The basic principle of traction control is rather simple. A tire is breaking free and starting to spin so we’re going to place a drag on that wheel so that two things can happen. One is we want that tire to have a chance to regain its grip on the road surface. And two, we want to transfer some of that wasted drivetrain energy at the “loose” wheel over to a wheel (of the same axle) that has a better grip on the road surface. This article will focus on Ford vehicles, but a lot of this will carry over to other makes, but specifics here will refer to Ford.
Before we can get into traction control, we need to talk about ABS. Typically, and for what we are going to talk about, traction control is an extension of the ABS system, which makes the ABS system the foundation. ABS and traction control together form the foundation for stability assist. But the key point to understand here is that ABS, traction control and stability assist are all functions of the ABS module.
You can have certain faults in stability assist that will not affect traction control or ABS. You can have certain faults in traction control that will not effect ABS, but faults in traction control will affect stability assist. Faults in ABS will affect both traction control and stability assist. If you don’t have ABS, you can’t have the others. Stability assist is built on top of traction control and ABS. Traction control is built on top of ABS. ABS is the root system. Let’s look at basic ABS fluid flow. The illustration labeled ABS1 represents brake fluid flow in a system that is under normal braking.
The illustration ABS1 shows the normal flow of brake fluid through the HCU when there is no lock-up condition. The fluid is pushed by the master cylinder though the inlet/isolation valve in the HCU and to the wheel hydraulics. However, as seen in illustration ABS 2, if wheel lock up is detected, the ABS module will command the inlet valve to close so that is blocks off fluid flow from the master to the wheel hydraulics.
This prevents the driver from adding more fluid pressure to the wheel and is sometimes enough strategy by the ABS module to prevent lock up and no other step is needed. At this point, the outlet valve is still closed. This is called brake pressure hold mode and is employed just before actual wheel lock up occurs. If the wheel does lock, then the ABS enters brake pressure relief mode. It then opens the outlet/dump valve while still keeping the inlet/isolation valve closed. The fluid is then bled off from the wheel hydraulics and enters the accumulator. The accumulator provides a place for the fluid to go. However, that is not all that happens in an ABS event. The valves are simply open or closed. Instead, they are cycling. When the wheel starts turning again, the ABS resumes the normal braking fluid flow, but when the wheel begins to lock, the hold and relief modes are entered again. This entire time, the driver’s foot is constantly on the brake pedal. Eventually, this fills the accumulator to capacity. If the accumulator fills completely, there will be no where for the fluid to go so ABS action will be lost. The accumulator must be emptied. This is where the pump and motor come into play.
In Illustration ABS 3, the pump motor is turned on. This pushes the fluid back into the master cylinder, pushing the driver’s foot pedal back up, and allowing the accumulator to empty.
This returns the fluid from the accumulator back into the master cylinder.
Traction Control Operation
OK, now that we see how the ABS system fluid flows, we can build upon that to look to see how traction control works. Look at the illustration labeled Traction control 1.
All of the components listed are the same as the ABS illustrations, except for the addition of the traction control inlet/isolation valve. In this illustration, normal braking is seen. The fluid fluids from the master down through the inlet valves to the wheels. The non-drive wheel has to pass through the traction control inlet valve as well as the ABS inlet valve to get to the wheel. Any ABS event that happens on this system will function the same as previously discussed.
Should the ABS module see the drive wheel spin faster than the non drive wheel, it will enter traction control mode.
In traction control mode, the ABS module then commands the TC inlet valve to close. That blocks off the fluid flow to the non drive wheels. It then commands the pump on. With the ABS inlet valve open to allow flow to the wheel hydraulics and the outlet/dump valve to the wheel hydraulics blocked off, the ABS module can now apply the brake to the spinning drive wheels. There is a little more to the plumbing work than what is drawn here.
For example some vehicles may have a gravity feed line from the master cylinder reservoir to supply the pump with fluid, but you can see what is going on with this.
The ABS module can also command ignition timing retard, transmission up-shifts, fuel shut down and, on some vehicles, throttle control measures to reduce the power from the engine to the wheels. Cutting the power to the wheels in this manner also prevents tire spinning.
