What causes the ABS warning light to illuminate in an old Jetta?

[StJoshua]

There are various states in which the ABS light may illuminate, with different causes and solutions for each state, as detailed below: State 1: ABS warning light stays on constantly (most common). Cause 1: The ABS wheel 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 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. Cause 2: Poor signal transmission due to loose system wiring connections or poor contact in the ABS relay, leading to system failure. Solution: Inspect wiring connections and reconnect any loose parts. State 2: ABS warning light intermittently illuminates but turns off during acceleration. Cause: When multiple electrical components are in use, the battery voltage drops below 10.5V. The ABS warning light turns off as the engine speed increases and voltage rises. This may 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). 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; poor electrical connection in the ABS hydraulic valve body wiring; ABS computer malfunction. Solution: Loosen and retighten the grounding screw on the hydraulic valve body, cleaning the contact surface if necessary. Check for enlarged gaps in the connectors; replace the ABS or ABS/ASR computer if needed. State 4: ABS warning light illuminates during high-speed driving. Cause: During high-speed driving, 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, typically found 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 in the brake light. Solution: With the engine running, press the brake pedal and use a finger to push the brake switch linkage down fully, then release the pedal to complete the self-setting process. 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, preventing the driver from slowing down or adjusting the vehicle's trajectory, posing significant safety risks.

[SanJune]

The ABS light coming on in an old Jetta can indeed be a headache. These aging vehicles are most prone to issues with wheel speed sensors. I once repaired one where the sensor probe was coated with a thick layer of mud, completely unable to detect wheel rotation. Another time, a rural driver had half their ABS wiring harness chewed through by rats. If the brake pedal kicks back with a buzzing sound during hard braking, it's likely the ABS pump motor failing. Actually, water splashing onto the rear wheel hubs after rain can oxidize sensor connectors - a spray of WD-40 might do the trick. The safest approach is hooking up a diagnostic scanner at the shop to read trouble codes and pinpoint the faulty wheel. I remember disassembling a rear wheel sensor on an old Jetta once - the magnetic ring was cracked like a spiderweb, requiring complete replacement.