Description of On-Board Diagnostics

ABS Control Unit Functions:

CONTROLLING FUNCTIONS
The ABS control unit consists of a main function section, which controls the operation of anti-lock brake system, and subfunction, which controls the pump motor and "self-diagnosis."

Main Function
The main function section of the ABS control unit performs calculations on the basis of the signals from each wheel sensor and controls the operation of the anti-lock brake system by putting into action the solenoid valves in the modulator unit for each front and rear brake.

Sub-Function
The sub-function section gives driving signals to the pump motor and also gives "self-diagnosis" signals, necessary for backing up the anti-lock brake system.

Self-Diagnostic Function
Since the anti-lock brake system modulates the braking pressure when a wheel is about to lock, regardless of the driver's intention, the system operation and the braking power will be impaired if there is a malfunction in the system. To prevent this possibility, at speeds above 6 mph (10 km/h) the self-diagnosis function monitors the main system functions. When an abnormality is detected, the ABS indicator light comes on.
There is also a check mode of the self-diagnosis system itself: when the ignition switch is first turned on, the ABS indicator light comes on and stays on for a few seconds after the engine starts, to signify that the self-diagnosis system is functional.

Fail-Safe Function
If an abnormality is detected, the ABS control unit turns off the fail-safe relays and motor relay. In this condition, the antilock brake system is prevented from functioning, yet the basic brake system continues to operate normally.

The ABS Indicator Light Comes On When:
- the fluid pressure pump runs more than 120 seconds.
- the parking brake is applied for more than 30 seconds while the vehicle is being driven.
- the rear wheel(s) is (are) locked more than a specified time.
- the wheel rotation signal is not transmitted due to faulty wire or sensor.
- the operation time of the solenoid valve(s) exceeds a predetermined value, and the ABS control unit finds an open in the solenoid circuit.
- the output signals from both main functions in the ABS control unit are not transmitted to the solenoid valve(s).


SELF-DIAGNOSTIC FUNCTIONS

Initial Diagnosis - Performed right after the engine starts until the ABS indicator light goes off.

Regular Diagnosis - Continuously performed (under some conditions) after the ABS indicator light goes off until the engine stops.

PROBLEM DETECTED
The Central Processing Unit (CPU) controls the following when it detects a problem during self-diagnosis:
- Turns the ABS indicator light ON.
- Turns the front and rear fail-safe relays off.
- Stops the ABS control.
- Stops the ABS pump.
- After the Diagnostic Trouble Code (DTC) is stored in the control unit, the CPU stops self-diagnosis.