Why does the ABS light come on when starting the car?

[Beatrix]

The ABS light on the car indicates a malfunction in the anti-lock braking system. If the wheels lock during emergency braking, the vehicle may lose control and skid, making it difficult for the driver to slow down or adjust the vehicle's trajectory, posing a significant safety hazard. There are several states in which the ABS light may illuminate, each with different causes and solutions, as detailed below: State 1: ABS warning light stays 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's speed or the wheel's slip rate, thereby 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 signal ring to restore normal function. Cause 2: Poor signal due to loose connections in the system wiring or poor contact in the ABS relay. Solution: Check the wiring connections and reconnect any loose parts. State 2: ABS warning light intermittently illuminates but turns off when accelerating. Cause: When multiple vehicle electrical devices are in use, the battery voltage may drop below 10.5V. As the engine speed increases, the voltage rises, and the ABS light turns off. This could also be 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, and 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; loose wiring connections in the ABS hydraulic valve body; ABS computer malfunction. Solution: Loosen and retighten the grounding screws on the hydraulic valve body, clean the contact surfaces if necessary; check for enlarged gaps in the connectors; 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, often listed on the fuel cap label. 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, press the brake pedal, push the brake switch linkage down fully with your fingers, then release the brake pedal to complete the self-setting process; replace the brake light switch if necessary.

[McAniyah]

After starting the car, the ABS light comes on, a situation I've encountered before. It's usually due to an issue with the brake system's sensors, such as the wheel speed sensor being covered by dirt or oil, interrupting signal transmission. It could also be caused by aging or loose wiring, or a fault in the ABS module itself. Unstable battery voltage during engine startup can also trigger it. Safety first—ABS is the anti-lock braking system, and the light coming on means the brakes may fail or become unresponsive, especially increasing the risk of skidding and accidents on slippery roads. It's recommended to turn off the engine and restart the car to see if the light goes away; if it does, it's a temporary issue. Otherwise, promptly check if the brake fluid level is sufficient and if the sensor connections are secure. When dealing with it myself, I usually start by cleaning around the wheels with a cleaner. If that doesn't work, it's best to send it for repair immediately without delay.