Scoping out engine sensors

“How do I know where to put my test leads and what settings to use on the scope?’ That’s the most common question that comes up while I am teaching a scope class. Digital Storage Oscilloscopes (DSOs) are not much different than what you have used for years to test electrical circuits. If you can use a test light, logic probe or multimeter, you can use a DSO. Scopes take accurate measurements and provide a picture that really is worth a thousand words, providing information that the other testers miss.

Remember the graphs you had to draw in school? That is essentially how DSOs work. They take millions of samples and plot them on an X and Y axis, plotting voltage over time. All DSOs measure voltage over time, whether they are a handheld or PC-based scope. Some DSOs have their graphs split up into eight grids of voltage and 10 grids of time, while other scopes might have a 10-by-10 grid display. It really does not matter, because you have control over the waveform. Some scopes allow you to zoom in and out, allowing the user to make the waveform as big or small as they like. If your scope does not have this feature, just adjust the voltage and time settings on your scope to achieve the same results.

The answer is different depending on what component we are going to test. You connect a scope the same way you would connect a multimeter to test for voltage. The difference is rather than looking at voltage numbers you are looking at a waveform.

Setting Up For Success

And just like your meter, you first have to pick between AC and DC voltage. AC should be selected to measure any circuit or component producing an AC voltage signal. Some examples include some speed sensors (vehicle and/or wheel), some cam and crank sensors and, of course, when looking for excessive AC ripple in the charging system. Selecting AC also plays a role in some DC tests. By selecting AC, you remove the DC component of the signal and can focus on the changes in amplitude of the waveform itself. A great example is when testing relative compression using a current clamp.

But for most testing, you’ll be setting your scope to read DC voltage. And you’ll be adjusting the voltage scaling on the meter’s X axis in the same way you need to select your measurement scaling on your multimeter.

The first step to take when using a scope to take a simple voltage measurement is to make sure we have a good engine ground. Connect the black (negative) lead of the

scope to a good ground (don’t forget to scrape the alligator clip to the ground), followed by connecting the scope positive lead to the battery positive post. With your scope set to read DC volts, you should see a straight line being displayed as the waveform on the scope equal to the battery’s Open Circuit Voltage (OCV).

Why don’t I suggest connecting the ground of the scope to the battery? Many times there is corrosion, interference or noise from the charging system. Another reason is that many lead sets only have about a six-inch ground lead that is connected to the main lead set and that’s not allowing the length needed to always ground to the battery. Use a ground close to the sensor or actuator. You can always verify the integrity of the ground path back to the battery by performing a voltage drop test with your multimeter or the scope.

A quick word on “floating” grounds. Many engine sensors are grounded through the Engine Control Module (ECM) and are purposely offset to avoid interference issues. If you measure the voltage drop on the ground conventionally, you’ll get a reading of around 0.70 volt. This also will impact the amplitude of the signal, making it look weaker than it really is. Check the schematic of the sensor you are testing to see where best to place your scope’s negative test lead.

Let’s Get Started Let’s try connecting to a very common sensor, a 4-wire O₂sensor. We have a 1 in 4 chance of connecting to the correct wire. Of the wires going to the sensor, one has 12 volts for the heater circuit, another provides the ground for the heater, a third is the ground for the sensor and the fourth is the signal wire that has will be sending out about 450 mV. Once you’ve confirmed you have the negative scope lead connected to a good ground, you can start back-probing the connector to find the correct wire.

Of course, you can look up the info on the appropriate wiring schematic and some scopes actually have a built-in library that will tell you which lead to connect to. Your scope setting should be about 200 mV per division or 2 volts per screen along with a time setting of 200 mS per division or 2 seconds per screen. Why did we pick those settings? What output would you expect to see from an oxygen sensor? What does the “Theory of Operation” section of your service information system tell you about how the oxygen sensor is tested by the ECM? These setting are a good starting point that will provide you with a waveform that oscillates from rich to lean as you raise the engine speed to 2,500 rpm.

Two wire sensors are easy to test, since we have a 1 in 2 chance of connecting to the correct wire. Once again let’s make sure that we baseline our scope and test as we did before making sure we have a good engine ground. Because this is a two-wire sensor, one wire will be ground while the other one will be signal. Some examples of two-wire sensors are the Engine Coolant Temperature Sensor (ECT), the Intake Air Temperature Sensor (IAT) and others. These sensors typically produce a varying voltage over time, and might use a “reference” voltage the ECM supplies. To set up your scope for the capture, start by setting up the voltage scale so a full range of system voltage can be covered (up to running charging voltage) and configure the time divisions so two seconds are displayed across the screen. Comparing the actual reading these sensors are providing to the corresponding data Parameter Identifier (PID) on your scan tool is a great way to catch a “lying” ECM.

