Rear Anti-Lock Brake System (RABS)

Location View:

Item Part Number Description
1. 2B120 Disc Brake Caliper (RH)
2. 2078 Front Brake Hose (RH)
3. 2261 Rear Wheel Cylinder
4. 2267 Rear Brake Tube (RH)
5. 2268 Rear Brake Tube (LH)
6. 2262 Rear Wheel Cylinder (LH)
7. 2282 Rear Brake Hose
8. 2C008 RABS Valve Output Tube
9. 2B373 RABS Valve
10. 2234 RABS Valve Input Tube
11. 2264 Front Brake Tube
12. 2078 Front Brake Hose (LH)
13. 2B121 Front Disc Brake Caliper (LH)
14. 2263 Front Brake Tube (RH)
15. 2140 Brake Master Cylinder
16. 2005 Power Brake Booster


PURPOSE
The Rear Anti-Lock Brake System II (RABS II) continuously monitors rear wheel rotation during braking and adjusts rear brake hydraulic pressure to prevent full rear wheel lock-up.

OPERATION
The (RABS II) continually monitors rear wheel speed with a sensor mounted on the rear axle housing (4010) while the brakes are applied. When the teeth on the speed sensor ring, mounted on the ring gear, pass the sensor pole piece, an AC voltage is induced in the sensor circuit with a frequency proportional to the average rear wheel speed. In the event of an impending lockup condition during braking, the RABS II modulates hydraulic pressure to the rear brake drums (1126). This inhibits rear wheel lockup.

When the brake pedal (2455) is applied, the anti-lock brake control module (2B373) senses the drop in rear wheel speed. If the rate of deceleration is too great, indicating that wheel lockup is going to occur, the anti-lock brake control module activates the electro-hydraulic valve causing the isolation valve to close. With the isolation valve closed, the rear wheel cylinders (2261) are isolated from the brake master cylinder (2140) and the rear brake pressure cannot increase. If the rate of deceleration is still too great, the anti-lock brake control module will energize the dump solenoid with a series of rapid pulses to bleed off rear wheel cylinder fluid into an accumulator built into the RABS valve. This will reduce the rear wheel cylinder pressure and allow the rear wheels (1007) to spin back up to vehicle speed. Continuing under RABS II module control, the dump and isolation solenoids will be pulsed in a manner that will keep the rear wheels rotating while still maintaining high levels of deceleration during braking.

At the end of the stop, when the operator releases the brake pedal, the isolation valve de-energizes and any fluid in the accumulator is returned to the rear brake tube (2267) and up to the brake master cylinder. Normal brake operation is resumed.

CONSTRUCTION
The system consists of a Computer Module (ECU), Rear Anti-Lock Brake System (RABS) Valve, vehicle speed sensor (VSS) (9E731) and rear axle exciter ring.