Application Cases of Oscilloscopes in Automobile Maintenance Diagnosis

Application of Oscilloscope in Maintenance Diagnosis

As a maintenance personnel, how to quickly and accurately capture abnormal phenomena and find the cause when diagnosing vehicle failures is the key to solving the problem, and the oscilloscope is an important helper to help maintenance personnel solve this problem (Figure 1). Here we briefly analyze the application of oscilloscopes in maintenance diagnosis.

1. The role of digital oscilloscope in automobile maintenance

Some signals of automotive electronic equipment change very quickly, with a change cycle of one thousandth of a second. Usually, the scanning speed of the test instrument should be 5 to 10 times that of the measured signal. Many fault signals are intermittent, sometimes appearing and sometimes not, which requires the test speed of the instrument to be higher than the speed of the fault signal. A digital oscilloscope can meet this speed requirement. It can not only capture circuit signals quickly, but also display these waveforms at a slower speed so that maintenance personnel can observe and analyze them at the same time. It can also record signal waveforms in a storage manner, and can go back to observe fast signals that have occurred, which greatly facilitates fault analysis. Whether it is a high-speed signal (for example: injector signal) or a low-speed signal (such as throttle position change and oxygen sensor signal), using an oscilloscope to observe can find clues from the waveform. An oscilloscope is like a ruler. It can measure the working status of a computer system. Through an oscilloscope, you can observe how the automotive electronic system works.

2. Application of oscilloscope in automobile failure cases

When a car's electronic equipment or circuit fails, maintenance personnel need to collect all relevant data. An oscilloscope can display the trajectory of the movement of electrons in a circuit by displaying the change of voltage over time in a curve. The magnitude of the displayed voltage depends on the current and resistance in the circuit. Based on the change of voltage over time on the oscilloscope, it is possible to determine what is wrong with the circuit. To maximize the effectiveness of the oscilloscope, it is necessary to compare the collected waveforms.

Here are a few examples to illustrate how an oscilloscope can help us find the cause of a fault.

①Diagnosis of intermittent flameout of Dongfeng Honda Civic

A 2006 Honda Civic 1.8VTi sedan has an intermittent flameout. Connect the oscilloscope to several lines of the engine control unit. In Figure 2, the red line (channel 2) is the camshaft position sensor signal, the green line (channel 3) is the crankshaft position sensor signal, and the blue line (channel 4), white line (channel 5), purple line (channel 6) and orange line (channel 7) are the control signals of the four injectors. This set of waveforms was recorded by the oscilloscope when the engine was about to shut down. Please note that the injection sequence of the injectors is messed up at this time. The purple and orange lines show that the injector control signals have the phenomenon of simultaneous injection. In addition, the injector represented by the orange line has been opened twice in the same period of time relative to the other injectors. One important point is that the camshaft position signal represented by the red line and the crankshaft position signal represented by the green line are not problematic at this time. Now that we have this information, let's analyze it to find clues for the next step of troubleshooting.

The two main timing signals of the engine control unit are the input signals of the camshaft position and the crankshaft position. These two signals are normal and have no faults, so it can be determined that these two signals are not the cause of the abnormal injection fault. Once the injection sequence of the injector is disordered and the number of openings increases, it means that the engine control unit is in a reset state. The reset of the control unit is caused by the disorder of the internal clock or timing signal. The internal clock or timing signal is used by the program to time and issue work instructions within a certain time. When there is a problem with the timing signal or the time signal of the logic circuit, the program that controls the main functional components of the engine will go wrong, resulting in a driving fault. The engine control unit usually enters the reset state for the following reasons: internal clock error, input timing signal error, power supply or grounding circuit failure, and interference signal entering the engine control unit. [page]

In order to find out the reason for the clearing, an oscilloscope is required to monitor the injection and ignition signals simultaneously.

In Figure 3, the yellow, red, green and blue waveforms are the control signals of the injector, and the white, purple, orange and brown waveforms are the signals of the independent ignition coil (COP) controlling the ignition. In this set of waveforms, the signal of the injector is synchronized with the engine ignition signal, which is the key. From this set of waveforms, we can see the cause of the fault. Please note that the yellow line has a negative spike pulse signal on the right.