And two sensors that we never want to see “lie” to the ECM are the Manifold Absolute Pressure (MAP) sensor and the Mass Air Flow (MAF) sensor. The ECM relies on one or the other, depending on the fuel control strategy used, to determine how much fuel to feed the engine. The MAP is a three-wire sensor that typically has a 5-volt reference supply line on one pin, a sensor ground on a second and a signal line back to the ECM on a third. The wire you want to connect your scope to is the signal wire. The waveform will be a straight line of voltage about 3.2 to 5 volts. Normal operating voltage with about 18 inches of vacuum should be about 1 to 1.4 volts. Your scope settings should be 1 volt per division (or 10 volts per screen) and the time setting set at 200 mS per division (or 2 seconds per screen). If the time is set as suggested and the engine speed is raised quickly, the waveform will go from low voltage to high voltage making a hump in the waveform. With the scope at these settings, you can find an analog MAP that is breaking down that you normally would not be able to identify otherwise.

MAF sensors provide similar information to the ECM along with temperature information. These sensors can be intimidating, because they typically have five wires. No matter if it’s an analog or digital sensor, the wiring is going to be the same; one wire will be the ground for the MAF, one will be the signal (this is where we connect the scope) and one will be the reference supply voltage from the ECM (typically 5 or 12 volts). The other two wires are for the IAT part of the sensor, with one being ground and the other reference. The scope will either display a straight line if it’s an analog sensor or a square wave if it’s a digital sensor. The scope setting should be 2 volt per division or 20 volts per screen and the time setting set at 2 to 5 mS or 20 to 50 mS per screen. The waveform on the analog sensor should look the same as a MAP described above.

The Throttle Position Sensor (TPS) is another three-wire sensor that has a 5-volt supply, a signal (where we connect the scope) and ground. The scope setting should be 1 volt per division or 10 volts per screen and the time setting set at 200 mS or 2 seconds per screen. The TPS is a potentiometer that produces a varying signal based on the position of the sensor. By rapidly opening and closing the throttle, you can look at the signal for any dropouts in what should otherwise be a smooth rise and fall. Other potentiometers can be tested similarly, like the Accelerator Pedal Position (APP) sensors used on many later model cars.

Crank (CKP) and Cam (CMP) sensors that are DC sensors are also three-wire sensors that use a reference voltage (typically 5 or 12 volts), ground and signal. We connect our scope leads to a good engine ground and the signal wires. The waveform will appear on the scope screen as a square wave that will increase or decrease in frequency as amplitude will always remain the same on a good sensor. The scope setting should be 2 volt per division or 20 volts per screen and the time setting set at 2 to 5 mS or 20 to 50 mS per screen.

Magnetic crank and cam sensors can be a two- or three-wire sensor depending on whether or not the sensor has a shielding wire. Because these sensors produce an AC voltage signal, we need to make sure that the positive lead of the scope is going to the positive lead of the sensor. The scope setting should be 1 volt per division AC or 10 volts per screen and the time setting set at 200 mS or 2 seconds per screen. The settings are a good starting point but may have to be change on a no start engine with a low battery. The amplitude and frequency of the signal changes with crankshaft speed. Similar settings can be used for the Antilock Brake System (ABS) wheel speed sensors that are the AC generator style.

The Knock sensor is an AC producing sensor that is mounted right on the engine intake manifold or block that is used to detect detonation and/or preignition. It does so by measuring engine vibration. Connecting to this 2 wire sensor is easy since it has one positive and one negative wire. The voltage signal on the screen to the right is too low and weak, causing by a defective sensor.

Starting From Scratch
Now let’s take a look at an eight-wire MAF sensor from a 2013 Caddy CTS 3.6L. If you were working on this vehicle and wanted to test the MAF sensor, it at first might look a little intimidating and very confusing. The first step is to look at a wiring diagram (Figure 11) as the one I use from MotoLogic that is already colored and allows me to highlight the wire I want to backprobe. This sensor has the IAT sensor 1, IAT sensor 2, Humidity sensor, MAF sensor and BARO sensor all in one housing. This frequency sensing signal will produce a square wave where the amplitude does not change but the frequency of the signal does as more or less air enter the intake manifold.

The two IAT signal wires will have only a few hundred millivolts on a hot engine while the ground will have little to no voltage, the Humidity sensor most likely will read the same and the BARO will have about 1.3 volts KOER, which leaves you with a ground and the signal wire for the MAF. Without using a wiring diagram you can backprobe all the wires until a square wave appears on the scope. When I was testing this sensor on the vehicle, I did not use a wiring diagram even though I was surprised to see an eight-wire (Figure 12) sensor. Breaking things down into pieces and knowing what the waveform should look like makes scoping a breeze.