Other Articles - October/November - 2021

Using Scopes for Diagnostics

This article is a bit of a teaser for the upcoming Powertrain Expo, coming this October in Las Vegas. We will demonstrate how to use various scopes (budget-friendly to pretty expensive) for automotive diagnostics. During that presentation, we will have a variety of scopes on hand, and we will perform live demonstrations showing their capabilities.

Scopes measure voltage over time, and it’s as simple as that. What makes them different than a Digital Multimeter (DMM) is that a scope traces the voltage changes on the screen, and it can do it very quickly! This is where scope usage gets confusing to the new user. It’s very important to understand the settings for the scope so that you’ll obtain a usable scope pattern, which is the most common complaint I hear from students and seminar attendees.

In this article, we will give examples of how to set your scope up perform a couple of different tests, and then we’ll leave you with a scope pattern that may or may not be good. Hopefully, you’ll attend Expo and join in the discussion!

Engine Analysis

The easiest and least intrusive test is the “relative compression” test using either amperage or voltage to determine if each cylinder pulls an even amount of current from the battery. Every time the engine compresses the air/fuel charge, the counter-resistance on the starter motor causes more current to travel through its windings, and as a result, the battery voltage dips a bit. We can use this attribute to evaluate if there’s a cylinder that’s not pulling its weight. Figure one shows a snippet of a scope image that shows this voltage/amperage relationship. Notice that they are an exact mirror image of one another.

With most PC-based scopes, the user can capture plenty of information on the screen and then zoom in to get detail. For this reason, we recommend setting the scope up at a high sample rate and plenty of time on the screen, such as 5 million samples per screen and 500 milliseconds per division. This setting displays 5 seconds on the screen and samples the voltage at an interval of 1 microsecond (that’s one-millionth of a second!). The scope image might look noisy at first, but once you zoom in, you’ll get a pattern that you can use. The benefit of capturing a larger amount of time on the screen and then zooming in is that you won’t miss anything when your screen refreshes, and you won’t have to jump between screens to see the whole event. This is a personal preference, and some techs might prefer to use a time base that shows the pattern perfectly without zooming, but we’ve found that when figuring things out initially, it makes more sense to collect a lot of information and then zoom in.

First things first – let’s hook everything up.

For this article, we are using a Pico 4423, which is not anywhere close to the newest model, but it works well and shows that you don’t always need the latest and greatest to do most automotive diagnostics. This article applies to any decent scope that can sample at a high rate and allows the user to zoom in. So to start, channel A is battery voltage where the red lead is battery positive, and the black lead is battery negative. Set the voltage on this channel to 20VDC.

Channel B is battery amperage measured by using a high current amp clamp. The amp clamp is directional, so you might need to flip the clamp if you don’t get the expected waveform during the measurement. If measuring the true amount of amperage is important to you, you’ll either need to select the proper amp clamp from the “probes” list or do the voltage to amperage conversion. This is because the scope doesn’t read amperage directly. It reads the voltage the amp clamp generates in relation to the amount of amperage it’s measuring. The amp clamp we’re using in our example outputs 1mv per 1 amp measured, so we can set the scope up to measure 1VDC – where 1A will display as 1mV and 1000A will display as 1000mV (1 volt).

Channel C is a cylinder’s ignition event. This could be a spark plug wire, or it could be the ignition coil signal. It can even be an injector, but it’s best to use an ignition component because there’s no doubt that a plug fires near the top-dead-center (TDC). You’ll need this information, along with the engine’s firing order, to determine which cylinder is not contributing if you have an uneven pulse.

So, you might wonder how this can be done with a two-channel scope since we’re using three channels in our example. Well, you don’t need to measure voltage AND amperage. One or the other will work. In the examples shown, we have both measured simply for the sake of proving that you can use one or the other. Many techs think it’s necessary to use an amp clamp, but voltage will work just as well if you can zoom in on the scope pattern. Plus, be honest, how many of you have a good battery in your amp clamp?

Here is an example of a 2016 Chevrolet Silverado. When performing the test, you will want to prevent the engine from starting, preferably by disabling fuel. With most GM products, you can use the “clear flood” feature, which inhibits injector operation, by cranking the engine over while holding the throttle position at wide-open throttle.

Figure two shows the scope pattern captured from the above scope settings of 500ms per division and 5 million samples per screen. Blue is the voltage trace, red is the amperage, and green is the ignition trigger signal for cylinder #1.

Figure three shows the zoomed-in scope pattern of that same capture. We zoomed in to the amperage and voltage between the ignition trigger by selecting the zoom feature on the top menu bar. Then, by selecting the channel boxes in the lower right and left corners, we zoomed in on voltage by 8x and amperage by 2x to get a bigger pattern.

