Handbook 652 Body Electrical Diagnosis Control Devices Most circuits on the vehicle are switched off and on to control when they operate. This switching is done through the use of relay contacts, transistors, and of course, mechanical switches. To limit or control the amount of current in a circuit, a series resistor or pulse width control can be used. Examples of this are found in A/C blower motor circuits and in interior light circuits. Several circuits are designed to be ON at all times. These include Theft Deterrent Systems, and components with electronic memory such as radios, clocks and ECUs (Electronic Control Units).
Circuits that are ON all the time create parasitic loads which draw current from the battery even when the engine is off and the vehicle is parked. Technical Training 13