During the last 10 years, I’ve worked in a variety of shops and, to date, have been the only tech that owned a DSO. When techs see it in use for the first time, they invariably ask what I’m doing and why — and then ask if they should own a scope of their own. While most have been capable of repairing problems without one, let me share a few examples of why they could have accomplished the same thing in less time and with more assurance their repair would be the correct one.
A Little Background
Automotive scopes have evolved significantly since the days of the old analog Sun models I grew up with. In those days, scopes were used primarily to view secondary ignition patterns in real time. I remember reading about techs around the country who were using the scope to test other electrical systems and sensors, and started playing with the techniques I learned from them. The only problem with the analog scope was the inability to freeze a capture for a closer inspection. If the fault was intermittent, it was that much more difficult to catch.
The DSO, or Digital Storage Oscilloscope, is akin to the Digital Multimeter (DMM) in that it doesn’t trace a voltage signal in real time. It samples the signal and reproduces that sample on the screen as a graph of voltage over time. The sample points then are connected by the DSO to form a pattern you can view. The faster the sample rate, the more accurate the screen trace will be.
This isn’t an article on exactly how a DSO works; there have been plenty of those already written. Suffice it to say, the sample rate on even the slowest DSOs is light years faster than any DMM on the market. Today’s DSOs also have the ability to record these samples over an extended period of time, allowing you to store and later review the patterns recorded, and that makes finding intermittent faults a lot easier.
DSOs are available as handheld units, or as a PC-based application. Some include databases of specific tests for specific components, and can preset the voltage and time divisions for you. There are two-, four- and eight-channel units available (a channel is one measurement — think of it as one voltmeter lead, with one reading displayed). DSOs vary quite a bit in their sample rates and buffer sizes. When choosing a scope, remember this bit of advice my Dad gave me when I was a teenager, wanting to buy a stereo.
“Son, there’s no sense in buying a $500 stereo if you only have a $50 pair of ears.”
In other words, match your scope to your ability and budget. As your skills in using one grow, so will your knowledge of exactly what you want in your next one.
Do You Really Need One?
There isn’t enough space in the pages of this issue for me to share all the instances where using a scope helped me diagnose and repair a problem. A DSO has the ability to measure circuits and signals that operate at speeds too fast for a conventional DMM. Consider an example: You want to verify the electrical integrity of a fuel injector circuit.
This circuit is only on for a matter of milliseconds — about the same amount of time it takes for an ignition spark to jump a plug gap. A DMM is useless for this test, but a DSO is more than capable and the resulting pattern can tell you everything you want to know about the electrical health of the tested circuit if you know what to look for.
Another common use for DSOs is crank and cam sensor testing.
Sure, you can check for power and ground to these sensors, but a DMM can’t tell you if the signal from the sensor is correct and steady. The DSO pattern can tell you both, and aids in finding an intermittent loss of any input signal the control module needs to carry out its duties.
A DSO is a useful tool for checking signal relationships, too. By measuring the signal of both sensors at the same time and comparing them to a known good pattern, you can tell if the sensors are in time with one another. Sensors found to be out of synch can be the result of a jumped timing belt and sensor signals that bounce around in relationship to one another can be an indication of a worn timing chain. There is a Ford TSB that uses this relationship to check for improperly installed cam sensor synchronizers on their models, and the TSB shows what a good pattern should look like. (In the old days, you had to collect your own, but today there are several on line resources available.)
Any voltage signal can be verified and tested using the DSO, whether it be input or output. This makes a DSO excellent for “If this, then that” troubleshooting. By monitoring an input signal and looking for the correct output signal, you can make sure that the control module is acting on its programming and test a given system while it is actually operating, just like the control module does.
It’s Not Just Voltage
A DSO is limited to reading voltage, but that doesn’t mean it can only measure voltage. There are scope accessories that convert current, pressure, vacuum and even temperature into voltage signals that the DSO can read and trace. This adds even more usefulness to the DSO as a diagnostic tool.
One of the first additions to my scope was a high and low amp probe. These accessories measure current flow, and convert it to a voltage signal the DSO can see. Monitoring current in an injector or ignition primary circuit aids in diagnosing problem coils or control module driver issues. I’ve used current monitoring to find intermittent A/C compressor clutch engagement problems, detect faults in relay controlled circuits and to help identify weak fuel pumps, just to name a few examples.
Current is what makes a component work, and watching what that current does as a component operates can speed up diagnosis. Can you measure current with your DMM? Sure, but by tracing that reading over time, I can tell a lot more about what is going on in that circuit than you can.
One of the first diagnostic tricks I learned with the DSO, and still use routinely, is the relative compression test that measures the current used by the starter. Simply, weak cylinders cause a lower current draw in the starter and that can be measured and traced on a DSO screen. If a weakness is detected, I next connect a pressure/vacuum transducer to the oil dipstick tube and repeat the test.
If the second pattern is OK, I know the fault is in the valvetrain…if not, it’s in the piston and/or rings. In a matter of minutes, I can get a quick picture of the engine’s mechanical health. Compare that to the time it would take to perform a conventional compression and cylinder leakdown test on a modern, transverse engine.
Measuring pressure and vacuum is nothing new, but a relatively new expansion on this idea is measuring the running pressure in a given cylinder. This is an exciting idea, allowing you to perform one test that will allow you to see exactly what is going on in an individual cylinder on a running engine. Intermittent sealing issues, exhaust backpressure problems and valve timing faults are just a few of the problems that can be easily identified and confirmed using this test. Here, there is no substitute test that I know of that can tell you so much in such a short time.
Do you need a DSO to fix today’s cars? No, you don’t. But the addition of a DSO to your diagnostic arsenal will help you fix more of the ones you just guessed at and missed, saving you time and money in the process.
