The ABS modulator is an air valve which controls the air pressure that streams into the brake chambers.
Located close to the engine, near the fuel filter, the ABS modulator is part of your car's hydraulic pump assembly.
The ABS modulator is sent messages from the ABS Control Unit ECU, when a wheel is likely to lock and skid, and it adjusts the air pressure in the brake chamber to stop this from happening. Basically, the ABS modulator, along with the ABS Control Unit ECU and pump, allows you to maintain control and brake safely and smoothly by regulating the air pressure in the wheel brakes.
If there is a fault with your ABS modulator the warning light will show on the display panel, perhaps having shown intermittently before, and the system will disable itself. The Check Engine Light may also show on the display and your speedometer could stop working. A faulty ABS modulator may affect braking when driving slowly.