What are the causes of ABS failure?

[MacChristopher]

ABS warning light may illuminate in various states, with different causes and solutions for each state, as detailed below: State 1: ABS warning light stays on continuously (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 the corresponding speed signal. This prevents the ABS control unit from determining the vehicle speed and calculating the wheel slip ratio, thus failing to issue appropriate commands to regulate braking. Solution: Clean the dirt from the wheel speed sensor and adjust the gap between the sensor and the tone ring to restore normal function. Cause 2: Poor signal transmission due to loose connections between system wiring or poor contact in ABS relays, leading to system failure. Solution: Inspect wiring connections and reconnect any loose parts. State 2: ABS warning light illuminates intermittently, turning off during acceleration. Cause: When multiple vehicle electrical devices are in use, battery voltage drops below 10.5V. As engine speed increases, voltage rises, and the ABS indicator turns off. This may also occur if the ABS system power supply voltage is too low, such as due to poor terminal contact or grounding issues. Solution: Check battery specific gravity; inspect the charging system; examine the power supply (e.g., voltage relays or poor power contacts). State 3: ABS warning light remains on after engine start until engine IGFF turns it off. Cause: Poor grounding contact in the ABS hydraulic valve body; poor electrical connector contact in the ABS hydraulic valve body; ABS control unit failure. Solution: Loosen and retighten the hydraulic valve body ground screw, cleaning the contact surface if necessary; check for enlarged connector gaps; replace the ABS or ABS/ASR control unit. State 4: ABS warning light illuminates during high-speed driving. Cause: During high-speed driving, the ABS control unit detects a significant discrepancy between rear and front wheel speed signals; incorrect tire or wheel rim specifications. Solution: Refer to the vehicle's tire and wheel rim specifications, as indicated on the fuel tank cap label. State 5: ABS warning light illuminates intermittently. Cause: Improper adjustment of the brake light switch; poor internal contact in the brake light. Solution: With the engine running, depress the brake pedal, manually push the brake switch linkage down fully, then release the brake pedal to complete self-positioning; replace the brake light switch if necessary. If the ABS warning light is on, it is advisable not to continue driving. An illuminated ABS light indicates a malfunction in the vehicle's anti-lock braking system. During emergency braking, if the wheels lock up, the vehicle may skid uncontrollably, preventing the driver from decelerating or adjusting the vehicle's trajectory, posing significant safety risks.