System Components

SYSTEM COMPONENTS

- Controller Antilock Brake (CAB)
- Hydraulic Control Unit (HCU)
- Pump Motor
- Four Wheel Speed Sensors/Tone Wheel assemblies
- ABS warning indicator
- TCS (ASR) event indicator
- TCS (ASR) warning indicator
- Brake Switch (BS)
- Brake Lamp Switch (BLS)
- TCS Switch (TCSS)
- K - ABS
- CAN C Bus
- Fuses, grounds, and wiring

ABS AND TCS (ASR) INDICATORS

This system is equipped with an ABS warning indicator, TCS (ASR) warning indicator, and TCS (ASR) event indicator to alert the driver of a malfunction/event it has detected. The CAB can request the illumination of the ABS warning indicator, TCS (ASR) warning indicator, and TCS (ASR) event indicator via CAN C BUS. The CAB controls the ABS warning indicator by:
- Light steady during an initial test at the beginning of an ignition cycle to function as a bulb check
- Light steady when a system malfunction exists (DTC)
- Light steady If you have not meet the speed required to reset/retest the ABS components

The Instrument Cluster (IC) controls the indicators. The Instrument Cluster transmits a message over the CAN C Bus relating to diagnostics and current lamp status for the ABS and TCS (ASR) indicators. The CAB can control the operation of TCS warning and TCS event indicators by:
- Both light steady with engine off and both go out with engine running
- TCS warning indicator lights steady when a TCS malfunction exists
- TCS event indicator will flash when TCS is in an active event

CONTROLLER ANTILOCK BRAKE (CAB)

The CAB is mounted directly to the Hydraulic Control Unit (HCU) that includes a microprocessor and twelve solenoids that control valves that control brake pressure during antilock braking or traction control events. The CAB also has circuits that monitor the following:
- Double brake switch outputs are monitored to determine whether or not to prepare for possible ABS braking
- Wheel Speed Sensors are monitored to determine when a wheel is tending to lock up. The CAB will operate the valves in the HCU to control braking pressure during ABS braking
- Detect ABS system related problems and take diagnostic action
- Able to execute self-tests and output control commands

HYDRAULIC CONTROL UNIT (HCU)

The HCU on the Bosch 5.7 has an integral valve body for controlling the front and rear brakes. Within the HCU are inlet, outlet, and shuttle valves, to release brake pressure as required to avoid wheel lockup, keeping the wheels rolling, and maintain optimum deceleration with stability. The Pump Motor is attached to the HCU which works with the ABS and TCS and is controlled by the CAB. The primary function is to provide extra amount of fluid when needed.

SWITCHES/SENSORS

BRAKE SWITCH (BS): This switch prepares the CAB for a possible antilock event. The CAB uses an output state voltage from the BS when the brake pedal is either released/depressed. The Fused Ignition Switch Output circuit supplies 12 volts to the BS. A released brake pedal will close the BS circuit and the BS Output circuit supplies 12 volts to the CAB. When the driver depresses the brake pedal, the BS Output circuit voltage drops to 0 volts and the CAB senses the brake pedal state. This tells the CAB what position the brake pedal is currently in to make an ABS event possible. When using the DRBIII in Inputs/Outputs, the BS and BLS will read opposite switch states. Note: The BS and BLS are in the same switch housing.

BRAKE LAMP SWITCH (BLS): This switch prepares the CAB for a possible antilock event. The CAB uses an output state voltage from the BLS when the brake pedal is either depressed/released. The Fused Ignition Switch Output circuit supplies 12 volts to the BLS. A depressed brake pedal will close the BLS circuit and the BLS Output circuit supplies 12 volts at the CAB. When the driver releases the brake pedal, the BLS Output circuit voltage drops to 0 volts and the CAB senses the brake pedal state. This tells the CAB what position the brake pedal is currently in to make an ABS event possible. When using the DRBIII in Inputs/Outputs, the BS and BLS will read opposite switch states. Note: The BS and BLS are in the same switch housing.

TRACTION CONTROL SYSTEM SWITCH (TCSS): This switch signals the CAB to either turn ON or OFF the TCS. The driver can toggle the TCSS, which receives 12 volts from the D (+) Relay Output circuit. Depending on the position of the TCSS, open or closed, the CAB receives the TCSS state voltage on the TCS Switch Sense circuit. When 12 volts are applied to the TCS Switch Sense circuit, the TCS is OFF. When no voltage is present, the TCS is ON.

WHEEL SPEED SENSORS AND TONE WHEELS: The Bosch 5.7 system uses one passive WSS on each wheel. The sensor measures the wheel speed by monitoring a rotating tone wheel. As the teeth of the tone wheel move through the magnetic field of the sensor an AC voltage and amperage is generated. This signal frequency increases or decreases proportionally to the speed of the wheel. The CAB monitors this signal to check for a sudden change in single or multiple wheel decelerations. If the deceleration of one or more wheels is not within a predetermined amount, the CAB takes control for antilock action through the HCU. Each WSS has a magnetic inductive pick up coil (WSS) that is mounted to a fixed component. There is an air gap between the tone wheel and the speed sensor assembly. Diagnostically, the coils of the Wheel Speed Sensors have the same amount of resistance. When measured across the CAB harness connector terminals, the resistance should be between 1100 - 1800 ohms.

Correct ABS operation is dependent on Tone Wheel speed signal from the WSS. The vehicle wheels and tires should all be the same size and type to get accurate signals. In addition, all tires should be at recommended tire pressures.