSS signal), the steering wheel should average around its physical center. If the steering wheel’s sensor is averaging more than 15 calculated degrees from center, then something must be wrong because it is not logical for the vehicle to be in a constant steer at highway speeds for long distances (not made for dirt track racers!).

Pulling up the steering angle sensor’s PID, with the steering wheel at a physical center shows the calculated angle is not within 15 degrees of physical center. In fact, it is at negative 302 degrees, or 302 degrees to the left (See LS2).

Inspection of the sensor found nothing wrong. It is possible that the steering wheel had been moved with the ABS module powered down… like from a dead battery maybe, or the key just barely turned enough to unlock the steering wheel and not power up the module. The steering angle needed to be retrained. To do this, an IVD initialization sequence needed to be performed. This is when the yaw rate sensor and the steering angle sensor are re-centered.

After accessing the steering angle sensor recalibration test (See IVD initialization 1 and 2), the tool instructs the technician to not bounce the vehicle and for him to rotate the steering wheel from lock to lock. When retraining the yaw rate and lateral accelerometer, is important to have the vehicle on level ground with no extra weight inside the vehicle… including the technician, so it will be necessary to reach in through an open window to operate the steering wheel. After retraining the steering wheel sensor’s center, the fault code is cleared and ready to return the vehicle to the customer. Anytime one of the following fault codes are cleared:

  • C1277 Steering Wheel Angle 1 and 2 Circuit Failure (Some years and models may list this as “Offset failure”)

  • C1278 Steering Wheel Angle 1 and 2 Signal Fault

  • C1279 Yaw Rate Sensor Circuit Failure

  • C1280 Yaw Rate Sensor Signal Fault

  • C1281 Lateral Accelerometer Circuit Failure

  • C1282 Lateral Accelerometer Signal Fault

  • C1285 Booster Solenoid Circuit Failure

  • C1287 Booster Pedal Force Switch Circuit Failure

  • C1288 Brake Pressure Transducer Input Circuit Failure

  • C1440 Pressure Transducer Input Signal Failure

  • C1516 Roll Rate Signal Fault

  • C1517 Roll Rate Sensor Circuit Fault

  • C1730 Reference Voltage Out of Range (+5 V)

  • C1991 Module Calibration Failure

  • C1996 Active Yaw Control Disabled

  • C2769 Longitudinal Acceleration Sensor Circuit Failure

  • C2770 Longitudinal Acceleration Sensor Signal Fault

  • C2777 Sensor Cluster Bus Failure

  • C2778 Sensor Cluster Power Supply Failure

The IVD initialization sequence will have to be performed. The ABS module is programmed to know the difference between clearing one of the above codes, and when you are clearing any other code that might be stored in the ABS module. When one of these codes is cleared, it is replaced with a C1998 for “Module Calibration error.” As seen in the screen shot labeled “Calibration Failure.”

A C1998 is a code that does illuminate the Advance Trac light and will display “Check advance trac” on the message center. Let’s say a customer comes into your shop with an Advance Trac light on with a “check Advance Trac” message on the display. Your tech pulls a fault code C1440. This may be a simple, straight forward pressure transducer replacement. The job itself is very easy. However, when the tech clears the code, a C1998 replaces it and the light and warning message still remain.

If you do not have a scan tool capable of performing an IVD initialization process, like the OE Ford tool, then you will have to outsource the job. As you know, this can be a sticky situation to be in. To answer this need, you’ll find that alternative scan tool makers are stepping up and adding this function to their tool (See IVD on AE).

Make sure your scanner will perform this function before you attempt to repair one of these systems. I hope you have enjoyed the information and have the opportunity to profit from it soon.

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