System Operation
CIRCUIT OPERATIONThe Anti-Lock Brake System prevents wheel lockup during braking operations. This enables the driver to maintain vehicle stability and steerability' while braking.
Anti-Lock Brake System ECU (Z108)-- is a computer that controls ABS system operation by constantly monitoring the vehicle's 4 wheel speeds when the ignition is in position II. If an impending wheel lock up is detected during this monitoring, the Anti-Lock Brake ECU will apply voltage to the various inlet and outlet valve solenoids contained in the ABS Booster Unit (Z103). Operation of the solenoid valves regulates the pressure applied to the each wheel brake caliper and therefore prevents wheel lock-up.
The Anti-Lock Brake System ECU (Z108) also has diagnostic capabilities that allow it to detect faults that may impair the system's efficiency. If a fault occurs, the ECU informs the operator of a problem by illuminating the ABS or the ETC warning light. The ECU also illuminates the warning light when the ignition is first placed in position II. The ABS warning light will remain illuminated until the ECU completes a self check of the system. When the ECU sees all wheels reach a speed of 7 kph (5 mph), the self check is completed and the ABS warning light turns off. If a fault is detected during the self check, the ABS or the ETC warning light will remain on and a fault code will be stored in memory to aid in servicing the system. The fault code can be retrieved using a diagnostic tester or by flashing the ABS warning light.
ABS Booster Unit (Z103) -- contains 2 isolation solenoid valves and 4 pairs of solenoid control valves which are grounded through the ground strap. The pairs of solenoid control valves each include a fluid pressure inlet and outlet valves that control ABS braking to 1 wheel. The Anti-Lock Brake System ECU (Z108) operates these valves by applying battery voltage to them. The valves are designed to decrease, hold or increase pressure to retain wheel rotation and optimum braking.
The 2 isolation valves consist of 2 solenoid valves that control fluid inlet and outlet. Their function is to disconnect or isolate the master cylinder from the servo cylinder and to connect the servo cylinder to the reservoir return during ABS functions.
Wheel Speed Sensors (X137, X140, X158, X161) -- located at each wheel. The speed sensors generate an AC voltage signal as a magnetic toothed ring rotates past the stationary sensor pick-up. The Anti-Lock Brake System ECU (Z108) calculates the wheel speed by measuring the frequency of the AC voltage signal generated by the sensors.
ABS Hydraulic Pump (M102) -- The hydraulic boost for the system is provided by the ABS Hydraulic Pump (M102), which is controlled by the ABS Pump Relay (K102) and the ABS Pressure Switch Unit (Z104). The Pressure Switch unit incorporates 3 electro-mechanical switches. The first operates the pump; the second illuminates the low pressure condition and that ABS functions should be curtailed. The Hydraulic Pump also contains a non-return valve, a low pressure inlet filter, and a pressure relief valve to protect the system.
When low pressure occurs in the brake system, a switch in the Pressure Switch Unit closes to ground the coil of the Pump Relay through the WO wire. The Pump Relay now energizes and applies battery voltage from fuse F4 to the Hydraulic Pump through the closed relay contacts and the NR wire. The Hydraulic Pump runs to increase pressure because it is grounded through the B wire. When sufficient pressure is developed in the system, the Pressure Switch opens to de-energize the Pump Relay and to turn off the Hydraulic Pump.
ABS Warning Relay (K103) -- The ABS Warning Relay (K103) circuit ensures that the ABS warning light illuminates if the Anti-Lock Brake System ECU (Z108) loses power. If the Warning Relay is not energized, the warning light illuminates because it is grounded at E300 through the BS wire and the relay's closed contacts. When the Warning Relay is energized, the relay's contacts open to interrupt the ABS warning light's ground path. The Warning Relay is energized whenever the ABS Load Relay is energized.
ABS Load Relay (K101) -- The Load Relay is energized when the ignition is in position II. Voltage from fuse F2 is now applied to terminal 9 of the ECU. This signals the ECU to energize the Load Relay by applying voltage from terminal 8 to the coil of the Load Relay. With the Load Relay now energized, fuse F1 applies voltage to the Warning Relay to energize through the NK wire.