Relays/Switches/Sensors

Relays: The main relay is attached to the outside of the power distribution center (PDC). The main relay has two functions. First, when the relay is energized, it supplies power to the valve/ solenoids in the valve block assembly and the pump/motor relay coil. Second, when the relay is deenergized, it supplies a ground to turn on the ABS warning lamp. The main relay is controlled by the CAB. The CAB energizes the main relay when it receives an ignition input signal.

The pump/motor relay supplies power to the pump/motor. It is attached to the outside of the power distribution center (PDC). The relay coil is powered by the main relay and the coil ground is controlled by the CAB. The pump/motor relay is a special five-terminal relay and should not be replaced with a standard relay. If either is defective, both relays must be replaced. They are serviced as an assembly.

Reservoir and Fluid Level Switch - FLS2 (LTCS only): The reservoir at the HCU includes a fluid level switch (FLS-2). This reservoir is connected to the master cylinder reservoir by a low pressure hydraulic tube. The CAB monitors the fluid level via the fluid level switch (FLS-2). When the reservoir is properly filled the fluid level switch is closed. If the fluid level falls below an acceptable level, the fluid level switch will open. When the CAB detects a low fluid condition (FLS2 OPEN), it will inhibit the antilock and traction control function, and turn on the ABS and LTCS warning lamps.

Traction Control Indicator Lamps (LTCS only): The green traction control function lamp is located in the message center. It is used to inform the driver that the system has entered the traction control mode. The lamp is controlled by the CAB. The CAB turns on the lamp by directly grounding the circuit.

The amber TRAC OFF lamp (warning lamp) is also located in the message center. It is used to inform the driver that the traction control function has been turned off by the CAB. This lamp will also come on if the traction control switch is used to turn off the system. The traction control warning lamp is also illuminated for approximately 2 seconds after the ignition is turned on. With the ignition in crank and the traction control system in thermal protection mode, the traction control warning and function lamps will be illuminated.

There are five fuses used by the antilock and traction control system. Three of these are located in the fuse block and two are in the power distribution center (PDC).

Vacuum Booster/Master Cylinder/Pedal Travel Sensor (LTCS only): The Teves Mark IV (LTCS) antilock brake system uses a special vacuum booster and master cylinder.

The vacuum booster used by the Teves Mark IV antilock brake system is the new "0" lost travel reaction disc type booster with the addition of a pedal travel sensor.

The pedal travel sensor (PTS) is a linear variable displacement transducer with seven steps that provide brake pedal position information to the CAB.

The master cylinder is a standard tandem design except that the compensation ports have been replaced by special center valves. The center valve is a spring-loaded ball and seat design that is unseated by a stop in the master cylinder when the brakes are released.
A fluid level switch is located in the master cylinder fluid reservoir. The switch closes when a low fluid level is detected. The FLS-1 turns on the brake warning lamp grounding the lamp circuit. This switch does not disable the ABS system.

Wheel Speed Sensors and Tone Wheels: One wheel speed sensor (WSS) is located at each wheel and sends a small AC signal to the control module (CAB). This voltage is generated by magnetic induction when a toothed sensor ring (tone wheel) passes by a stationary magnetic sensor (wheel speed sensor). The CAB converts the AC signals into digital signals for each wheel.

The front wheel sensor is attached to a boss in the steering knuckle. The tone wheel is an integral part of the front axle shaft. The rear speed sensor is mounted in the caliper adapter plate and the rear tone wheel is an integral part of the rear rotor hub. The wheel speed sensor air gap is not adjustable. The correct wheel speed sensor air gap is from 0.52mm to 1.64mm. All wheel speed sensors have a resistance between 950 and 1250 ohms.

The four wheel speed sensors are serviced individually. The front tone wheels are serviced as an assembly with the outer C.V. joint housing. The rear tone wheels are serviced as an assembly with the rotor.

Correct antilock and traction control system operation is dependent on tone wheel speed signals from the wheel speed sensors. The vehicle's wheels and tires should all be the same size and type to generate accurate signals. In addition, the tires should be inflated to the recommended pressures for optimum system operation. Variations in wheel and tire size or significant variations in inflation pressure can produce inaccurate wheel speed signals; however, the system will continue to function when using the minispare.