Technician Handbook ! 673 Electronic & Computer Controlled Systems Pulse Width An ECU’s electronic circuits have the ability to open and close a circuit very rapidly. The ECU can switch a circuit on for a fraction of a second at very precise intervals. Modulation When a circuit is switched ON and then OFF, the momentary change in voltage creates a voltage pulse .
(The pulse can be either a momentary increase or decrease in voltage depending on whether the circuit is ground- side switched or power-side switched and where the voltage is measured.) When the voltage is viewed on an oscilloscope, the voltage pulse’s width represents the amount of time the circuit is switched ON and can be as brief as 1 millisecond or less. In some circuits, the ECU uses the amount of ON time to regulate component operation. When the ECU varies the width of the voltage pulse (the ON time) to control a component, the process is called pulse-width modulation . NOTE In the above example, the frequency of the pulses changes as well as the pulse width.
In some circuits, the frequency of the pulses is constant. 14 Technical Training