This past October, I presented a class at the ATRA Powertrain Expo in Las Vegas. The event was amazing! I’d personally like to thank everyone who was involved in putting this event together. It was a necessary component for technicians to train and stay at the top of their games.
The class I presented covered digital storage oscilloscope use, specifically the testing of permanent magnet generators and Hall Effect sensors. During the presentation, some questions arose and I believe there was a little confusion.
In the last issue of GEARS I said we’d be covering vacuum transducer use over the next few months. If you can bear with me for one more month, I promise to come back to that topic. This month I’d like to clear up any issues from my presentation while the material from Expo is still fresh.
The question was about digital signal sensors, specifically the wiring and voltages expected on each wire. To tackle the issue, let’s start from scratch with how these sensors work.
You can think of digital signal sensors as switches that open or close using magnetism. There are a variety of sensors that fall into this category, including Hall Effect sensors and magneto-resistive sensors.
Hall Effect sensors are simply transistors with a fixed magnet. A rotating interrupter wheel alternately opens and blocks the space between the transistor and the magnet. When the space opens, the transistor allows current through to the signal wire; when it closes, the transistor shuts off the flow of current. The result is a digital, on-off waveform that varies in frequency based on the speed of the interrupter wheel.
Magneto-resistive sensors are a little different. They combine a permanent-magnet, AC-generating sensor with an internal IC chip. The IC chip requires power and ground to create a digital signal, based on the AC signal from the sensor. An example of magneto-resistive sensors was covered in detail in the June 2012 issue of GEARS.
Take a look at the diagram (figure 1). The transistor symbol inside the sensor represents the IC chip; the switch is mainly for visual clarity. The inside of the magneto-resistive sensor is actually more complex than this.
The usual harness configuration consists of three wires: one power, one ground, and one signal. The power and ground wires supply the electrical current the sensor needs to function, just like a module. A module has to be powered up to work and so do these sensors. In some cases, the sensor takes its ground through the chassis instead of a separate wire, so they’ll have only two wires to the sensor.
The signal wire is how the sensor transmits its data to the computer. There’s also a pole piece, or reluctor, in front of the sensor. It’s made of ferrous metal, and it mounts to a rotating component, such as the crankshaft. The rectangle represents a tooth on the reluctor. In this case, the presence of the metal causes the switch to open.
As the metal pole piece moves past the sensor, the IC chip opens and closes the circuit (figure 2). This opening and closing continues as the teeth on the reluctor move toward and away from the sensor. The on-and-off action of the IC chip is what generates the digital signal to the computer.
In some systems, the sensor creates the signal voltage from the voltage that’s applied to power the sensor. The IC chip uses the applied voltage to power up and to create a signal. This is the case with Nissan cam and crank sensors.
In other systems, the voltage level of the digital signal generated is determined by the circuitry inside the computer (figures 1 and 2). In this case, we have a 5-volt reference voltage. This reference voltage could be any value, but is most often five volts or system voltage.
To enable the system to pull the voltage from power to ground, the next component in line is a resistor. In this configuration, the resistor is considered a pull-down resistor. The value of the resistor isn’t important as it only provides a load for the circuit.
After the resistor comes the voltage-measuring circuitry in the computer. This is where the computer actually reads the sensor’s input. Finally, the signal wire leaves the computer and connects to the sensor, where it may or may not be pulled to ground.
Now that we’ve examined the configuration, let’s look at how these sensors generate the voltage signal.
The signal generation is actually a basic electrical concept. Using a simple illustration (figure 3), you can easily apply basic electrical knowledge to understand the signal generation. In this case, the reference voltage is system voltage, the bulb is the resistance, the voltmeter is the computer’s measuring circuitry, and the switch is the magnetoresistive sensor.
When the switch is closed, current flows and voltage drops across the bulb. Since the bulb is the only load in this simple circuit, all of the source, or reference, voltage gets used. The remaining voltage would be near zero. The switch pulls the voltage low, which is why this wiring configuration is called a pull-down circuit.
Conversely, when the switch opens (figure 4), the circuit opens and no current flows. Since there’s no current flow, no voltage drops. As a result, the voltmeter shows the available, or reference voltage; in this case, system voltage. If the switch continues to toggle on and off, the voltage will continue to switch between system voltage and 0 volts.
In the sensor diagrams (figures 1 and 2), the voltage toggles between 5 and 0 volts because the reference voltage isn’t system voltage, but the circuit still functions the same.
In the case of a pull-up circuit, the reference voltage and ground in the diagram are reversed. When the switch is open, the voltage is 0 volts. When the switch closes, the voltage rises to reference voltage, or is pulled up. Even though pull-down is more common, the results are the same: an on-and-off digital voltage signal that conveys speed and position.
The actual digital pulses vary in frequency with the reluctor speed and each pulse can be a different length if the reluctor doesn’t have equally sized or spaced teeth on the reluctor. With a variety in reluctor designs, digital sensors can transmit both speed and position data.
For example, an input shaft speed sensor will most likely have evenly spaced reluctor teeth because those sensors are only used to measure speed. On the other hand, camshaft position sensors mainly identify position, so they’re usually designed with fewer teeth that vary in length and spacing.
When diagnosing digital sensors, always check power and ground. As long as the sensor has what it needs to power up, you’re ready to check the signal. A voltmeter isn’t the appropriate tool for this task. The only way to see the actual working voltages of a digital sensor on its signal wire is with a digital storage oscilloscope.
Now that the basics, function, and design are out of the way, let’s discuss the statement that was presented in class. Using figures 1 and 2, the same images I used in class, someone made this statement: “If the digital sensor receives system voltage as its a power feed, then the waveform will be toggling between 0 and system voltage.”
Hopefully you’ll see that this statement is false. While many of these sensors require system voltage to function, the signal voltage is often the direct result of the reference voltage applied by the PCM to the sensor on the signal wire, or in this case, five volts.
Keep these concepts in mind while examining wiring diagrams during diagnosis. Other inputs function in a very similar manner. One example would be a typical mass airflow sensor, either analog or digital. An analog MAF sensor receives system voltage to power up but outputs a 0-to-5-volt analog signal, not a 0-to-system-voltage digital signal.
A MAF sensor that outputs a digital signal also has system voltage for its power feed, yet the on-and-off digital signal will be 0 volts to reference voltage, not system voltage, unless the reference voltage happened to be at system voltage level as well.
Engine or electrical diagnostic issues you’d like to see addressed? Let Scott know. Send him an email at email@example.com and you just may have your question covered in a future issue of GEARS Magazine.