In this issue, we’ll cover some of your questions on the topics of diagnostics, inspection tools and scopes. This area probably represents the greatest frequency of change, making it the most difficult to keep up with. Technology doesn’t stand still and neither should you in your quest to do your job effectively, efficiently, and accurately—the first time. Let’s get started.
Q. I see lab scopes mentioned every now and then for diagnosing electronic circuits. How is a lab scope different from older types of oscilloscopes?
A. First of all, a lab scope is a lot like the ignition scope you’re probably already familiar with. The lab scope also displays voltage on the vertical axis and time on the horizontal axis. The lab scope differs in that it measures more minute quantities of voltage and time than the ignition scope. The lab scope also allows you to select various voltage and time thresholds, giving you complete control over the waveform. Consequently, a lab scope's sensitivity, speed and control let you see problems as they happen in what's called a real-time format. This is priceless when tracking down a glitch in a system that may not appear on other types of diagnostic equipment.
Q. What exactly can I see in vehicle electronic systems with a lab scope?
A. A lab scope really shines when checking for the presence and condition of signals used in various onboard electronic systems. The scope also has several other uses including checking for noise on a signal line, but you will mainly check the signal waveform integrity of the two most common types of electronic signals. Analog signals generally come from variable-reluctance, magnetic-based position sensors, such as wheel speed sensors on antilock brake systems and some engine position sensors. The output of this signal is a variable, AC voltage that continuously changes in polarity. The strength of an analog signal depends on the rotating speed of the sensor's trigger wheel. The faster the speed the trigger wheel goes, the stronger the signal.
Digital signals, on the other hand, appear as a series of squares known as a square wave. Unlike the analog signal, the digital signal represents voltage turning on and off. Digital signals are typical of Hall-effect sensors such as those used in cam and crank sensors. The best way to get a feel for the type and importance of the signals used on different cars and systems is by experimenting with your scope. If possible, sample different cars that roll into your shop to get an idea of the different signal “signatures.”
Q. What are some tips for choosing the right lab scope?
A. Whatever you do, don't be hasty and buy the first thing that comes along. Talk with as many manufacturers’ reps as possible and ask a barrage of questions. Then, get as much hands-on time with different scopes and note your likes and dislikes. If possible, sign up for a training clinic or take a class at a local technical college on using a lab scope. Lab scopes with multiple channels allow you to compare two signals and compare their timing to make sure that events are properly synchronized. You can also use a two-channel scope to check cause-and-effect relationships, like checking the input signal to a certain component and seeing if it makes the proper response. Make sure the manufacturer recommends and stands behind a given scope in the arena of automotive testing. Since there's a lot more radio frequency interference (RFI) present in your shop than in a laboratory environment, the right scope will need some healthy shielding.
Q. During a scan tool discussion, someone brought up the topic of Mode 6 diagnostics. What exactly is it and what can it do?
A. In a vehicle’s Powertrain Control Module (PCM), Mode 6 performs on-board monitoring of test results for non-continuously monitored systems. Since Mode 6 data is live and not recorded, it can yield real-time information about system faults like ignition misfire, fuel control, oxygen sensors and more. Because of this, you can actually see things happening with your scan tool before they are recorded as a diagnostic trouble code (DTC). Of course, not all problems get to the point where they actually set a code, so any data that can help you sniff out problems is always a plus. In other words, you get the ability to look inside the PCM and look at values that may represent a given trend in the performance of a certain component or subsystem. The bottom line is, Mode 6 data can reveal a problem before it trips a DTC or sets the MIL.
Q. Do all scan tools display Mode 6 data?
A. As valuable as Mode 6 data can be, not all scan tools have the capability to display it. When shopping for a scan tool, check with your equipment rep to find out if the model you’re considering can access and display Mode 6 data. There may also be updates available for your current scan tool, which will help upgrade its capabilities. Ask a lot of questions and try different scan tools to see how they work when pulling out Mode 6 information. You will also find that scan tool manufacturers vary on how you access the data, by labeling menus in different ways.
Not all vehicle systems will reveal Mode 6 data, so it’s not an altogether universal diagnostic strategy—at least yet. Over the last several years, the automakers have added increased Mode 6 functionality, along with placing increased emphasis on the value of what the data can reveal.
Q. I’ve heard that too much AC voltage from the alternator can cause strange electrical problems. How much AC voltage is too much and how do I check for it?
A. No question, AC leakage from the alternator can confuse electronic controllers, so it's a good idea to check for too much AC when chasing weird, ghost-like problems. To test for this, connect a digital voltmeter to the battery with the engine running. With the meter set to the AC range and on the millivolts scale, you should see no more than 400 millivolts (0.4 volt) AC.
Q. We occasionally see cars with battery drain problems that leave the owner stranded. How do we diagnose these sorts of problems to eliminate the guesswork?
A. Key-off current drains can be diagnosed with a Digital Volt Ohm Meter (DVOM). Connect your DVOM in series between the negative battery terminal and its cable. With the meter set to the 10-amp scale, note the amperage reading. You may need to switch to a lower range after connection to get an accurate reading. In most cases, the current draw should be less than 150 milliamps. If it's higher, start by pulling fuses while watching the meter. When the current draw drops, you've located the group of accessories that may be causing the load. For exact specifications about normal key-off current drains, refer to a shop manual.
Q. We’re considering getting into reprogramming of Powertrain Control Modules (PCMs). How do we get started?
A. The best place to start is at the website of the National Automotive Service Task Force (NASTF), www.nastf.org. NASTF is a cooperative effort among the automotive service industry, the equipment and tool industry and automotive manufacturers, to ensure that automotive service professionals have the information, training and tools needed to properly diagnose and repair today's high-tech vehicles. Once on the site, click on the link in the left-hand navigation bar, “Reprogramming Information” that will then show a list of links for each manufacturer. When clicked, each manufacturer’s specifics for reprogramming will be shown, each compliant with SAE standard J2534. You can also download a spreadsheet containing a rundown of where you can locate J2534 programing information on manufacturer websites. Just click on the link toward the top of the page. Of course, this doesn’t mean that reprogramming has to be done exclusively through OE equipment. The aftermarket has a growing selection of J2534-compliant equipment. Just know that when it comes to reprogramming vehicles, it’s a two-part equation. The first part is the equipment itself and the second part is access to reprogramming information—the actual software code that you’re updating. Reprogramming subscriptions vary in price and availability, based on the manufacturer. You’ll want to weigh the access and costs of these subscriptions into your selection criteria.
In the next issue of PTEN, we’ll address your questions covering tools and equipment for Tire Pressure Monitoring Systems (TPMS), and tire and wheel service. See you then.