Pull-up Voltage and How the ECM Uses it to Determine Diagnostic Conditions

PULL-UP Voltage

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Pull-up voltage is a voltage supplied from within an ECM through an internal resistor (typically 22K 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.

To be able to effectively troubleshoot and diagnose switches and sensors, it is important for the service technician to understand the basic principles of pull-up voltage in an electronic control system. This illustration shows a typical example of a sensor input.

The signal sensing circuitry in the control is electrically connected in parallel with the input device. Basic electrical circuit analysis reveals that the voltage drop across, or the voltage output of, the input device will be seen by the signal sensing circuitry inside the control.

A series circuit can be compared to the pull-up voltage circuit. One lamp (1) represents a sensor or switch. The other lamp (2) represents the pull up resistor. This resistor is internal to the ECM. The battery (3) represents the pull-up voltage from the ECM. This voltage will vary depending on application, but is typically 6.5 Vdc for active analog sensors.

The series circuit above is constructed with two loads of the same size. The voltage measured between each load to ground is 6 volts, which is the voltage that will be detected by the signal sensing circuitry inside the ECM. It is important to remember that the signal sensing device is always measuring the voltage drop of the load farthest from the power supply (sensor or switch), so the point at which we measure voltage in the above series circuit with a voltmeter can be compared to the point where the signal sensing device is measuring voltage within the ECM.

The above series circuit is constructed with two loads of different sizes. 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 an 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 and therefore has no voltage drop.  Since there is an open, there is an infinite resistance. This infinite resistance will have all circuit voltage dropped across it. The voltage measured in this example will be 12 volts.

The code for this condition with a sensor will be Voltage High (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 in effect measuring from ground to ground. The voltage measured will now be 0 volts. Current takes the path of least resistance. In this case, that path is the short to ground; therefore, no voltage gets to the second load. All of the voltage is then dropped across the first load.

The code for this condition with a sensor will be Voltage Low (FMI 04).

If a closed switch is inserted in place of Load 2, the signal sensing device will not measure a voltage (0 volts). 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 measure 12 V. 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.

The illustration above depicts a pull-up circuit for a three-wire sensor. Looking at the above ECM circuit, we can see that three-wire sensor circuits are monitored by the signal sensing device between the B and C terminals in the same manner as a switch or two-wire sensor circuit.

NOTE: It is important to remember that a pull-up circuit is only monitoring the B and C portion of a sensor circuit. Supply power for the sensor is independent of pull-up voltage.

If an open occurs in the circuit, full voltage will be measured by the signal sensing device. The ECM is programmed so that this measured voltage, 6.5 volts in this example, is beyond the acceptable operating range for the signal. Cat ET would display an active "Voltage High" diagnostic code (FMI-03).

If a short occurs in the circuit, no voltage will be measured by the signal sensing device, as the full voltage drop is across the internal pull-up resistor. The ECM knows that this measured voltage, 0 volts in this example, is beyond the acceptable operating range for the signal. Cat ET would display an active "Voltage Low" diagnostic code (FMI-04).


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NOTE:
Pull-up voltage values are application specific and may vary between ECMs. Pull-up voltage can be the same value as the voltage source that powers the sensor, but does not have to be.

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.

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 uses pull-up voltage to diagnose the signal circuit.







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