Pull UpVoltage SLI

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Pull-up Voltage


What is Pull-up Voltage?

Pull-up voltage is a voltage supplied from within an ECM through an internal resistor (typically 2K ohms). This reference voltage is used to monitor the state (open or shorted) of a signal circuit. Pull-up circuits

are used on most sensor and switch inputs of electronic controls.


How Does it Work?

A series circuit can be compared to the pull- up voltage circuit.


This series circuit is constructed with two loads of the same size. The voltage measured between each load to ground is 6 Volts.


If an open is created between the two loads, the voltage measured will now be 12 Volts.


If a short is created between the two loads, the voltage measured will now be 0 Volts.


When comparing this circuit to a sensor circuit of an ECM, Load 1 represents the resistor inside the ECM and Load 2 represents the Switch

or Sensor.


The point at which we measured voltage in the series circuit with a voltmeter can be compared to the point where the signal sensing device

is measuring voltage.


The Signal Sensing Device is always measuring the voltage drop of Load 2.


The resistance of Load 2 can change according to the parameter measured and the voltage value read at the Signal Sensing Device will

change.


If Load 2 is not present or a open occurs in the wires going to Load 2, full voltage will be seen by the Signal Sensing Device because Load 1

has no path to ground.


The code for this condition with a sensor would be Voltage high or Open circuit (FMI 03).


If a short to ground occurs after Load 1 and the Signal Sensing Device, no voltage will be seen by the Signal Sensing Device because it is now

measuring from ground to ground.


The code for this condition with a sensor would be Voltage low or Short circuit (FMI 04).


If a closed switch is inserted in place of Load 2, the Signal Sensing Device will see 0V.


This is usually a desirable signal in a switch input circuit.

For example: The switch would close when oil pressure is present.


If an open switch is inserted in place of Load 2, the Signal Sensing Device will see 12V.


This is usually an undesirable signal in a switch input circuit.For example: The switch would open when oil pressure is not present or

an open has occurred.


Typical Switch Circuit

Here is how the circuit would look on a machine.


953C Switch Circuit

This is an example schematic of a switch type input that a technician would see on a machine.

Note: COMPONENTS ARE SHOWN INSTALLED ON A FULLYOPERABLE MACHINE WITH THE KEY AND ENGINE OFF


Typical 2-Wire Sensor Circuit

2-wire sensors are monitored in the same manner as a switch, with the exception of a variable resistor in place of switch contacts. The resistance

changes according to the measured parameter.



The signal sensing device will measure the voltage drop across the variable resistor of the sensor. In this example, the desirable signal is

somewhere between 0 and 5 volts, but never 0 and never 5. If 0V or 5V is sensed a fault is activated.


This is the operating range for an Engine Temperature

Sensor


Engine Temperature Sensor

This is an example schematic of a 2 wire sensor type input that a technician would see on a machine.



3-wire sensor circuits are monitored on the B and C terminals in the same manner as a switch or 2-wire sensor circuit.



If an open occurs in the circuit full voltage will be received by the Signal Sensing Device. The technician would receive a “Voltage high”, or

“Open” diagnostic code (FMI-03)



If a short occurs in the circuit, no voltage will be received by the Signal Sensing Device. The technician would receive a “Voltage low”, or

“Short” diagnostic code (FMI-04)


This is an example schematic of a 3 wire sensor type input that a technician would see on a machine.

Engine Oil Pressure Sensor


NOTE: Pull-up voltage values are application specific and may vary between ECMs. Pull-up voltage sometimes is the same value as the

voltage source that powers the sensor, but does not have to be.


NOTE: It is not important to know what the pull-up voltage value is for a given signal circuit. Measuring voltage on an open circuit will provide no indication as to the integrity of the signal circuit in terms of excessive

resistance.


NOTE: Measuring voltage on an open circuit will give you an indication of whether or not a circuit is open or closed, a function already provided

by the ECM. What is important is understanding how the ECM usespull-up voltage to diagnose the signal circuit.


Questions?


OK, I have a few…


Question:

With the key on, what would the Signal Sensing Device see in the condition shown?


Question:

What would the Signal Sensing Device see in the condition shown?


Question:

Why does E.T. call this a “Voltage high” code?

Pull UpVoltage SLI

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