The Digital Multimeter (DMM) and Digital Storage Oscilloscope (DSO) both can be used to measure voltage, but the way each displays that voltage reading, and what you can learn from it, is very different. If you can use a multimeter, you can use a scope, so hold on and let’s see where you might want to use the scope over the meter during your troubleshooting.
The computer doesn’t know that the sensor signal dropped out for a millisecond or two. It only knows it didn’t see what it needed to see.
Try taking your DMM and your DSO and connecting both of them to the battery post until you see a straight line of voltage. Now you get to see the number from the meter and the straight line of voltage on the scope, so what’s the difference? While both are use digital processors to sample the voltage being measured, the meter shows only the average of the samples while the scope shows voltage over time. Additionally, most DMMs sample the input voltage at a significantly lower rate than even the cheapest scopes do. Today, many handheld and PC-based scopes are checking the input signal millions of times per second. That means that even momentary losses of the voltage signal will be seen and displayed in a form that is easy for the user to recognize.
Scopes measure time horizontally and voltage vertically. So you might be wondering what good does this do you, and why would you want to use a scope. Hang in there and see where it would be beneficial using a scope. Let’s take a look at a common sensor that produces a square wave like a Hall Effect sensor. Now you will get to see the difference between the DMM and DSO first hand. Take a look at the square wave voltage on the voltmeter. Do you notice that the voltage is about 2.5 volts? Now take a look at the scope reading that you measured from the zero line noting that each line is 2 volts per division going up from zero. Now you are looking at the true voltage of the sensor that is being displayed.
The other difference between the scope and meter is clearer when we are trying to find out why the engine is dropping out. Since a meter can only update 40 samples a second depending on the meter while a scope can typical sample 25 million samples a second. When you are trying to locate a drop out in a sensor you need a tool that is fast enough. So with all the theory out of the way let’s take a look at the example. The problem here is not that hard to see if you look at the cam signal to the left that is good and the one to the right that is bad. The top of the waveform was dropping down in the middle of the square wave causing a drivability problem. The fix for this vehicle was to replace the defective sensor.
Any scope platform (like the one in the Snap-on Modis shown here) can be used to perform this relative compression test right at the DLC.
Relative Compression from the Driver’s Seat
This easy concept came to me as I was explaining relative compression to a class that I was teaching in Massachusetts. I was showing and explaining relative compression with the scope connected to the battery B+ and Neg - post while cranking the engine over. I continued on to explain that this test can also be performed using an amp clamp attached to one of the battery cables as well. Then one of my students asked the following question.
“Since you said this was testing a voltage drop that we were observing with the scope, how about if we connected the scope directly to the Diagnostic Link Connector (DLC) using the breakout box you showed us?” I thought about it and responded back to Jeffery Allan Clark, the tech who asked the question that I believed he was on to something. I had never really thought about performing the test using a DLC breakout box. In fact, I have never seen anything about this in print or knew of anyone performing this test. I always say in all my classes and seminars that there is no such thing as a stupid question, and everyone knows something that we don't know yet. If we share our ideas we all can learn, and we did.
This test is so easy if you own a DLC breakout box and a scope and can be accomplished with a couple of minutes with any other equipment. First couple (select) your scope to AC volts followed by selecting a range on your scope of 500mv AC per division and a time base of 50 ms per division. Many of today’s automotive scopes are set up as voltage and time per screen instead of division, so you may have to take the settings I gave you and multiply them by 10.
Next insert the red (B+) to pin 16 (that’s B+) and the black (-) to pin 4 (that’s ground) of the breakout box. If the model you’re testing uses “Clear Flood” mode, you can disable the fuel system for testing by pressing the gas/throttle pedal to the. If the vehicle does not have a “Clear Flood” fuel management strategy, you will need to remove the fuel pump relay or fuse to prevent the engine from starting. Take a look at the waveform shown in the accompanying image. What we are looking for is an equal voltage height (amplitude) and time. If there is a difference in height or time, this will indicate a problem in one of the cylinders.
Unusual but not improbable, this Air Injection system code was a simple carbon clean to correct.
This test can tell you quickly if there is an engine mechanical issue requiring further diagnosis. If you are wondering how you can tell what cylinder has the problem, you will need to remove your butt from the driver’s seat and open the hood. Your next step would be to connect another channel of the scope to one of the cylinders, preferably an ignition source, so you can sync the relative compression signal to the engine’s firing order.
For example, if the firing order is 1-6-5-4-3-2, and you’re second channel is attached to the number 2 cylinder’s secondary lead, you can use this as a “landmark” for precisely locating the weak cylinder. In our case, if the weak peak is two to the left of the peak we’ve synched as cylinder number 2, the weak cylinder would be number 6. If you add this quick test to your diagnostic game plan you can quickly check to make sure that the engine is good or bad before proceeding with your diagnosis. This test is so important that Ford and Toyota have both included this test in their factory scan tools.
A No Parts Repair
This 1999 Saturn SL 1.9L vehicle came in with an illuminated Malfunction Indicator Lamp (MIL) and a P0410 (AIR Pump System Performance). This DTC can be caused by a blown 30 amp fuse that is located in the underhood fuse box, vacuum line, air hose, check valve and possible restriction in the air hoses or pipes. Where do you start? Using a scan tool that supports bi-directional testing of these components is one option. The GM Tech II is the factory scan tool and certainly has the functionality to check the operation of the secondary air injection system. Unfortunately, not all aftermarket scan tools have the bi-directional functionality to perform these tests.
The next easiest place to start is at the load. In this case, the electric air pump is the main component in this system. First turn the ignition key off and remove the connector at the pump, turn the ignition key back on and test which side of the connector has power. If either side is “hot,” you just confirmed that the fuse is good. With power to the pump confirmed, it’s time to check the PCM side that supplies the ground needed to complete the circuit and operate the pump. To check to see if the pump can operate, connect a Power Probe or a set of fused jumper wires to supply power and ground to the AIR pump. This should confirm if the pump is good or bad.
In the case of this SL, the pump was good. Our quick test eliminates a short to ground or a faulty pump, all in less than a minute. Because the pump was able to supply air pressure we checked the hose and pipes to the check valve to make sure they were free and clear. Next we removed the hose from the other side of the check valve and found the problem. Take a look at the carbon buildup on the check valve port that goes to the cylinder head in the included photo. The fix for this vehicle was to use intake cleaner along with a drill and pick to clean the passages of carbon.
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