Isuzu Cars and Trucks 1981-1991



Anti-Lock Braking Systems (ABS) are designed to prevent locked-wheel skidding during hard braking or during braking on slippery surfaces. The front wheels of a vehicle cannot apply steering force if they are locked and sliding; the vehicle will continue in its previous direction of travel. The four wheel Anti-Lock Brake Systems found on the Isuzu Impulse hold the individual wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways.

There are conditions for which the ABS system provides no benefit.

Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface and rendering the vehicle totally uncontrollable until road contact is regained.
Extreme steering maneuvers at high speed or cornering beyond the limits of tire adhesion can result in skidding which is independent of vehicle braking.

For this reason, the system is named antilock rather than antiskid.

Under normal braking conditions, the ABS system functions in the same manner as a standard brake system. The primary difference is that power assist is gained from hydraulic pressure rather than a conventional vacuum booster. The system also prevents excessive pedal travel in the event of a hydraulic leak.

The system is a combination of electrical and hydraulic components, working together to control the flow of brake fluid to the wheels when necessary. The Electronic Brake Control Module (EBCM) is the electronic brain of the system, receiving and interpreting speed signals from 4 sensors at the wheels. The EBCM will enter anti-lock mode when it senses impending wheel lock at any wheel and immediately controls the line pressure(s) to the affected wheel(s). The hydraulic assembly contains the wheel circuit valves used to control the brake fluid pressure to each wheel circuit.

During anti-lock braking, line pressures are controlled or modulated by the rapid cycling of electronic valves within the hydraulic assembly. These valves can allow pressures within the system to increase, remain constant or decrease depending on the needs of the moment as registered by the EBCM.

The operator may hear a popping or clicking sound as the pump and/or control valves cycle on and off during normal operation. The sounds are due to normal operation and are not indicative of a system problem. Under most conditions, the sounds are only faintly audible. If ABS is engaged, the operator may notice some pulsation in the body of the vehicle during a hard stop; this is due to suspension shudder as the brake pressures are altered rapidly and the forces transfer to the vehicle.

Although the ABS system prevents wheel lockup under hard braking, as brake pressure increases, wheel slip is allowed to increase. This slip will result in some tire chirp during ABS operation. The sound should not be interpreted as lockup but rather than as indication of the system holding the wheel(s) just outside the point of lockup. Additionally, since the ABS system turns off around 4 mph, the final few feet of an ABS engaged stop may be completed with the wheels locked.

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Fig. Four wheel ABS system diagram Impulse