What Could Be the Reasons for ABS Failure in a Sedan?
4 Answers
ABS light can illuminate in various states, with different causes and solutions for each state, 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 control unit from determining the vehicle speed and wheel slip rate, 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 signal ring to restore normal function. Cause 2: Poor signal transmission due to loose connections between system wires or faulty ABS relay contacts, leading to system failure. Solution: Inspect wire 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. Another possible cause is 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 of the ABS hydraulic valve body; poor electrical connector contact of the ABS hydraulic valve body; ABS control unit malfunction. Solution: Loosen and then retighten the grounding screw of the hydraulic valve body, cleaning the contact surface if necessary. Check if the connector gap has widened. Replace the ABS or ABS/ASR control unit if needed. State 4: ABS warning light illuminates during high-speed driving. Cause: During high-speed driving, the ABS control unit detects a significant discrepancy between the rear and front wheel speed signals. Incorrect tire or wheel rim specifications may also cause this issue. 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 of the brake light. Solution: With the engine running, press the brake pedal and push the brake switch linkage down fully with a finger, then release the brake pedal to complete the self-setting position. 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, making it difficult for the driver to slow down or adjust the vehicle's path, posing significant safety risks.
Last time I was driving an old Passat when the ABS light suddenly came on, which scared me into getting it checked immediately. The mechanic said the issue might be with the wheel speed sensor, as it’s particularly prone to getting dirty with mud or being interfered with by metal shavings. If there’s been any impact near the tires, it could also easily get damaged. Additionally, if the ABS pump relay has poor contact or is burnt out, the system will also throw an error. Oh, the mechanic also reminded me to check that the tire pressure difference among all four tires shouldn’t be too large—if the difference exceeds 0.5 bar, it could cause the ABS to misjudge the wheel speed. The worst-case scenario is if the control module gets water damage or the circuit board burns out, which would require an expensive replacement of the entire system. The advice was to stop driving immediately if the ABS light comes on and to get the fault codes read with professional equipment as soon as possible.
My neighbor's Corolla, which had run 100,000 kilometers, had its ABS fail last week. After being towed for inspection, it was found to be a typical wiring aging issue. If the brake fluid line connectors leak and cause the fluid level to be too low, it can trigger a fault, which is often overlooked during maintenance. The wiring harness for the wheel speed sensors is particularly fragile; the mechanic said it tends to become brittle and crack after five or six years in the high-temperature environment of the engine bay. There's also the less common possibility of excessive wheel bearing wear, which can damage the position accuracy of the tone ring. I recommend rinsing the inside of the wheels every time you wash the car to prevent sand and dirt buildup from affecting the sensors. For older cars, it's especially important to check if the ABS master pump power connector has oxidized and turned green.
ABS failure can easily lead to accidents if left unresolved. I personally experienced a dangerous situation where the steering wheel shook violently during braking. The most common cause is a faulty wheel speed sensor—the small black square on the hub can malfunction if damaged by stones or water ingress. If the tread depth varies too much across the four tires, it can also interfere with ABS operation, especially when tires are replaced individually rather than as a set. Issues with the control module itself usually require professional diagnostic tools to confirm, and small repair shops might misdiagnose them as sensor problems. Last time, my ABS warning light came on because I forgot to install the sensor spacer when modifying the wheels—a lesson I won’t forget. It’s recommended to have a technician check the ABS data stream with a computer every 30,000 kilometers.