Figure four shows the zoomed-in scope pattern with some filtering added to clean the image up. The filtering feature is nice to remove some of the high-frequency interference. You can add filtering by clicking the down arrow by the channel and selecting “activate filtering.” By default, the filtering usually filters out noise above 1,000Hz, which works well for this pattern. The engine’s firing order is also added to that scope capture to identify the compression stroke of each cylinder.

Figure five shows a cranking pattern from a different vehicle. See if you can identify anything wrong with the capture. What would you conclude and what would be your next steps in diagnosis? The firing order for this 6-cylinder engine is 1-5-3-6-2-4 and the ignition pulse is on cylinder #1.

We’ll continue this scope discussion regarding engine analysis at Expo by covering in-cylinder pressure and pulse sensor diagnostics. If you saw an issue with Figure four, we’ll have the in-cylinder and pulse tests to help us evaluate this engine issue. We’ll also demonstrate some simple shop-made tools that you can make to help ease the costs when just starting scope-based diagnostics.

In the next example, we’ll show how using a scope can help with network communication diagnostics. We’re sure at some point you’ve experienced the “no communication” screen on your diagnostic tool like shown in Figure six. Most vehicles produced within the past 15 years utilize the Controller Area Network (CAN) to deliver information between control modules.

Here is a description of the high-speed (HS) CAN in few paragraphs. The HS CAN is a two-wire network that is NOT “fault tolerant.” This means that a failure in one circuit will potentially cause no communication to one or more modules or fail the entire network. We need this network during diagnostics because that’s how our scan tool communicates to the control modules! An HS CAN may have up to 13 modules, but like shown in Figure seven, two of the modules will contain 120 ohm terminating resistors separating the two HS CAN wires: CAN + and CAN -. When testing the HS CAN, resistance between pins 6 and 14 at the DLC. The result should be about 60 ohms (two 120 ohm resistors in parallel measure 60 ohms), as shown in Figure eight.

Be aware that many vehicles have more than one CAN bus. You must look at a wiring schematic to determine what modules are on the network and how CAN + and – circuits are accessed. Also, when testing the resistance of the HS CAN bus, manufacturers typically have you disconnect the battery since any stray voltages will mess with your ohmmeter reading. While this is true, we also know the issues that might arise from disconnecting the battery, ranging from lost radio and seat memory features to adaptive controls resetting. We prefer first to try to measure the HS CAN with the battery connected. If the readings are erratic, wait a couple of minutes to let the HS CAN shut down, then once everything is stabilized, you’ll likely get your valid measurement. If the measurement is 60 ohms, you can move on with the knowledge that at least the modules with two terminating resistors are present. If the measurement isn’t 60 ohms or is not a believable value, then disconnect the battery and try again. This Ford Focus example took about 20 seconds for the CAN to shut down and give us about 62 ohms of resistance, as shown in Figure eight.

The CAN + and – wires have approximately 2.5 volts when no messages are being sent. When a message is being sent, a module on the network will elevate CAN + about 1v (so the CAN + circuit raises to about 3.5 volts) and simultaneously, the module will pull CAN – down about 1v (so the CAN – drops to about 1.5 volts). Figure nine shows a typical CAN bus message found on a 2013 Ford Focus. Every module in the network will see the voltage change and start deciphering the message.

The message is very fast. You can see in Figure nine that to capture just a couple of messages, the scope is set up to 50 microseconds per division. That scope screen is only showing 1/2000th of a second!! Now, just like the relative compression example, we prefer to set the scope up with plenty of time on the screen and zoom in. So, if your scope is powerful enough, try setting the scope up to 200ms/div and cranking the sample rate up to 10 million samples per screen (10MS) while measuring voltage on the 5-volt scale (figure 10). Then start zooming in a little at a time. Even though the pattern initially looks like two big blocks of red and blue, once you start zooming in, the individual HS CAN messages are there as clean as can be (figure 11).

Now for a diagnostic example: this Ford Focus has no communication to the TCM, but it communicates to other modules. The technician checked the HS CAN at the DLC and found the pattern displayed in figures nine, ten, and eleven. Then, the technician checked the HS CAN at the TCM by back probing the connector. After zooming in, they found the pattern in figure 12. The technician verified the power and ground circuits to the TCM by load testing the wires and testing voltage drop. They all tested good. So, what would you conclude? And what would your next steps to diagnosis be? Please join us at Expo on Wednesday evening to discuss these patterns and more. Between the two of us, we plan on sharing information and demonstrating scope setup and measurements on scopes ranging from $60 to $2000!