Antilock Brakes / Traction Control Systems: Description and Operation
4WAL EHCU Features:
4WAL Hydraulic Control Unit Components:
Using cycling action, the 4WAL system hydraulic control unit quickly changes the LF, RF, or rear channel hydraulic pressure during antilock braking. The control unit contains:
^ LF, RF, and rear isolation solenoid-operated valves (ISO valves)
^ LF, RF, and rear pulse-width modulation solenoid-operated valves (PWM valves)
^ A check valve in the passage between each ISO and PWM valve
^ A front and rear low-pressure accumulator (LPA)
^ A front and rear high-pressure accumulator (HPA)
^ A relay-operated electric motor that drives a front and rear channel pump
^ Internal electrical connections to the ECU within the EHCU
^ Service air bleed passages and valves for the accumulators
The Isolation (ISO) Valve
The isolation valves are normally in the open position, allowing unrestricted fluid flow to the wheel circuits. When the ECU determines antilock intervention is needed, the valve closes to isolate the hydraulic circuit. Hydraulic fluid is trapped in the isolated circuit and the system is in the pressure maintain mode.
The Pulse-Width Modulation (PWM) Valve
The PWM valve is computer modulated ON and OFF by the ECU. The PWM valve will cycle only if the isolation valve is in the pressure maintain mode, isolating the circuit.
When the valve is ON, it directs fluid to the low pressure accumulator for temporary storage. This causes the hydraulic channel pressure to decrease; this is the pressure decrease mode.
When the valve is modulated OFF, fluid is directed to the wheel cylinder.
The Low Pressure Accumulator (LPA)
The LPA is an accumulator used to temporarily store fluid purged from the hydraulic circuits during the pressure decrease cycle. When the system is not in an antilock state, the spring-loaded accumulator is empty.
The High Pressure Accumulator (HPA)
The HPA stores hydraulic fluid under pressure to be used when needed during a pressure increase mode. The accumulator is empty when not in use. It fills when the ECU commands the pump to operate during an antilock stop.
The Reset Switch
The reset switch monitors the hydraulic circuits for an imbalance in system pressure. The valve is centered in its bore by equalized pressures and a return spring. When the valve body has a difference in pressures, it will be forced to slide in its bore. When the valve makes contact with the end bore, it opens the electrical contacts and alerts the ECM of a pressure imbalance. When the valve returns to its normal position, the ECM resets the system and prepares it for the next hydraulic circuit pressure modification.
In the non-antilock mode, the reset switch alerts the ECM to any system pressure imbalance, as in the case of a hydraulic failure in the base brake system. At this point, it will disable the ABS function and illuminate the "Antilock" light.
The Pumps
During antilock braking control, the ECU operates the electric motor that drives the pumps for front and rear channels. The EHCU uses a relay within the EHCU to complete the pump motor circuits. A fusible link-protected feed wire and ground wire provide a power-supply circuits for the pump motor that is external of the ECU. The pumps provide fluid movement under high pressure for the two HPA circuits. They also create low pressure needed for LPA circuit operation.