Pull Up Voltage 3c186

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 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 through an internal resistor which "pulls up" the signal circuit on the connector of the control input shown in this illustration. The signal is held at this Pull-Up Voltage (high) except during the time that the sensor (or switch) drives the signal to a low level (low). The , "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-Up Voltage." 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 frequency inputs can be programmed by the VIMS configuration software to have a Pull-Up Voltage while others never use a Pull-Up Voltage. (Ref: SENR6059).

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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 if the 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, the control knows when there is no PWM oscillation because the input is usually fixed at the Pull-Up Voltage value. This is the condition the control identifies as FMI 03-Voltage Shorted High. The opposite condition may occur on a signal input that is expected to be oscillating as from a PWM sensor but is shorted to ground. Again, there will be no change in level or voltage value of the input so the control knows that a signal input that is continuously held at ground (low) is abnormal. The control recognizes this condition as FMI 04-Voltage Shorted Low.

Question:

Under What Conditions Will I See "Pull-Up Voltage"?

When the sensor or switch is disconnected at the harness connector, the signal should be pulled up to the value (high) of the pull-up circuit. When a voltmeter is connected between the signal and signal ground on the harness connector (control side), the voltmeter should read the value of the Pull-Up Voltage.

Question:

What Value Is "Pull-Up Voltage" to the Service Technician?

The purpose of Pull-Up Voltage is to allow an electronic control to determine what is happening on its signal input in the outside world. This ECM design is of great value to the service technician, it allows a quick test of the entire control input circuit. For example: •

If the sensor or switch is disconnected and the Pull-Up Voltage is measured to be the specified value, the harness and control are very likely to be working as expected.

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If the sensor or switch is disconnected and the Pull-Up Voltage is measured to be ABOVE the specified value, the failure is a harness short to a voltage source higher than the pull-up value or, the control has failed internally.

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If the sensor or switch is disconnected and the Pull-Up Voltage is measured to be near or at ground (0 Volts), the harness is likely to be open, or shorted to ground, or the control has failed internally.

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Question:

How Much Voltage Can I Expect From Pull-Up Circuits?

This voltage is determined by the electronic control design and may vary between controls. Use the service manual for the control to find the voltage. Pull-Up Voltage sometimes is the same value as the voltage source that powers the sensor. , Pull-Up Voltage is on the SIGNAL input to the control for a given sensor (or switch) and most often HAS NO relationship to the voltage that POWERS the sensor. PWM sensors most often have a Pull-Up Voltage value different than the voltage that powers them. Analog sensors, as used with engine controls, most often have a Pull-Up Voltage that is the same as the voltage that powers them. Typical values you can expect are 5V, 6V, 8V and 12V but can include other values also.

Question:

What is Meant By Analog and Digital Sensors?

Analog sensors provide a signal that varies smoothly over the measured range such as a resistive fuel level sender or engine oil pressure sensor as used with an engine control module. The value of the fuel sender is in Ohms and is measured on the resistance scale of a multimeter. The signal of the oil pressure sensor is proportional to the sensed pressure (condition) and can be measured with a DC voltmeter. Digital sensors provide a signal that has only two voltage levels: at ground (low) or at some positive value (high) and oscillates between these two levels. The actual value of the measured condition is proportional to that condition and is represented by either the duty cycle (use duty cycle meter) or frequency (use frequency meter) of the signal. The electronic control module knows (with internal software) what type of signal is expected on each input and how to "read" the signal.

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