This older Thunder Bird throttle body had two sets of throttle plates. This was a primitive way of giving the ABS module a way to reduce throttle if the wheels spin. Normally, the second set would be open at all times and throttle control would be performed by the set connected to the throttle cable. But, the ABS module could control the throttle by use of this early throttle-by-wire motor design to return the engine to the desired RPM.
Stability Assist/Roll Stability Control
Now you see how the ABS module is achieving ABS functions and traction control functions. Next comes the stability assist function. To achieve this, the ABS module is given enough valves to provide ABS and traction control ability to each individual wheel plus some new sensor inputs and and software tweaks. One new sensor input that is given is a Yaw rate sensor. What is a yaw rate sensor? The text book definition is a sensor that measures the vehicle’s rate of rotation around its vertical axis. I’m not too proud to admit that when I heard that definition for the first time, I needed it to be defined a little clearer. So to be more clear, first imagine a car sitting in your shop. Now, in your mind, take a bird’s-eye view of that car and find the dead center of that vehicle. Imagine driving a long spike down through the center of that car, down through the roof, then the floor, and into the ground pining the car to the earth. Now, with a bird’s-eye view still, spin that car like a propeller. That is what the Yaw rate sensor is measuring.
Another new sensor input is the lateral accelerometer. A lateral accelerometer is used to measure lateral sideways force. Any spinning ride at the fair where you sit in the seat and the ride throws you to the other side of the seat, is a good example of what the lateral accelerometer is measuring.
A Roll Rate sensor is another sensor that is added to stability assist systems, but only if the vehicle has roll stability control added. Usually this is only found on vehicles that are prone to tip over in high speed turns, like SUVs, trucks and vans. It is measuring the body roll.
The steering angle sensor is not so new, but is present on stability assist vehicles. It is used by the ABS module as a reference for the driver’s intended course. It is a laser optic sensor that pulses as the laser passes through windows in a plate. All the other sensors are compared to the steering angle sensor to determine if the body is following the driver’s desired path. Let’s hope the driver’s steering is correct in the situation then.
Now all the ABS module has to do is use its new sensor inputs to see what the vehicle’s body is doing in terms of yaw, lateral force, and sometimes roll rate, and compare those to the steering angle sensor. If the body is not tracking in the driver’s desired manner, then the ABS module can apply ABS and traction control functions as needed to various wheels to brake-steer the body accordingly, as well as needed measures to reduce power to the wheels.
Wheel Speed Sensors
There are two main types of magnetic wheel speed sensors — passive and active. Both use a tone ring to reference against and each look very similar to one another. As the raised portions of the tone ring pass by the tip of the sensor, the sensor creates a signal.
The passive type of sensor is a simple AC signal generator. As the teeth pass the sensor, it produces a sine wave. As the wheel speeds up, the frequency of the sine increases as well. The ABS module reads the frequency to determine the speed of the wheel. The downfall with this design is that the frequency is not the only thing to change with rotational speed. The amplitude also changes.
The faster the wheel turns, the greater the peak voltages are. The high end of that aspect is not a problem. The problem lays in the low end. When the wheel is turning at very low speeds, such as under 3 mph, the sensor becomes unreliable because it is barely producing a readable signal to the ABS module, especially when wiring resistance and RF noise are factored in. You may notice that a scan tool will show the wheel speeds starting at 3 mph for these types of sensors. Some scan tools might report 3 mph when sitting still, and others might report 0 mph until a vehicle speed of 3 mph is achieved and suddenly the PID will jump from 0 to 3 mph. That is normal for a vehicle with passive wheel speed sensors. These sensors can be tested for coil circuit integrity with an ohm meter. More preferred methods of testing are AC volt meter for voltage output and oscilloscope.
To get reliable readings at very slow speeds, the active wheel speed sensor is employed. An active wheel sensor has a tiny processor chip in it that reads the tone ring with the same signal generator method as the passive speed sensor, but does not use that conventional signal to share with the ABS module.
