What are the reasons for ABS system failure after vehicle inspection?
2 Answers
ABS warning light can indicate various conditions, each with different causes and solutions as follows: Condition 1: ABS warning light stays on (most common). Cause 1: The ABS wheel speed sensor's sensing area is covered by mud, sludge, or other contaminants, affecting its ability to detect wheel speed signals. This prevents the ABS control unit from determining vehicle speed and wheel slip rate, thus failing to issue appropriate commands for brake adjustment. Solution: Clean contaminants from the wheel speed sensor and adjust the gap between the sensor and tone ring to restore normal function. Cause 2: Loose system wiring connections or poor ABS relay contact causing signal issues. Solution: Inspect wiring connections and reconnect any loose parts. Condition 2: ABS warning light intermittently illuminates but turns off during acceleration. Cause: When multiple electrical components are in use, battery voltage drops below 10.5V. The light extinguishes when engine speed increases and voltage rises. Possible low voltage supply to ABS system due to poor terminal contact or grounding issues. Solution: Check battery specific gravity; inspect charging system; examine power supply (voltage relay or poor power contact). Condition 3: ABS warning light remains on after engine start until engine IGFF. Cause: Poor grounding contact of ABS hydraulic valve body; faulty wiring connection of ABS hydraulic valve body; ABS control unit malfunction. Solution: Loosen and retighten hydraulic valve body ground screw, clean contact surface if necessary; check for enlarged terminal gaps; replace ABS or ABS/ASR control unit. Condition 4: ABS warning light illuminates during high-speed driving. Cause: ABS control unit detects excessive speed difference between front and rear wheels; incorrect tire or wheel rim specifications. Solution: Refer to vehicle's specified tire and wheel rim sizes (check label near fuel filler cap). Condition 5: ABS warning light intermittently flashes. Cause: Improper brake light switch adjustment; internal contact failure in brake lights. Solution: With engine running, depress brake pedal, manually push brake switch linkage downward, then release pedal to complete self-adjustment; replace brake light switch if needed. When the ABS warning light is on, continued driving is not recommended as it indicates anti-lock braking system malfunction. During emergency braking, wheel lock-up may cause loss of vehicle control and skidding, preventing speed reduction or trajectory adjustment, posing significant safety risks.
I've been in the car repair business for a long time, and the most common reason for the ABS light coming on after an inspection is that the wheels were lifted and rotated during the testing process. The ABS system relies on wheel speed sensors to monitor speed. When the vehicle is on a lift with the tires spinning in the air, the sensors can't detect the correct speed, which easily triggers a fault code. Additionally, inspectors might accidentally loosen the sensor wiring or get it dirty with dust and grime, causing signal interruption. Another possibility is that the testing equipment itself interferes with the ABS control module, such as causing a false alarm when connecting the diagnostic port. In such cases, a simple system reset usually solves the problem—just connect to a computer to clear the fault code. However, don't take it lightly. If the light stays on after a reset, promptly check whether the sensor is damaged or the wiring is broken to avoid compromising brake safety.
