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ECU hardware has input and output circuits, while the software consists of programs and data.
To check whether the ECU is functioning properly, we should focus on these three aspects. Because the pins and functions used by the ECU are numerous, it is advisable to create a project list and measure each item one by one. Measurements require observing results, but where do we look for results? There are two channels: 1) change the sensor signal and directly observe whether the ECU’s output reaction is valid, and 2) check through the diagnostic interface and data stream whether the ECU has received the input sensor or switch signals, and whether the displayed data is reasonable; or give commands to the ECU through the communication port to see if the changes in output control signals are reasonable. Both methods include a basic check of the ECU program. I don’t know what advanced techniques ECU testers have. In fact, the most practical method is to confirm that the input sensor signals are sent to the ECU but it does not receive them, indicating an issue with the internal input circuit; if the expected outputs are not present or the parameters are unreasonable, it can indicate a potential issue with the internal output circuit. This method is fundamental and requires a certain depth of understanding of circuits and control strategies; it may seem a bit challenging, but the key technical focus of the diesel engine electronic control technology lies here—if you want to stay in this field, you must learn. Of course, there is also the simplest method: some fault codes in the ECU’s fault code table are specifically related to internal circuit diagnostics, but the ECU is ultimately a controller, not a detector, so the range of these fault codes is limited.
If we say the ECU is the heart of the electronically controlled diesel engine, then electricity is the blood of this heart.
Whether in household appliances or industrial electrical systems, whether in products or system engineering, the design of the power supply is crucial. Even in electrical repairs, the first step is usually to check whether the power supply of the main unit and various circuits is normal. The same applies to diesel engines; to ensure the ECU operates reliably, the power supply sent to the ECU must be normal and reliable. If the ECU’s power supply is abnormal, it is like a patient in a coma due to excessive blood loss—it will make mistakes and say the wrong things, and at this time, the fault codes it reports are not trustworthy. If there are no fault codes, they are naturally not trustworthy either. All of this is due to excessive blood loss; the body has no organic damage, meaning the ECU’s hardware and software are not damaged.
Aside from the ECU, we know from the relevant materials provided by the ECU manufacturer that the working voltage range of the ECU is quite wide, but abnormal power supply conditions can still occur during use. We can categorize ECU power supply anomalies into two major types: overvoltage and undervoltage. Overvoltage is rarely seen in vehicles because the battery itself acts as a very good voltage buffer; as long as the connections are good, most overvoltage energy can be absorbed without causing the power supply voltage to rise too high. However, undervoltage situations are somewhat more complex. The simplest form of undervoltage is a drained battery. When external electrical devices, including the ECU, stop working, the voltage measured across the battery with a multimeter does not represent the battery’s charge, which we commonly refer to as floating voltage. Once any electrical device operates, especially a large load like a starter motor that draws hundreds of amps, if the battery is drained, the voltage across the battery will drop significantly. If it falls below the lower limit of the ECU’s operating voltage, the ECU may enter a state of confusion due to excessive blood loss and will not function normally. Large current loads also include preheaters, and the inductive electrical energy generated by relay switches can cause momentary voltage drops in the power supply, which are difficult to capture with a multimeter. If necessary, an oscilloscope may be required for observation; some decoder models come with oscilloscope functions that can be useful.
In addition to the ECU, external active sensors also require power supply, and their importance is on par with that of the ECU. The term ‘active’ here means that they need an external power supply. These types of sensors include rail pressure, air pressure, and oil pressure sensors; camshaft and crankshaft speed sensors are active if they have three wires; the accelerator pedal is also active; while temperature sensors such as oil temperature, air temperature, and water temperature may seem to be integrated with other signals, they can actually be considered passive, not requiring an independent power supply line. Even so, do not assume they can work without power. In fact, within the ECU, a resistor is connected from the temperature signal line to the internal sensor power supply, forming a voltage divider circuit with the external temperature sensing resistor. However, this potentiometer has no knob; instead, the invisible temperature acts as the knob. When the external temperature sensing resistor is open and disconnected, you can use a digital multimeter to measure the voltage between the signal line and the sensor ground, and this voltage can be considered the voltage of the internal sensor power supply, which is the same as directly connecting the red probe to the positive terminal of the ECU’s internal sensor power supply.
Reliability of Control Strategies Inside the ECU, there are some power supply module circuits that stabilize voltage, generally at 5V, because sensors convert physical parameters into voltage signals and require a certain degree of precision; the need for voltage stabilization is quite understandable. On the other hand, from the perspective of reliability and safety of control strategies, the power supply used by these sensors cannot be a single line but rather multiple lines that may be independent and isolated from each other. Even if they share a common ground, the wiring must be independent externally to prevent signal crosstalk. Additionally, these power supply module circuits must implement their own safety protection, allowing the sensor power lines to short-circuit with the sensor ground or chassis ground without damage. Furthermore, the ECU must have hardware functions for detecting and diagnosing these sensor power supplies. If two sensors share a certain power supply, when the power supply for sensor A shorts to ground, causing the voltage to drop or lose power, it will affect the normal operation of sensor B. At this point, the ECU may report a series of fault codes related to sensor B, leading us in the wrong direction, while in reality, sensor B and its circuit are functioning properly. Therefore, to facilitate future diagnostics, we need to understand the power supply sharing situation of each electronic control system. The detection method is simple: with the ignition switch turned off, use a multimeter in resistance mode to measure which sensor grounds and sensor power supply pins are interconnected. In summary, when diagnosing and repairing electronic control systems, we must first focus on checking the power supply to the ECU and each sensor, and it can even be used as a routine inspection method, just like cutting off a cylinder when calibrating a pump. This can help us avoid detours and prevent some so-called strange faults from occurring.
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