Why is the ABS light on in a Citroen C-Quatre?

[AdamDella]

When the ABS light is displayed on the car, it indicates a malfunction in the vehicle's anti-lock braking system. If the wheels lock during emergency braking, the vehicle may skid uncontrollably, preventing the driver from decelerating or adjusting the vehicle's trajectory, which poses significant safety risks. There are several conditions under which the ABS light may illuminate, each with different causes and solutions as follows: Condition 1: ABS warning light remains constantly 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 speed sensor and the signal ring to restore normal function. Cause 2: Loose connections between system wires or poor contact in the ABS relay result in poor signal transmission, causing system failure. Solution: Check the wire connections and reconnect any loose ones. Condition 2: ABS warning light intermittently turns on 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 and the voltage rises, the ABS indicator light turns off. The ABS system's power supply voltage is too low, possibly due to poor wire 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). Condition 3: ABS warning light remains 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 contact in the ABS hydraulic valve body's wire connector; ABS computer malfunction. Solution: Loosen the grounding screw on 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. Condition 4: ABS warning light turns on during high-speed driving. Cause: During high-speed driving, the ABS computer detects a significant discrepancy 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. Condition 5: ABS warning light intermittently turns on. Cause: Improper adjustment of the brake light switch; poor internal contact in 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 the self-setting position; replace the brake light switch.