What are the reasons for the ABS warning light being on in an old Jetta?

[Elaine]

There are various states of the ABS light being on, with different causes and solutions for each state as follows: State 1: ABS warning light stays on (most common). Reason 1: The ABS wheel speed sensor's sensing part 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 speed and the 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. Reason 2: Poor signal due to loose connections in the system wiring or poor contact of the ABS relay, causing system failure. Solution: Check the wiring connections and reconnect any loose parts. State 2: ABS warning light intermittently turns on but goes off when accelerating. Reason: When multiple vehicle electrical devices are in use, the battery voltage drops below 10.5V. As the engine speed increases and voltage rises, the ABS warning light turns off. The ABS system's power supply voltage is too low, possibly due to poor terminal contact or grounding issues. Solution: Check the battery's specific gravity; inspect the charging system; 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). Reason: Poor grounding contact of the ABS hydraulic valve body; poor electrical connection of the ABS hydraulic valve body; ABS computer malfunction. Solution: Loosen the grounding screw of the hydraulic valve body, then retighten it, cleaning the contact surface if necessary; check if the connector gap has widened; replace the ABS or ABS/ASR computer. State 4: ABS warning light turns on during high-speed driving. Reason: At high speeds, the ABS computer detects a significant difference 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, as indicated on the label near the fuel tank cap. State 5: ABS warning light intermittently turns on. Reason: Improper adjustment of the brake light switch; poor internal contact of the brake light. Solution: Start the engine, press the brake pedal, push the brake switch linkage down fully with a finger, then release the brake pedal to complete self-setting; replace the brake light switch. When the ABS warning light is on, it is advised not to continue driving. The 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, preventing the driver from slowing down or adjusting the vehicle's trajectory, posing significant safety risks.