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 each front wheel brake, the third is assigned to the rear wheel brakes. This system is known as a ``Three-channel system.''
The modulator valve receives its instructions from control module. The modular valve regulates brake fluid pressure based upon road conditions as interpreted by the control module, regardless of master cylinder output pressure. The modulator valve can maintain or reduce brake fluid pressure to the wheel calipers. However, the modulator valve can not increase the pressure above that sent by the master cylinder. The modulator valve alone can not apply the brakes.
Two relays are mounted on top of modulator valve. These are the only replaceable parts of the modulator valve. The pump motor relay controls the return pump and the solenoid relay 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.