Why is the ABS light on the car dashboard illuminated?

[StephenFitz]

When the car displays ABS, it indicates a malfunction in the vehicle's Anti-lock Braking System. During emergency braking, if the wheels lock up, the vehicle may lose control and skid, preventing the driver from slowing down or adjusting the vehicle's trajectory, which poses significant safety risks. There are various states in which the ABS light may illuminate, 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 the corresponding speed signal. This prevents the ABS computer from determining the vehicle speed and the wheel's slip rate, thus failing to issue appropriate commands to adjust braking. Solution: Clean the dirt from the wheel speed sensor and adjust the gap between the sensor and the signal ring to restore normal function. Cause 2: Loose connections between system wiring or poor contact in the ABS relay result in poor signal transmission, leading to system failure. Solution: Inspect the wiring connections and reconnect any loose parts. State 2: ABS warning light intermittently illuminates but turns off during acceleration. Cause: When multiple vehicle electrical devices are in use, the battery voltage drops below 10.5V. As the engine speed increases, the voltage rises, causing the ABS light to turn off. This may also occur due to insufficient power supply voltage to the ABS system, such as poor terminal contact or grounding issues. Solution: Check the battery's specific gravity; inspect the charging system; examine the power supply (e.g., voltage relay or poor power contact). State 3: ABS warning light remains on after engine start and only turns off when the engine is turned off (IGFF). Cause: Poor grounding contact in the ABS hydraulic valve body; poor electrical connector contact in the ABS hydraulic valve body; ABS computer malfunction. Solution: Loosen and then retighten the grounding screw on the hydraulic valve body, cleaning the contact surface if necessary; check if the connector gap has widened; replace the ABS or ABS/ASR computer. State 4: ABS warning light illuminates during high-speed driving. Cause: At high speeds, the ABS computer detects a significant discrepancy between the 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 label near the fuel tank cap. State 5: ABS warning light intermittently illuminates. Cause: Improper adjustment of the brake light switch; poor internal contact in the brake light. Solution: Start the engine, depress the brake pedal, push the brake switch linkage down fully with your finger, then release the brake pedal to complete the self-setting position; replace the brake light switch if necessary.