What does the BMW ABS warning light indicate?

[StFrank]

When the ABS light is displayed on a vehicle, it indicates a malfunction in the Anti-lock Braking System. During emergency braking, if the wheels lock up, the vehicle may skid uncontrollably, preventing the driver from slowing down or adjusting the vehicle's trajectory, which poses significant safety risks. There are several scenarios in which the ABS light may illuminate, each with different causes and solutions, as detailed below: Scenario 1: ABS warning light remains constantly illuminated (most common). Cause 1: The ABS speed sensor's sensing area is covered by dirt, mud, or other contaminants, affecting its ability to detect the vehicle's speed signal. This prevents the ABS control unit from determining the speed and calculating the wheel slip rate, thus failing to issue appropriate commands to regulate braking. Solution: Clean the dirt from the speed sensor and adjust the gap between the sensor and the signal ring to restore normal function. Cause 2: Poor signal transmission due to loose system wiring connections or faulty ABS relay contacts. Solution: Inspect wiring connections and reconnect any loose parts. Scenario 2: ABS warning light intermittently illuminates but turns off during acceleration. Cause: When multiple electrical components are in use, the battery voltage may drop below 10.5V. As engine speed increases, voltage rises, causing the ABS light to turn off. This can also occur if the ABS system's power supply voltage is too low due to 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). Scenario 3: ABS warning light remains illuminated after engine start-up until the engine is turned off. Cause: Poor grounding of the ABS hydraulic valve body, faulty wiring connections in the hydraulic valve body, or ABS control unit malfunction. Solution: Loosen and then retighten the grounding screw of the hydraulic valve body, cleaning the contact surface if necessary. Check for enlarged gaps in connectors. Replace the ABS or ABS/ASR control unit if needed. Scenario 4: ABS warning light illuminates during high-speed driving. Cause: The ABS control unit detects a significant discrepancy between rear and front wheel speeds. This may also occur due to incorrect tire or wheel rim specifications. Solution: Refer to the vehicle's recommended tire and wheel rim specifications, typically found on the label near the fuel tank cap. Scenario 5: ABS warning light intermittently illuminates. Cause: Improper adjustment of the brake light switch or internal contact issues in the brake light. Solution: With the engine running, press the brake pedal and manually push the brake switch linkage downward to its limit, then release the pedal to complete self-positioning. Replace the brake light switch if necessary.