Before using this section, it is important that you have a basic knowledge of the following items. Without this knowledge, it will be difficult to use the diagnostic procedures contained in this section.
The antilock braking system (ABS) consists of a conventional hydraulic brake system plus antilock components. The conventional brake system includes a vacuum booster, master cylinder, front disc brakes, rear leading/trailing drum brakes, interconnecting hydraulic brake pipes and hoses, brake fluid level sensor and the BRAKE indicator.
The ABS components include a brake pressure modulator valve, an electronic brake control module (EBCM), 2 system fuses, 4 wheel speed sensors, one at each wheel, interconnecting wiring, the ABS indicator, the EBD indicator, which is connected to the parking lamp, and the rear disc brakes. For the general layout of this system, refer to Wheels/Vehicle Underbody Component Views .
The brake pressure modulator valve with the attached EBCM is located between the surge tank and the fire wall on the left side of the vehicle.
The basic brake pressure modulator valve configuration consists of hydraulic check valves, 2 solenoid valves for each wheel, a hydraulic pump, 2 accumulators, and 2 dampers. The brake pressure modulator valve controls hydraulic pressure to the front calipers and rear wheel cylinders by modulating hydraulic pressure to prevent wheel lockup.
Nothing in the brake pressure modulator valve or the EBCM is serviceable. In the event of any failure, the entire ABS unit with the attached EBCM must be replaced. For more information, refer to Base Braking Mode and Antilock Braking Mode.
The baseline braking mode of the ABS system used in this vehicle is a diagonal split system. In this system, one master cylinder circuit supplies pressure to the right front and the left rear brakes; the other circuit supplies pressure to the left front and the right rear brakes. All valves in the hydraulic modulator are in their normal, non-energized positions.
If a wheel speed sensor detects a wheel locking up, the electronic brake control module (EBCM) closes the normally open inlet valve for the brake on that wheel to prevent adding more hydraulic pressure to that brake.
The illustration shows this for the right front brake.
If the wheel locking tendency continues, the EBCM releases the hydraulic pressure at that brake by opening the outlet valve for that wheel.
As an add-on logic to the antilock brake system (ABS) base algorithm, electronic brake force distribution (EBD) works in a range in which the intervention thresholds for ABS control are not reached yet.
EBD ensures that the rear wheels are sensitively monitored for slip with respect to the front axle. If slip is detected, the inlet valves for the rear wheels are switched to pressure hold to prevent a further increase in pressure at the rear-wheel brakes, thus electronically reproducing a pressure-reduction function at the rear-wheel brakes.
Tire size is important for proper performance of the antilock brake system (ABS). Replacement tires should be the same size, load range, and construction as the original tires. Replace tires in axle sets and only with tires of the same tire performance criteria (TPC) specification number. Use of any other size or type may seriously affect the ABS operation.
Important: There is no serviceable or removable electrically erasable programmable read only memory (EEPROM). The electronic brake control module (EBCM) must be replaced as an assembly.
The EBCM is attached to the brake presure modulator valve in the engine compartment. The controlling element of ABS is a microprocessor-based EBCM. Inputs to the system include the 4-wheel speed sensors, the stop lamp switch, the ignition switch, and the unswitched battery voltage. There is an output to a bi-directional serial data link, located in pin K of the data link connector (DLC) for service diagnostic tools and assembly plant testing.
The EBCM monitors the speed of each wheel. If any wheel begins to approach lockup and the brake switch is closed, brake pedal depressed, the EBCM controls the solenoids to reduce brake pressure to the wheel approaching lockup. Once the wheel regains traction, brake pressure is increased until the wheel again begins to approach lockup. This cycle repeats until either the vehicle comes to a stop, the brake pedal is released, or no wheels approach lockup.
Additionally, the EBCM monitors itself, each input, except the serial data link, and each output for proper operation. If it detects any system malfunction, the EBCM will store a DTC in nonvolatile memory (EEPROM). Diagnostic trouble codes (DTCs) will not disappear if the battery is disconnected.
The front wheel speed sensors are of a variable reluctance type. Each sensor is attached to the steering knuckle, close to a toothed ring. The result, as teeth pass by the sensor, is an alternating current (AC) voltage with a frequency proportional to the speed of the wheel. The magnitude of the voltage and frequency increase with increasing speed. The sensor is not repairable, nor is the air gap adjustable.
The toothed ring mentioned above is pressed onto the wheel-side, outer, constant velocity joint. Each ring contains 47 equally spaced teeth. Exercise care during service procedures to avoid prying or contacting this ring. Excessive contact may cause damage to one or more teeth. If the ring is damaged, the wheel-side constant velocity joint must be replaced.
The rear wheel speed sensors operate in the same manner as the front wheel speed sensors. They incorporate a length of flexible harness with the connector attached to the end of the harness. The rear wheel speed rings are incorporated into the hub assemblies and cannot be replaced separately, but require replacement of the rear hub/bearing assembly.
The valve relay and the motor pump relay are located inside the electronic brake control module (EBCM) and are not replaceable. If one should fail, replace the EBCM.
The wiring harness is the mechanism by which the electronic brake control module (EBCM) is electrically connected to power and to ground, to the wheel speed sensors, the fuses, the switches, the indicators, and the serial communications port. The components, considered part of the wiring harness, are the wires that provide electrical interconnection, and connectors, terminals, pins, contacts, or lugs, that provide an electrical/mechanical interface from the wire to a system component.
The electronic brake control module (EBCM) continuously monitors itself and the other antilock brake system (ABS) components. If the EBCM detects a problem with the system, the amber ABS indicator will light continuously to alert the driver to the problem. An illuminated ABS indicator indicates that the ABS system has detected a problem that affects the operation of ABS. No antilock braking will be available. Normal, non-antilock brake performance will remain. In order to regain ABS braking ability, the ABS must be serviced.
Important: The electronic brake force distribution (EBD) indicator lamp wiring is connected to the parking brake lamp. If the parking brake lamp is turned ON when you are driving, check on whether the parking brake lever is engaged or the brake fluid level is low. If the system has no problem, the EBD system is working improperly. The EBD system must be serviced.
The red BRAKE indicator will be illuminated when the system detects a low brake fluid level in the master cylinder or when the parking brake switch is closed, the parking brake is engaged, or the EBD system is disabled.
