What Causes the ABS Light to Stay On?

[AnnieFitz]

ABS light illumination can occur in various states, with different causes and solutions for each state, as detailed below: State 1: ABS warning light stays on constantly (most common). Cause 1: The ABS wheel speed sensor's sensing area is covered by mud, sludge, or other contaminants, affecting its ability to detect vehicle speed signals. This prevents the ABS control unit from determining wheel speed or calculating wheel slip rate, thus failing to issue appropriate commands for brake modulation. Solution: Clean contaminants from the wheel speed sensor and adjust the gap between the sensor and 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 reconnect any loose terminals. State 2: ABS warning light illuminates intermittently, turning off during acceleration. Cause: When multiple electrical components are in use, battery voltage drops below 10.5V. The light extinguishes when engine RPM increases and voltage rises. Possible low voltage supply to ABS system due to poor terminal contact or faulty grounding. Solution: Check battery specific gravity; inspect charging system; examine power supply (e.g., voltage relay or poor power contact). State 3: ABS warning light remains illuminated after engine start until engine IGFF position. Cause: Poor grounding contact in ABS hydraulic valve body; loose wiring connections in hydraulic valve body; ABS control unit malfunction. Solution: Loosen and retighten hydraulic valve body ground screw, cleaning contact surfaces if necessary; check for enlarged terminal gaps; replace ABS or ABS/ASR control unit. State 4: ABS warning light activates during high-speed driving. Cause: ABS control unit detects excessive speed difference between front and rear wheels; incorrect tire or wheel rim specifications. Solution: Verify correct tire and wheel rim specifications per vehicle requirements (refer to fuel filler cap label). State 5: ABS warning light flashes intermittently. Cause: Improper brake light switch adjustment; internal contact failure in brake lights. Solution: With engine running, depress brake pedal and manually push brake switch lever downward to reset position; replace brake light switch if needed. When the ABS warning light is illuminated, continued driving is not recommended. An active ABS fault indicates compromised anti-lock braking functionality. During emergency braking, wheel lock-up may occur, potentially causing loss of vehicle control and inability to decelerate or steer, posing significant safety risks.