Pull Up Voltage

Pull-Up Voltage
Application: All Machines with Sensor and Switch Inputs to Electronic Control Modules (ECM's) References: • Service Manual article for system under test • Service Magazine SEPD0318"What Is Pull-Up Voltage?" (2 Aug 96) • To download a current version of this document, please check page 4 of this Newsletter for instructions.

The following is an explanation of "Pull-Up Voltage"found on switch and sensor inputs of electronic controls and ECM's.

Question:

What is "Pull-Up Voltage"?
"Pull-Up Voltage" is a Voltage supplied from within an electronic control module throughan internal resistor which "pulls up" the signal circuit contact on the connector of the control input shown in this illustration. The signal contact is held at this Pull-Up Voltage (high) exceptduring the time that the sensor (or switch) drives the signal to a low level (low). The terms, "low" and "high" relate to the expected extreme values of the signal input voltage level.

Question:Where are Pull-Up circuits used?

Pull-up circuits are used on most sensor and switch inputs of electronic controls. Frequency sensor inputs to a control module do not typically have a "Pull-UpVoltage." One exception to this rule is the suspension cylinder pressure sensor inputs to VIMS interface modules. These sensor inputs DO require a Pull-Up Voltage. Some VIMS interface module frequencyinputs can be programmed by the VIMS configuration software to have a Pull-Up Voltage while others never use a Pull-Up Voltage. (Ref: SENR6059).

-6-

Question:

What Is The Purpose of "Pull-Up"Voltage"?

This voltage provides a known "signal" on the switch or sensor input for the electronic control. For example, the control knows (by the computer software program within the control) that ifthe input is open (no sensor is present or, the switch on this input is open) the "signal" on this input should be equal to the value of the Pull-Up Voltage. If a PWM sensor belongs on this input,...