In order to correctly analyze this spike pulse signal, the oscilloscope needs to be set to dual-channel display mode, so that the waveforms can be superimposed, making it easier to see the relationship between the waveform signals (Figure 4). Now, it can be clearly seen that the spike pulse signals on the white line (channel 5) and the yellow line (channel 1) are aligned, and the falling edge of the white square wave signal is the signal for controlling the ignition of the coil.

The cause of the vehicle's failure is the presence of carbon marks between the primary and secondary windings of the ignition coil. When the resistance between the spark plug electrodes is greater than the resistance of the carbon marks, the current will take a shortcut, that is, flow through the carbon marks of the primary winding. Since the primary winding of the ignition coil is connected to the power supply circuit and the control circuit, the induced high voltage electricity generated in the secondary winding can find a circuit from the integrated circuit of the engine control unit. The high voltage electricity entering the engine control unit will affect the operation of the internal clock in the processor, resulting in the disruption of the injection sequence and injection time of the injector, thus causing driving failures.

Please note: Negative spikes only occur in the control circuit of one injector. This problem is common when there is carbon traces between the primary and secondary windings of the ignition coil. This negative spike usually appears in the waveform signal of the injector. The ignition coil is a negative discharge because its winding method makes the center electrode of the spark plug negative and the side electrode positive. The reason is that the insulated center electrode can maintain a very high operating temperature, and the high temperature makes it easy for electrons to break through the gap between the spark plug electrodes and ignite.

When checking the noise or spike pulses in the waveform, it is usually necessary to superimpose the measured waveform signals or display the signals in a strip chart (or curve chart), so that they can be compared with other waveform signals in real time to find out the cause of the fault. The oscilloscope cannot display the spike pulse signal in the trigger mode. The spike pulse signal in Figure 3 appears after the injection pulse signal ends. The negative spike pulse signal in the waveform signal is mostly caused by a problem with the ignition coil. The fault of this car was caused by an abnormal ignition coil. After replacing the ignition coil, the fault was eliminated.

②Buick Regal stalling during driving fault diagnosis

A 2006 Buick Regal 2.5L sedan has an intermittent flameout fault. The engine of this car will flame out while the vehicle is driving, and then it can start again. The data of the engine control unit in Figure 5 is collected when the engine is about to flame out. The yellow line is the IC control signal (between the ignition module and the engine control unit, providing the engine with the signal required for fuel control and calculation of the ignition advance angle under normal operation), the red line is the bypass control signal (providing the engine control unit with the signal required for fuel control and calculation of the ignition advance angle when starting or when the IC circuit is abnormal), and the green, blue, white, purple, orange and brown waveforms are all injector control signals. This set of waveforms shows that the injector works normally at first, then there is a problem, and the injection pulse width increases. This shows that the engine control unit has entered the reset mode. What exactly causes the engine control unit to enter the reset state? This is very important.

By setting this set of waveforms to superimpose mode, we can see the relationship between them (Figure 6). Zoom in on the waveform before the fault occurs, and note that the bypass control signal (red line) has a problem first, followed by the IC control signal (yellow line). Combined with the working principle of the vehicle's ignition system, this sequence indicates that the fault is caused by an internal fault in the engine control unit, or by a fault in the power supply circuit or the grounding circuit.

If the IC control circuit fails before the bypass control circuit, it means that there is a problem with the sensor. To determine the reason why the engine control unit enters the reset state, it is necessary to connect the oscilloscope probe to the power supply circuit or ground circuit of the engine control unit and record the waveform when the fault occurs. If there is no interference signal or spike pulse signal on the power supply and ground circuit, the cause of the fault is that the engine control unit is broken. If there is an interference signal or spike pulse signal on the power supply and ground circuit, then the problematic circuit needs to be repaired. There was no problem with the power supply and ground circuit of the vehicle when the fault occurred. Replacing an engine control unit and matching it solved the problem.

These are just two examples of using an oscilloscope. There are many other uses for oscilloscopes in maintenance. As long as there is voltage in the circuit, an oscilloscope can be used. Once you use an oscilloscope, it can help you understand many things about electricity and electronics. An oscilloscope can help you quickly understand the working status of an electronic circuit. More importantly, when an electronic circuit has a problem, it can help you find the root cause of the problem.