What Causes the ABS Light to Illuminate While Driving?

[Susanna]

ABS light illumination can occur in various states, each with different causes and solutions, as detailed below: State 1: ABS warning light remains constantly on (most common). Cause 1: The ABS wheel speed sensor's sensing area is covered by dirt, mud, or other contaminants, affecting its ability to detect wheel speed signals. This prevents the ABS control unit from determining vehicle speed or calculating wheel slip rates, thus failing to issue appropriate braking commands. Solution: Clean the wheel speed sensor and adjust the gap between the sensor and the tone ring to restore normal function. Cause 2: Loose connections in system wiring or poor contact in ABS relays causing signal interference. Solution: Inspect wiring connections and secure any loose terminals. State 2: ABS warning light intermittently illuminates but turns off during acceleration. Cause: Battery voltage drops below 10.5V when multiple electrical components are in use. The light extinguishes when engine RPM increases and voltage rises. Possible low voltage supply to ABS system due to poor terminal contact or grounding issues. Solution: Check battery specific gravity; inspect charging system; examine power supply components (voltage relay or loose power connections). State 3: ABS warning light stays illuminated after engine start until engine is turned off. Cause: Poor grounding connection at ABS hydraulic valve body; faulty wiring harness connection at valve body; ABS control module malfunction. Solution: Loosen and retighten hydraulic valve body ground screw, cleaning contact surfaces if necessary; check for connector pin gap expansion; replace ABS or ABS/ASR control module. State 4: ABS warning light activates during high-speed driving. Cause: ABS control module detects excessive speed difference between front and rear wheels; incorrect tire or wheel rim specifications. Solution: Verify proper tire and wheel rim specifications as indicated on vehicle's fuel filler door label. State 5: ABS warning light flashes intermittently. Cause: Improper brake light switch adjustment; internal contact failure in brake lights. Solution: With engine running, fully depress brake pedal while manually pushing brake switch linkage downward, then release pedal to reset position; replace brake light switch if necessary. When the ABS warning light is illuminated, continued driving is not recommended. An active ABS fault indicates potential loss of anti-lock braking function, which may cause wheel lock-up during emergency braking, resulting in loss of vehicle control and compromised ability to decelerate or steer - presenting significant safety risks.