Nissan 240SX/Altima 1993-1998 Repair Guide

General Information


See Figures 1 and 2

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Fig. Fig. 1: ABS component locations-240SX

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Fig. Fig. 2: ABS component locations-Altima

The Anti-Lock Brake System (ABS) is 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 anti-lock brake systems found on these vehicles hold the wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways while braking.

There are conditions for which ABS provides no benefit. Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface. This renders 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.

Under normal braking conditions, ABS functions in the same manner as a standard brake system. The system is merely a combination of electrical and hydraulic components, working together, to control the flow of brake fluid to the wheels when necessary.

The ABS control unit is the electronic brain of the system, receiving and interpreting speed signals from the speed sensors. The control unit will enter anti-lock mode when it senses impending wheel lock at any wheel, and immediately control the brake line pressure to the affected wheel.

The actuator assembly is separate from the master cylinder and booster. It contains the wheel circuit valves used to control brake fluid pressure to each wheel circuit.

The wheel speed sensors monitor decelerating wheel speed and provide data to the control module.

The Altima uses a four-channel ABS system with independent wheel speed sensors/controls at each wheel. The 240SX uses a three-channel system with a single wheel speed sensor for the rear wheels (located on the differential) and independent wheel speed sensors/controls for each of the front wheels.


Certain components within the Anti-lock Brake System (ABS) are not intended to be serviced or repaired individually. Only those components with removal and installation procedures should be serviced.
Do not use rubber hoses or other parts not specifically identified for an ABS system. When using repair kits, replace all parts included in the kit. Partial or incorrect repair may lead to functional problems and require the replacement of components.
Lubricate rubber parts with clean, fresh brake fluid to ease assembly. Do not use lubricated shop air to clean parts; damage to rubber components may result.
Use only specified brake fluid from an unopened container.
If any hydraulic component or line is removed or replaced, it may be necessary to bleed the entire system.
A clean repair area is essential. Always clean the reservoir and cap thoroughly before removing the cap. The slightest amount of dirt in the fluid may plug an orifice and impair the system function. Perform repairs after components have been thoroughly cleaned; use only denatured alcohol to clean components. Do not allow ABS components to come into contact with any substance containing mineral oil; this includes used shop rags.
The ABS control unit is a microprocessor similar to other computer units in the vehicle. Ensure that the ignition switch is OFF before removing or installing controller harnesses. Avoid static electricity discharge at or near the controller.
If any arc welding is to be done on the vehicle, the control unit should be unplugged before welding operations begin.