Abs Circuit Operation
When the ignition switch is turned to RUN position, battery voltage is applied through the In-Line Brake Fuse to the Over-Voltage Protection Relay (OVPR). The OVPR is energized and applies battery voltage through its contacts from Fuse R4 to the Electronic Brake Control Module (EBCM) at terminal No. 1 and the Hydraulic Modulator at terminals No. 3 and 4.See WIRING DIAGRAMS . Battery voltage is also applied to the Brake Pump Relay, but the pump does not run until the relay receives a ground from the Hydraulic Modulator.
As soon as battery voltage is applied to the EBCM, it begins an initialization check which tests various circuits for normal operation. Before completing the initialization check, the EBCM grounds terminal 28, which signals the Hydraulic Modulator to energize the Valve Relay located within the Hydraulic Modulator. Terminal No. 28 (terminal No. 7 at the Hydraulic Modulator) is held at 5 volts through a pull-up resistor in the Hydraulic Modulator. As long as the input at terminal No. 7 on the Hydraulic Modulator is at 5 volts, the Valve Relay is not energized. When the EBCM pulls terminal No. 28 low (terminal No. 7 at the Hydraulic Modulator), the Valve Relay is energized.
When the Valve Relay is energized, battery voltage is applied through its contacts to the five valve solenoids and terminal No. 32 of the EBCM (through terminal No. 8 of the Hydraulic Modulator). However, none of the solenoids operate until commanded to do so by the EBCM.
The EBCM operates the four individual wheel valve solenoids during the Anti-lock Brake System (ABS) process by allowing a controlled current to flow through the solenoids to at the EBCM. The four wheel valves are three position valves and the EBCM determines the positions of the valve by setting the magnitude of the current flow. In this manner, the four valves are used to modulate the wheel brake pressures individually during ABS control.
The fifth valve (Replenishing Valve) serves to maintain brake pedal height during ABS operation or extremely hard braking without ABS operation by redirecting hydraulic fluid within the booster/master cylinder assembly. Battery voltage at terminal No. 32 of the EBCM acts as a feedback to the EBCM regarding the position of the Valve Relay. If the Valve Relay does not operate, battery voltage is not present at terminal No. 32, which indicates a failure condition to the EBCM.
The Diode between terminals No. 29 and 32 of the EBCM prevents battery voltage from the Valve Relay contacts from being directly applied to terminal No. 29 of the EBCM. This could happen if the EBCM detects a failure and attempts to ground terminal No. 29 when battery voltage is present from the Valve Relay Contacts.
When the vehicle begins to move, the Wheel Speed Sensors produce voltage signals which are input to the EBCM. There is one Wheel Speed Sensor at each wheel. The signals consist of small A/C voltage pulses having a frequency proportional to the wheel rotational speed. At start ups, begins the ABS correct operation the initial start up of the vehicle after Ignition turn on, the EBCM completes a dynamic test of the valve solenoids when the speed of the vehicle reaches approximately 4 to 8 MPH.
The dynamic test may be felt by the driver through the brake pedal. On subsequent vehicle , the dynamic test does not take place, although there is a test routine performed each time which is not felt by the driver.
If the EBCM does not receive the correct response during the test it will ground terminal No. 29 signalling the BCM that the Anti-lock Brake System is not working. The BCM will then display the ANTI-LOCK DISABLED message on the CCDIC to alert the driver that the ABS is not operating. Specific failures in the ABS will be indicated by a trouble code. Once most failures are detected, the anti-lock braking function will remain disabled until the next time the ignition switch is turned to RUN position, when the process will be repeated.
The Wheel Speed Sensor signal from each wheel is sent to the EBCM which continuously processes the information. If during braking, one or more wheels start to lock up, the EBCM process and determines the of the individual valves and the replenishing valve. The typical sequence is as follows: Upon imminent lock-up, the appropriate solenoid valve is energized to the hold position, which maintains pressure at the associated wheel. If the Speed Sensor for the wheel indicates that it is continuing to lock-up, the solenoid valve is shuttled to the release pressure mode to reduce or remove brake fluid pressure at the wheel. When the wheel begins to resume normal rotation, the pressure is allowed to increase.
The above process occurs in millisecond time increments allowing up to 15 control cycles per second to take place. There are four Solenoid Valves in the Hydraulic Modulator, one for each wheel. All of them are three position valves which are positioned by the current level in the coil. The current level is determined by the EBCM. The four valves are controlled separately by the EBCM, however, the front wheels are controlled individually while the rear wheels are controlled as a pair by identical signals. The rear wheels are controlled together to avoid undesirable vehicle yaw tendencies during anti-lock control actions.