Antilock Brakes / Traction Control Systems: Description and Operation
ABS Components:
Split system
The operation of the ABS system remains unchanged; however, the system is now split similar to the XJS Range vehicles. The ABS hydraulic actuation unit and reservoir assembly are mounted on the brake and accelerator pedal housing. The motor/pump unit is located separately on the right side of the front bulkhead. A steel pipe carries high pressure fluid between the two assemblies.
ABS electronic control unit
The ABS ECU is new and incorporates one new function. The road speed input to the instrument pack is provided by the ECU from the right front wheel speed sensor. The previous differential mounted road speed sensor has been deleted. In addition to this new function, the ECU has an improved warning indicator circuit that allows deletion of the former ECU relay and one diode. The input for brake application is sourced from the high mounted stop light instead of from the brake switch as in previous model years.
System fuses
The ABS fuses are incorporated into the left fuse panel on the driver's A post, deleting the former in-line fuses.
Anti-lock System:
General System Description
The anti-lock braking system eliminates wheel lock by controlling hydraulic pressure applied to the brake calipers. This system, which is assisted by the power hydraulic system, also reduces tire wear and improves vehicle stability during severe braking conditions. Modulation of hydraulic pressure is achieved by a modulator containing three solenoid actuated valves; one valve in each hydraulic line to front calipers and one in a common hydraulic line to rear calipers. Each valve is controlled by an Electronic Control Unit (ECU) which also activates a hydraulic pump to return used fluid back to the modulator. The ECU contains electronic circuits which actuate the valve solenoids from inputs generated by speed sensors comprising variable reluctance coils located close to an exciter disc attached to each wheel hub. Each wheel deceleration rate is then calculated and pressure is applied to the calipers.
The hydraulic components necessary for the anti-lock control are integrated with a servo-hydraulic booster and its operating components; brake fluid is used as the single working fluid. An energy source consisting of an electric motor, a pump, a pressure switch and an accumulator provides high pressure fluid to operate the booster during normal braking as well as providing a means of increasing brake line pressure when required during anti-lock braking. Also contained in this assembly are the master cylinder the solenoid valves and the reservoir.
In the installation the front hydraulic circuit is operated conventionally by the master cylinder, assisted by the booster, but the rear circuit is operated directly by the controlled, pressurized fluid in the booster. The front circuit is static and the rear circuit dynamic. However when the anti-lock control is required, the front also becomes dynamic. Then inlet and outlet solenoid valves operate in each of the three circuits to control the pressure as required to prevent wheel-lock.
Sensors are installed at each wheel. Their wheel speed related signals are processed by an electronic control module which operate the solenoid valves in the hydraulic system. The front wheels are controlled individually and the rear wheels, which are on a single hydraulic circuit, are controlled together on the select low principle. Therefore a tendency for one wheel to lock result in control of both wheels according to the need of the "locking" wheel.
The state of the anti-lock system is continuously monitored by the control module, which switches off the system automatically if a failure is identified, illuminating a warning lamp and leaving full, boosted braking to all wheels. Warning lamps would also indicate low accumulator pressure or low fluid level in the reservoir.
Should the front hydraulic circuit fail, pedal TRAVEL will increase. If the rear circuit fails, pedal EFFORT will increase.