Pull-Up resistors are used on some inputs to allow both high (12V) and low (0V) driving transducers to be connected. Pull-up resistors are connected between the input pin and supply voltage pin. This effectively pulls the input pin to a high voltage (hence the name "Pull-up") when nothing is connected to the input, or it is driven high by an external device.

 

Example

Use of a pull-up resistor with a simple mechanical switch. If the switch is connected in series with the input pin and ground, without a Pull-Up Resistor Enabled, opening or closing the external switch will not change the voltage at the input pin. With the Pull-Up Resistor enabled, when the external switch is open, the input pin will be at +12V (connected to +12V through the Pull-Up resistor) and when the switch is closed, the input pin will be at 0V (Ground). This provides the necessary two distinct voltage levels for the ECU to detect that the external switch has been changed.

 

Example

Connection of a Digital Input to a vehicle speedometer digital output. This output provides (in most cases) a square wave type signal. As this signal changes between 0V and 12V by itself, the Pull-Up resistor is not required to be enabled (again, in most cases).