ABS False Modulation

Subject: ABS False Modulation

Vehicles Involved: 1996-2001 Audi A4, 1998-2002 Audi A8, 1998-2004 Audi A6, 2000-2005 Audi Allroad Quatros

False Activation can occur for a few reasons:
  1. Damaged Sensor Ring (missing tooth or cracked tooth)
  2. Rust/Corrosion under wheel speed sensor increasing air gap
  3. Damaged wiring
  4. ESC alignment angles
  5. Excessive bearing play
Condition: The base brake system is in good working order. The ABS activates under light braking when the wheels are not locking.
Repair Procedure:
  • Check for damaged wheel speed sensors or mismatched tires.
  • Check specification of wheel speed sensors air gaps and cleaning sensor contact removing rust if present.  The most likely problem will be tiny cracks in the ABS tone rings found at the outer CV joint, or inside the rotor or wheel hub.  The best way to identify a crack or damaged ring is to use a lab scope. 
  • With the scope attached to the sensor turn the ABS ring.  A normal wheel speed sensor will show a smooth round uninterrupted sine wave that increases in amplitude and frequency as the wheel speed increases.  If a crack in the wheel speed ring is present a notch or a flat spot on the wave form will be present.  If no scope is available, clean tone ring and inspect carefully with bright light. Cracks normally form at base of tooth.