What does it mean when the ABS and ESP lights are on in a Mercedes-Benz?

[ChristopherLee]

When the ABS light is displayed in a car, it indicates a fault in the vehicle's anti-lock braking system. If the wheels lock up 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 significant safety risks. 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 speed sensor's sensing area is covered by dirt, mud, or other contaminants, affecting the sensor's ability to detect the corresponding speed signal. This prevents the ABS computer from determining the vehicle's speed and the wheel's slip rate, thus failing to issue appropriate commands to adjust 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: Loose connections between system wires or poor contact in the ABS relay can cause signal issues, leading to system failure. Solution: Check the wire connections and reconnect any loose parts. State 2: ABS warning light intermittently turns on 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 can also occur if the ABS system's power supply voltage is too low due to poor 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 stays 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, or a faulty ABS computer. Solution: Loosen and then retighten the grounding screw on the hydraulic valve body, clean the contact surface if necessary, check for enlarged gaps in the connectors, and replace the ABS or ABS/ASR computer if needed. State 4: ABS warning light turns on during high-speed driving. Cause: At high speeds, the ABS computer detects a significant discrepancy between the rear and front wheel speed signals. This may also occur due to incorrect tire or wheel rim specifications. Solution: Refer to the vehicle's tire and wheel rim specifications, as indicated on the sticker near the fuel tank cap. State 5: ABS warning light intermittently turns on. Cause: Improper adjustment of the brake light switch or poor internal contact in the brake light. Solution: Start the engine, press the brake pedal, push the brake switch linkage down with your finger, release the brake pedal to complete the self-setting position, or replace the brake light switch if necessary.