System Characteristics and Components
SYSTEM CHARACTERISTICS AND COMPONENTSAs mentioned before, the Anti-Lock Brake System (ABS) system consists of three basic components. These are the hydraulic assembly, ACBM, and sensors. It is important that before diagnosing the ABS system a basic knowledge of the individual parts be achieved. The paragraphs that follow will help with the understanding of the components and build on a quick systematic way of diagnosing the ABS system.
The Hydraulic Assembly
The hydraulic assembly will be discussed as three separate components and then as a whole. The first portion of the assembly to be discussed is the pump/motor assembly. This is a piston type pump driven by an electric motor. The pressure from the pump is delivered to the accumulator where it is stored for use by the booster. The accumulator is a two chamber device with one chamber charged with nitrogen and the other for the brake fluid. The chambers are separated by a rubber bladder which acts as a piston to deliver the needed fluid to the booster and rear brakes. Pressure is controlled by the pressure switch, a combination pressure and warning switch. The pressure switch is located on the pump assembly and is identified by its five pin connector. This assembly has essentially three switches operated by a single piston. The first switch is for pressure maintenance in the accumulator which is kept at a level between 1885-2755 psi. The second switch is a normally closed switch which is directly connected to the red BRAKE warning lamp on the dash. And the last is a normally open switch which is used by the controller to indicate that enough pressure is in the accumulator for normal ABS operation. In the event of loss of sufficient pressure to operate the ABS system, the red BRAKE warning lamp and the amber ANTI-LOCK warning lamp will illuminate. This warning will occur at 1524 psi.
The second assembly in the hydraulic assembly is the booster with tandem master cylinder. When the brake pedal is depressed, high pressure fluid from the accumulator is ported through a small spool valve and into the boost chamber. The pressure in the chamber is regulated by the spool to maintain a pressure proportional to the pedal force applied by the driver. The fluid is also delivered from the boost chamber to the rear brakes. The master cylinder is a tandem type, with separate circuits for the front right and left brakes. Also part of the master cylinder booster assembly is the main valve. This valve is located in the front of the master cylinder and is identified by the two pin connector. The function of this valve is to refill the master cylinder and reset the pedal to thirty percent of travel during the anti-lock modulated. stop. This is accomplished by porting high pressure fluid into the master cylinder and into a resetting piston. This is responsible for the pulsating pedal during the anti-lock modulated stop.
The third assembly is the valve block that is attached to the master cylinder. The valve block controls the pressure to the wheel circuits in the anti-lock mode. Consisting of six solenoid valves, the valve block controls circuit pressure by either blocking or releasing fluid from the wheel circuits. Each wheel circuit has a pair of valves, one normally open (NO) and the other is a normally closed (NC). In the normal operation without anti-lock, fluid from the master cylinder is ported through the NO valve to the proper wheel circuit. During anti-lock, the valve block works in three phases. These phases are the pressure holding, reducing and increasing. While in the pressure holding phase, the normally open valve is closed so that no additional pressure can enter the system through the master cylinder. In the pressure reduction phase, the NO valve closes and the NC valve is pulsed so that pressure can be removed from the circuit and the fluid returned to the reservoir. Pressure increase is achieved by pulsing open the already closed NO valve.
Wheel Speed Sensors and Tonewheels
The wheel speed sensors operate as a generator. A permanent magnet located in the sensor is passed by a toothed sensor wheel (tonewheel). This causes a disturbance in the magnetic field pushing the signal high as a tooth passes and drawing the signal low in the spaces. This creates an AC signal that is sent to the anti-lock brake module (ABCM) to be processed. The ABCM monitors the frequency or time between the teeth to determine the speed of the vehicle.
Anti-Lock Brake Control Module (ABCM)
Operation of the anti-lock brake system is controlled by the microprocessor in the ABCM. The ABCM is responsible for the entire electrical side of the anti-lock brake system. Signals are sent from the ABCM to the various components of the system to insure their function is normal. If any disturbance is noted by the ABCM, the amber ANTI-LOCK warning lamp will be illuminated. The ABCM also monitors the wheel speed sensors to determine if the input from them is valid. Should a sensor begin to send false signals due to misadjustment or shorting sensor leads, the amber light will be lit.