Hydraulic Control Assembly - Antilock Brakes: Description and Operation

Fig. 2 Modulator valve:

The modulator valve, Fig. 2, consists of three rapidly switching solenoid valves, two accumulator chambers (one for each brake circuit), and a return pump. One solenoid valve is assigned to the left and one to the right front wheel brake. The third solenoid valve is assigned to the rear wheel brakes. This system is known as a ``3-channel system.'' Modulator valve receives its instructions from control module.
Independent of pressure in tandem master cylinder, modulator valve can maintain or reduce brake fluid pressure to wheel brake cylinders during regulation. The modulator cannot increase pressure above that transmitted to master cylinder, and can never apply brakes by itself.
Two relays are mounted on top of modulator valve that can be replaced. One controls return pump and other controls power supply to solenoid valves. Modulator valve is mounted in rear storage compartment located behind drivers seat. The modulator valve itself cannot be repaired and must be replaced as an assembly.