ABS System Description

DESCRIPTION
This specification applies to Hydraulic and Electronic Control Unit (HECU) of the Anti lock Braking System (ABS) and Traction Control System (TCS).

This HECU has the functions as follows:
- Input of signal from the wheel speed sensors attached to each wheel.
- Control of braking force and traction force.
- Fail-safe function.
- Self diagnosis function.
- Interface with the external diagnosis tester.

OPERATION
When the ignition key is turned ON, the HECU energizes and begins an initialization process, then is fully energized. Software in the ECU controls the ABS electrical and hydraulic operation by input from various sensors, including the four wheel speed sensors, transmitted to the control unit. The signals are converted to square - wave patterns and transmitted to the processor.

Because of the detailed circuitry in the system, the circuits are continuously monitored for opens and/or short circuits. In the event of a failure, ABS will disable and normal braking will continue to function.

The solenoid(s) operate when power is applied to one side of the valve coil, and ground to the other. The function of the solenoid(s) is monitored by the valve test pulse. When over voltage is detected (16v or more), the HECU switches off and valve and disables ABS, although normal braking functions will continue to operate. When voltage returns to normal, the HECU will go through an initialization process and the ABS will operate again.

If the HECU detects a low voltage situation (under 11 V), the ABS function will disable and the ABS light on the instrument panel will illuminate. Normal braking functions will continue to operate.

The pump motor only operates when the ABS is operating. When driving, the HECU performs a pump motor test when the vehicle reaches a speed of 20 km/h, once on each ignition cycle. The customer may hear the pump motor actuate at this time, this is a normal operating condition of the self - test.

Any malfunctions or failures detected by the HECU are encoded and stored in the EEPROM that can be retrieved using the scan tool.