It has been stated that a picture is worth a 1,000 words; this is perhaps an understatement when working on a vehicle. In order to properly diagnose vehicle systems, a picture into the electronics is a necessity. Modern vehicle systems are based on electronic controls over mechanical systems. To diagnose failures within these systems, we must first separate these systems from one another by analyzing the electronic control system. We need to look at a vehicle’s circuits for voltage change over time. This will be accomplished with an oscilloscope.
Defining the DSO
What is an oscilloscope? Webster’s Dictionary states: an oscilloscope is an instrument in which the variation in fluctuating electrical quantity appears temporarily as a visible waveform on a fluorescent screen of a cathode-ray tube.
What is important with this statement is “variations in a fluctuating electrical quantity” or “variations in fluctuating voltage.” So what is voltage? Voltage is a unit of measurement based on an electrical force or the electrical pressure difference between two points of a conductor. What this indicates is the scope or any voltage-measuring instrument is only displaying the difference of electrical pressure. So if both the positive and negative leads are connected to the positive post of the battery, the display would show 0 volts. On a charged vehicle battery, both leads would have 12.6 volts on them, so in this case there would not be a voltage difference between the leads, thus a display of 0 volts. If the negative lead was then moved to the negative post of the battery, the display would now show the source voltage of the battery, or 12.6 volts. Be aware the measuring device only shows the difference of voltage between the two leads. Where the leads are placed is the test! When first connecting a voltage-measuring instrument to the vehicle, it will be important to connect the negative lead to the negative post of the battery; this will ensure that you are on the lowest point of potential of the vehicle, thus providing you with accurate test data.
The oscilloscope is a tool used to enter into another dimension of time. In our dimension of time, you can move from one point to another point fairly quickly. For instance, a Top Fuel Dragster can travel ¼ mile in less than 4 seconds. This takes a nitromethane supercharged engine that produces more than 10,000 horsepower. Electricity, however, can go around the earth 7.5 times in just 1 second. This is far faster than the human brain can recognize. Therefore this electrical movement must be altered so we can recognize or see it. This is done by having the force that is pushing the electrons (voltage) leave a trace behind it as it changes over time (Figure 1). This trace that is left behind is now in our dimension of time. This allows us to physically see the voltage movement in time, and is what will be displayed on the screen of the oscilloscope.
It is important to understand what the voltage is indicating to you. Since voltage is only present to push electrons through resistance, if the voltage changes, then the resistance within the circuit is also changed. This can be demonstrated by analyzing a good circuit from the point where the source voltage is present on the power side of the circuit to the point of the load or resistance. As soon as you pass the point of the load or resistance on the ground side, there is very little voltage remaining. Voltage in a circuit is only present with resistance within that circuit or if the circuit is open. In the case of an open circuit, source voltage is present to the point where the circuit is open. Each voltage change displayed on the scope screen will be indicative of what is occurring in the circuit.
The oscilloscope display is shown in Figure 2. As can be seen, the screen is divided into two planes, that of a horizontal plane (X axis), and that of a vertical plane (Y axis). The vertical plane — or up and down plane — will display the voltage level. The horizontal plane — or side-to-side plane — will display time. These planes will be divided into 10 grids, referred to as gradicules. A gradicule is a network of lines representing meridians and parallels, on which a map can be represented. The voltage changes over time will be mapped on the oscilloscope display. Each line will represent a value; those vertical division lines will show voltage and those horizontal division lines will show time. These divisions can be adjusted by the controls on the oscilloscope. To adjust the scope settings, simply watch the waveform on the display and make changes to the voltage and time settings until the waveform is clear on the scope display. This is very similar to taking a picture with your camera. In order to take a good picture, you adjust the lens while viewing the object. This lens movement will focus the light on the object for a clear view. It will not be necessary to know exactly where you will need to adjust the camera lens. Once the lens is adjusted, the settings can be looked at to see where they are. This is similar to the scope settings; you do not need to know exactly where the scope needs to be adjusted to, just get a clear picture and then look at the setting to determine what the voltage and time of the signal being viewed is. A scope is a very simple tool to set up, don’t over complicate it.
Another adjustment of the oscilloscope is how the data will be displayed. This can be set up with a trigger mode, roll mode or non-trigger mode. The trigger mode is used to stabilize the waveform being viewed. These setting are adjusted by the channel, voltage level, voltage slope and display position. The channel will represent which channel will be used for the trigger. The voltage level or trigger level will represent the voltage needed to break a threshold that will allow the screen sweep to occur. This is usually set to half of the voltage of the waveform being viewed. In other words, if the voltage is moving from 12 volts to 0 volts the level would be set at 6 volts. The voltage slope will be set for a failing edge or a rising edge. In other words, if the slope is set to a failing edge, the trigger voltage that is set will have to be at a higher level and drop to a lower value in order for a screen sweep to occur. If the voltage slope is set for a rising edge, the trigger voltage that is set will have to be at a lower level and rise to a higher value in order for a screen sweep to occur. The display position is the point on the display the voltage trigger level will start from. It is important to understand that a trigger can hide a failing circuit. This occurs when the set voltage level is not broken, and thus a screen sweep will not occur. On all oscilloscopes the last triggered event will remain on the scope display. Let’s say the waveform is present on the scope display and is active. If the circuit driver fails intermittently, the trigger level is not broken and therefore does not make a screen sweep. However, the last screen sweep remains on the scope display. This will hide the failing circuit. This is why it is best to avoid the use of triggers. It will be best to use the strip chart roll mode.
The strip chart roll mode acquires all of the data and displays it on the scope screen. The difference between the triggered mode and strip chart roll mode can best be understood by comparing a single shot camera with a video camera. If you are moving on a highway at 200 mph and take a single picture of the buildings passing by, then reset the camera and take yet another picture of the buildings it would be clear that there would be missing data between the points that the pictures were taken. However, if this same scenario took place but a video camera was used then no data would be missing between the buildings. All of the data would be present for your review. This is why it is best to use the scope in a roll mode and then go back through the acquired data to determine what caused the failure.
Understand what you are connecting to
It is important to have an understanding of what you are connecting to on the vehicle. The vehicle has many physical events that will need to be monitored in order to control a system. In order to monitor these events, electrical sensors are used. An electrical sensor converts a physical quantity into an electrical output. The microprocessor can then use these electrical outputs from the sensors to control the system. Electronics are about timing. In order to see where the timing events are you will need more than one channel. One example would be using the oscilloscope to see the ignition coil fire. With only one channel you could see the coil fire but could not tell where in space this occurred. For instance if the coil fired at BDC the engine would not start, but with no other reference you would not be able to see where this event occurred, only that it had occurred. The oscilloscope takes the voltage and displays this in a graph format. In order to take advantage of this graphing format multiple channels will be used. These channels will then be compared to one another in order to determine if the events occurred at the correct time and in the correct sequence.