* Wheel Speed Sensor(s): This is the most probable cause. The ABS system relies on wheel speed sensors to detect wheel slippage and modulate braking pressure accordingly. A faulty sensor provides inaccurate or no data, causing the ABS to activate erratically or not at all (resulting in the shudder). The AWD system also uses wheel speed sensors to distribute torque effectively. A bad sensor will confuse the AWD system's computer, triggering the AWD light. The shuddering starts *before* the light because the ABS system attempts to compensate for the faulty sensor input before it finally throws a fault code and illuminates the light. This is very common.
* ABS Module: The ABS module itself could be failing. It receives signals from the wheel speed sensors and controls the ABS hydraulic system. A malfunctioning module can lead to similar symptoms.
* Brake System Issues: While less likely to trigger *both* lights, significant problems in the regular braking system – such as worn brake pads, warped rotors, or low brake fluid – can indirectly affect the ABS and AWD systems. The shuddering strongly points to this being a possibility. Uneven brake pad wear or a warped rotor would cause the braking to be uneven causing the shudder. The ABS then tries to correct for this uneven braking causing the ABS light to come on. The AWD system might then be tripped because the sensors detect that not all wheels are braking correctly.
* Wiring/Connectors: Damaged wiring or corroded connectors between the wheel speed sensors and the ABS module can also cause intermittent or complete signal loss.
Troubleshooting Steps:
1. Check Brake Fluid: Ensure your brake fluid level is sufficient and clean. Low fluid is a serious problem that needs immediate attention.
2. Inspect Brake Pads and Rotors: Visually inspect your brake pads and rotors for wear and tear. Worn pads or warped rotors are common causes of brake shuddering.
3. Have the codes read: Use an OBD-II scanner to read the trouble codes stored by the vehicle's computer. These codes will pinpoint the likely source of the problem. This is crucial for accurate diagnosis.
4. Professional Diagnosis: Based on the codes and your inspection, you'll likely need a professional mechanic to diagnose and repair the issue. The problem could involve replacing a wheel speed sensor, the ABS module, or performing more extensive brake work.
Do not drive your vehicle with these warning lights illuminated. Continued driving with a faulty ABS system could lead to loss of braking control in an emergency.
