The described vehicles use ABS with three sensors - one on the front wheels, as well as one sensor on the two rear wheels. The rear wheel sensor is located in the extended part of the transmission housing on two-wheel drive vehicles, and in the transfer case housing on all-wheel drive vehicles. Thus, the electronic unit of the system provides separate control of the front wheel brakes and general control of the rear brakes.
Electrohydraulic modulator (EHCU)
The ABS modulator is located on the left side member of the frame. This element controls the pressure in the calipers, which prevents wheel locking during emergency braking (see illustration). This is ensured by the presence of a pressure control valve in the brake system (BPMV) and electronic brake system control unit (EVSM). When registering in the ЕВСМ block of wheel locks (based on wheel sensor signals) the BPMV valve is activated and the pressure in the corresponding caliper is reduced. After the wheel speed has stabilized, primary pressure is again created in its caliper. This cycle is repeated many times in one second, which is manifested by pulsation in the brake pedal.
2.2 Electrohydraulic ABS modulator (EHCU) located longitudinally on the left frame side member, under the driver
Note: The modulator can create pressure in the hydraulic system that exceeds the pressure. created by the brake cylinder. The possibility of activating the brakes directly from the modulator is excluded.
Along with performing the function of receiving and processing data from wheel sensors, as well as monitoring pressure in the hydraulic system, the EVSM also monitors the state of the system and registers diagnostic codes corresponding to specific problems that may occur in the brake system.
Wheel sensors
There is a speed sensor on each front wheel. The sensors are located on the front hubs/bearings. The sensor pulse wheel is integrated into the bearing. If the ring is damaged, the entire hub/bearing must be replaced. If the sensitive element of the sensor, which is bolted to the hub, fails, the element should be replaced separately. The sensors cannot be adjusted or disassembled.
Rear wheel speed is monitored by the VSS, which is located in the transmission housing extension on two-wheel drive vehicles and in the transfer case housing on four-wheel drive models. A more detailed description of the VSS sensor is provided in Chapter 6.
The wheel rotation speed in the sensor is determined by the revolutions of the toothed pulse wheel. When a wheel tooth passes near the sensitive element, an alternating pulse is generated. The frequency of alternating pulses directly depends on the wheel speed. When sensor pulses enter the ЕВСМ, changes in wheel speed are monitored. When a wheel lock is detected from the ЕВСМ, the ABS system is activated.
Alarm indicators
ABS has a self-diagnosis function. Every time the engine is started, the system is diagnosed in the EVSM. There are two emergency brake indicators on the instrument panel: red indicator «Brake» and amber «ABS». Each of the indicators has specific functions. Turning on the indicator «Brake» occurs when a general problem occurs in the brake system, for example, a drop in the brake fluid level. This indicator also turns on when the parking brake is applied. If the indicator does not turn off after releasing the parking brake, you need to check the brake fluid level in the master cylinder reservoir (see chapter 1).
Amber indicator «ABS» turns on when problems occur in the anti-lock braking system that do not apply to the entire braking system. When the indicator turns on, it indicates that the ABS function has been lost, but the braking system remains operational in normal mode. In this case, diagnostics and repairs are carried out in a proprietary station.
Examination
Diagnosis should be carried out at a service and repair station using special equipment, but a car mechanic can carry out a number of simple checks before sending the car to the station.
- A) Inspect the appropriate fuses.
- b) Make sure that the electrical connectors of the control unit and the ABS hydraulic regulator are securely connected.
- With) Make sure that the wires are intact and that the connectors of the wheel sensors and brake light sensor are securely connected.
- d) Inspect the hydraulic lines, calipers and wheel cylinders.
If during the above-described checks the cause of the malfunction was not identified, the vehicle should be sent to a service and repair station for diagnostics using special equipment. Due to the complexity of this system, repair work should be entrusted to highly qualified specialists.