The Car Care Guide

ABS modulator valves regulate the air pressure to the brakes during ABS action. When not receiving commands from the ECU, the modulator valve allows air to flow freely and has no effect on the brake pressure. The ECU commands the modulator valve to either:

• Change the air pressure to the brake chamber, or
• Hold the existing pressure.

However, it cannot automatically apply the brakes, or increase the brake application pressure above the level applied by the driver.

The modulator valve typically contains two solenoids. The modulator valve and relay valve may be incorporated into a single unit. The modulator valve may also be separate, inserted into the service line to the brake chamber(s) after any relay valve, located as close as practicable to the chamber(s) itself.

When the modulator valve is separate, it has to control more air flow and, therefore, includes two larger diaphragm valves which are controlled by the solenoids. It usually has three ports: the supply port, the delivery port and the exhaust port

• The supply port receives air from a quick release or relay valve.
• The delivery port sends air to the brake chambers.
• The exhaust port vents air from the brake chamber(s).

Typically, when an ECU controlling a separate modulator valve detects impending wheel lockup, it activates the solenoids to close the supply port and open the exhaust port. When enough air is vented to prevent wheel lockup, the exhaust valve will close and the ECU will - depending on the situation - either:

• Keep the supply port closed to maintain existing pressure, or
• Open the supply port to allow brake application pressure to increase and repeat the cycle.