Technician Handbook ! 673 Electronic & Computer Controlled Systems Voltage ON/OFF The diagrams above illustrate a ground-side switch connected to an ECU. The ECU supplies battery voltage to the switch circuit and provides the circuit’s load (a resistor). The ECU’s electronic circuits detect when the voltage after the load is high (near battery voltage) or low (near ground voltage). (Switch) Input While the switch is open, no current is flowing and the available voltage after the load is near battery voltage. When the switch is closed, current flows and most of the battery voltage is dropped across the load. The available voltage after the load is now near ground voltage.
In this example, the switch controls a lamp, but is not actually part of the lamp circuit. When the ECU senses a voltage drop in the switch circuit, it supplies five volts to the transistor. This in turn closes the lamp circuit, lighting the lamp. SERVICE TIP You can detect the same high or low voltage the ECU is detecting by measuring voltage at the appropriate ECU terminal. If the switch is closed and the voltage remains high, you’ll know there is an open in the circuit between the ECU and the switch. NOTE The actual wiring inside the ECU is extremely complex.
The ECU circuit details shown in the diagrams above and the diagrams on the following pages are to illustrate concepts, not actual internal connections. Technical Training 9