Instead, the ABS module sends the sensor a 9 volt power supply and the active sensor produces a square sine, also referred to as a modified sine wave. It is a digital high/low toggle rather than an analog voltage ramp-up like with passive sensors. Since the chip can read the minute voltage generated at the coil end of the sensor and convert that to a signal to the module, it can read speeds below 3 mph reliably.
The amplitude of this signal to the ABS module does not change with speed, only the frequency does. An ohmmeter can not be used to test the coil side of the sensor. If you use an ohmmeter on it, you will get a measurement because the ohmmeter will find a path down one wire, through the chip, and back up the other wire. But that will not be an indicator of the coil windings at the sensor tip. The preferred methods are with an amp meter that can measure in the 7mA to 14mA range and an oscilloscope. A DC voltmeter can be used to test the ABS module’s ability to send voltage to the wheel speed sensor. In the picture here, labeled active wss, the housing of the sensor has been ground away so that you can see the tiny little processor chip inside.
The pump is typically an part of the HCU (Hydraulic Control Unit) on modern ABS/Traction control systems.
The pump motor is typically only sold as a part of the HCU, but if you were to remove the pump from the HCU (which I don’t recommend due to lost of core value and damage), you’ll find that the shaft end of the pump has an elliptical lobe. That can be seen in the photo labeled pump shaft.
That offset lobe pushes two… valves in the HCU. It is the action of these valves that serve as a pump. You can see the ends of them in the photo labeled pump bore.
The HCU houses all of the inlet and dump valves, as well as the pump on modern vehicles. The ECU (Electronic Control Module) houses the coils for the valves. Again, this is modern vehicles such as upper 90s and newer. The coils are electro magnets. They slip over the bores of the valves. The valves are inside these little housings and it is magnetism from the coils that make the valves move in their bores.
The coils make the valves move one direction, and return springs inside the bore make the valve return when the coil is de-energized. You can see the ECU to HCU coil and valve relationship in the photo labeled HCU with ECU.
Let’s take a look now at what happens when things don’t work like they should.
Below, in the figure labeled LF WSS, this LF wheel speed sensor was weak and would die out at low speeds. It worked fine at higher speeds, but would cause a false ABS activation. You can see it here dropping to zero mph intermittently. A bad tone or severely worn wheel bearings might also cause a reading like this.
In the next screen shot below, is a 2003 Crown Vic that had an ABS light, a red brake light and a check engine light on. It set PCM failure codes P0500, P1502 and U1039 all pointing a loss of vehicle speed information, as seen in the screen. Ignore the P0401 (EGR low flow) code for this discussion. That is an unrelated issue.
The PCM on this vehicle gets its vehicle speed information from the ABS module. Since the warning lights were on with no ABS fault codes reporting in, yet the PCM set several VSS codes, this pointed to an ABS module that was not communicating. A network test found that the ABS module did not respond to the scanner. That can be seen in the screen shot labeled network test.
To test this, first the ABS ECU was accessed. The connector was unplugged. Power supplies and grounds were tested — they passed. Next, the network communication wires were tested with an ohmmeter from the ABS module’s connector to the DLC inside the vehicle. The network wires were good. A new module fixed this vehicle. This ABS module on this vehicle was “plug and play,” no programming was required.
This 2001 Windstar came in with a blown fuse causing no communication with the ABS module. The odometer read all dashes and the ABS light and red brake lights were on. The ABS module would not communicate with the scanner until the fuse was replaced. Replacing the fuse seemed to fix the symptoms, but is it fixed? Inspection of the ABS module found fluid leaking from out of the connector. That fluid could not be seen until the unit was unplugged from the harness connector as seen below.
Where did it come from? The cruise control disconnect switch at the bottom of the master cylinder is where as you can see in the photo below.
The switch leaked at the pins in the connector area. The weather seals, meant to keep water out, held the brake fluid in the connector. The fluid, forced by gravity, traveled into the wiring. The wiring acted as plumbing and piped the brake fluid down under the vehicle and into the ABS module. That is not all. It also flowed into the speed control module as well, as can be seen in the photo below of the speed control module.
As with the ABS module, the wetness couldn’t be seen until the module was unplugged. The fix for this vehicle was a new disconnect switch, ABS module, speed control module, and wiring harness. This ABS module must be programmed.