Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Description & Operation
The Rear Anti-Lock Brake System (RABS) is designed to prevent rear brake lock-up. This is done by controlling the amount of hydraulic fluid pressure to the rear wheel cylinders. The system consists of 2 warning lights (Red BRAKE and Yellow REAR ANTI-LOCK), a computer module, an electro-hydraulic valve, speed sensor and exciter ring. See Fig 1 -Fig 6 .
The control module continually monitors rear wheel speed. Wheel speed is sensed by speed sensor mounted on rear axle. As vehicle moves, teeth on exciter ring, located on ring gear inside the differential case, pass by the speed sensor probe. This induces an AC voltage signal in sensor circuit.
When brakes are applied, control module senses reduced wheel speed. If rate of deceleration is too great, indicating lock-up is occurring, the control module activates the electro-hydraulic valve. The valve, located on frame rail, then closes an internal isolation valve, isolating rear wheel cylinders from master cylinder. This stops rear brake pressure from increasing further.
If rate of deceleration is still too great, control module will activate dump solenoid with a series of rapid pulses, bleeding rear wheel cylinder fluid into the electro-hydraulic valve accumulator. This allows rear wheels to spin back up to vehicle speed. Dump and isolation valves will continue to be pulsed in a way that allows rear wheels to rotate, while maintaining maximum deceleration during braking.
When vehicle comes to a complete stop and brake pedal has been released, control module de-energizes isolation valve. Any fluid in accumulator is returned to master cylinder and braking is returned to normal.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.