The vacuum pump is a small electric motor-driven pump which works with the car's ECU (Electronic Control Unit) to provide the extra vacuum required for the car's cruise control system to operate.  

The vacuum pump is usually located in the left front corner of the engine compartment and is connected to a vacuum line running from the pump to the inside of the car. 

When the accelerator pedal is pressed, the vacuum pump is activated. The vacuum pump works by creating a vacuum in the actuator, which operates the throttle valve. The throttle valve regulates the flow of air and fuel into the engine and ultimately the speed the car travels at. So the cruise control on the car takes charge of this process and, with the vacuum produced by the vacuum pump, manages the flow of air into the engine and in turn the speed of the car. Once the car has reached the desired pre-set speed, the cruise control system will maintain this constant speed and the vacuum pump shuts off.  

The vacuum pump has several components that can fail over time, including the electric motor. If this part malfunctions it is imperative that you replace it with a vacuum pump that will be compatible with the make and model of your car.