Shuttle Valve Switches
The ABS modulator houses two shuttle valves. The valves are actuated when brake pedal pressure is applied via the master brake cylinder. The shuttle valves have a micro switch attached to them and a set of resistors located between them. The resistance of the circuit controlled by these shuttle valve switches will change depending on the position of the shuttle valve. With no brake pressure applied, the current flows through all three resistors.With one switch closed, the current flows through two of the resistors. And when both valves are closed, current flows through one resistor. This gives a circuit resistance of approximately 3 K, 2K and 1K respectively. The signal from the shuttle valve is used by the ABS ECU to detect the correct operation of the brake master cylinder and the integrity of the braking circuit.

Shuttle Valve Switch Failures

The bad news is that shuttle valve switch failure messages are more or less a death sentence for the ABS modulator. The failure may start off in the begninning as a intermittent reoccurance but will eventually progress into a full blown permanent failure. I would first recomend following the check list outlined in the repair options below; make sure that the failure is not an electrical connection failure. The next course of action is to then clear the fault and take the vehicle for a test drive. If the fault returns, then consider installing the Shuttle Valve Switch Kit (200 USD). The shuttle valve switch kit is unlikely to repair the problem, but it is a relative inexpensive repair option. If the fault continues to return, then the replacement of the entire ABS modulator assembly is required. This replacement can cost roughly 2000 USD and should be performed by a trained Land Rover mechanic.

Wheel Speed Sensors
The sensors used on the Discovery 2 are incorporated into the wheel bearing on both the front and rear hubs. This is a sealed unit and has no replaceable parts. The wires from the wheel speed sensors is a twisted pair. The wire twists provide improved resistance to electrical interference. The signal produced from the wheel speed senor is an AC sine wave. The signal is generated in the inductive sensor by a 60 tooth reluctor ring. This reluctor ring is machined into the wheel bearing inner race. The frequency of the sine wave signal supplies the ABS ECU with the information it needs to determine the speed of the individual wheels. Each wheel speed sensor has a resistance of 950-1100 ohms.

Wheel Speed Sensor Failures

Wheel speed sensor faults are a very common occurance. Where as the actual failure of a wheel speed sensor is not a common event. The most common Logged ABS wheel sensor faults are caused by worn brake pads. A worn brake pad or even some new non-Land Rover Brake pads can set up a resonance vibration in the wheel hub. This vibration can interfear with the ABS sensor signal. When the ABS sensor signal is interrupted an ABS wheel speed sensor fault can be logged. Best advice is to clear the fault and take the vehicle for a test drive. Other causes of the wheel speed sensor fault can be caused by a bad wheel hub. Recomended approach is to inspect the wheel hub and make sure the hub does not have excessive play. A bad wheel hub can cause the ABS reluctor ring to deviate from the sensor range.

Anti-Lock Brake System Operation (ABS)
The SLABS ECU is the Brains behind all of the vehicle Anti-Lock Braking operations. The SLABS ECU is a small rectangular black box that is located under the left hand seat. The ECU monitors various vehicle sensors and conditions and determines through a set of internal software controls, what is happening when you press the brake pedal. The SLABS ECU is constantly comparing the overall calculated vehicle speed to each individualy measured wheel speed. detects a variation in speed of both wheels on a single axle when the brakes are applied, it operates the inlet solenoids in the modulator unit relative to that axle. This has the effect of removing the brake pressure through the brake system on the axle with the slipping wheels.

Traction Control Operation (TC)
The SLABS ECU also manages all of the Traction Control Operations of the vehicle. In many ways, the TC functions are simply an extention of the existing ABS system. The same physical valves and sensor assemblies that can detect when your wheels are locked up, can also detect is your wheels are slipping too much. The primary difference between the TC and the ABS is the software pathway that is triggered inside the SLABS ECU. The Discovery 2 Vehicle does not employ a Viscous Coupling between the front and rear drive lines. Effectively the Discovery 2 vehicle has and open center, rear and front differential. This means that if any one of the four wheels looses traction, all of the engine power goes to the slipping wheel. This brings your forward progress to a complete halt. The vehicle relies solely on the TC system to modulate the open differentials and help control wheel slippage. The ABS ECU monitors each individual wheel speed sensors and the overall road speed. If one wheel on one axle or both wheels on one axle increase in speed compared to the other axle, the ABS pump will trigger. The ABS ECU then diverts hydraulic pressure to either one wheel or both wheels on the axle that has lost traction. The ABS system will operate up to 32 MPH. All ETC functions are suspended when the brake pedal is depressed. The TC can be active for an indefinite period of time.

Electronic Brake Distribution (EBC)
The EBC system is another extention of the ABS subsystem. Previous vehicle designs would limit the brake pressure to the rear of the vehicle through mechanical pressure regulators. It is important to regulate the amount of rear brake pressure during emergency braking situations. If the rear brakes are not limited, the rear of the vehicle could begin to overtake the front and cause a spin. The distribution of brake pressure is managed electronically. The ABS system electronically modulates the rear brake valves to limit the ammount of brake force applied.

Hill Descent Control (HDC)
The purpose of the HDC system is to provide control when descending steep gradients. The system applies the brakes automatically to assist the engine braking. The amount will depend upon the gear and the amount of throttle applied. The system give security in circumstances where the wrong gear has been selected. The ECU can accomplish this by distributing the braking pressure primarily to the downhill axle using the principles of the ABS system. Additionally, wheel slippage can be controlled by employing some of the ABS principles. The driver activates the system by selecting the low range on the transfer box and pressing the HDC switch. It is important that the brake pedal is not pressed as HDC mode will be canceled. HDC operates in all gears when in low range and can also function in neutral. There is no direct signal from the gearbox to the ABS ECU as to which gear has been selected. The ECU calculates this by using signals from the engine management system, engine speed signal and its own calculated road speed. If the ECU detects that no gear is selected or that the clutch is depressed then a fixed target speed is selected and the system will operate for 1 minute only. When the HDC system is in operation, the brake lights are illuminated by the ABS ECU regardless of the brake pedal position.


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