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Matters needing attention in the maintenance of electronically controlled diesel engines:

- Oct 20, 2018 -

Matters needing attention in the maintenance of electronically controlled diesel engines:

When the capacitance of the diesel battery is insufficient, the diesel engine can not start with the fast start power supply, but can start with the auxiliary battery.


In the process of diesel engine fault inspection, the electrical connectors and components should not be plugged in casually, but should be carried out after the ignition switch is closed, and attention should be paid not to measure directly with the universal meter pen at the front end of the plug-in connector, but to adopt special connectors or measure according to the technical manual requirements. In addition, attention should be paid to the cleaning of joints and components, and not to let water, fuel or dust enter.


The electric welding of the vehicle equipped with the electronic control diesel engine can not be carried out directly. When the electric arc welding is carried out on the vehicle body, the power supply of the electric control unit should be disconnected first. Special attention should be paid to body repair work near ECU or sensor.


If it is necessary to disassemble the fuel system, the pipelines and components must be disassembled after the diesel engine has been shut down for a period of time. The specific time varies with the vehicle type, the diesel engine type and the electronic control system. When assembling, attention should be paid to keeping the joint clean and tightened after sealing, according to the removal of empty. For some diesel engines, the first step is to empty the fuel tank to the filter, and then the filter to the pump, that is, the exhaust plug or the exhaust port on the pump body is switched open, with manual vacuum pump to empty the gas in the pump body.


The traditional method can not be used to diagnose the faults of the new type of electronic diesel engine. The technicians trained by the system professional knowledge should apply the appropriate diagnostic equipment and special tools to diagnose the faults of the electronic diesel engine system. At the same time, the operation instructions and technical instructions of the diesel engine manufacturer should be read in detail before fault diagnosis.


The fault diagnosis of electronic control diesel engine system mostly adopts the method of inverse source diagnosis. First, it uses the diagnostic equipment to find out the possible cause of the fault, then it finds out the location of the fault step by step from the peripheral equipment to the control unit, and finally solves the problem.


Do not turn off the ECU power at will. In the process of engine operation, the self-diagnostic system will store the corresponding fault code when the electronic control system fails, so that the maintenance personnel can read the fault code by the diagnostic instrument or the self-diagnostic system on-board, and then find the fault reason and location according to the fault information. If the battery cable is removed or the main fuse is removed before reading the fault code, the ECU power is cut off and the fault code stored in the ECU RAM is automatically eliminated.


For engines with intermittent faults or unable to start at all, cutting off the ECU power will lead to difficulty in obtaining or even unable to obtain fault codes, which also lost a very important fault information. Therefore, in the maintenance of electronic-controlled engine, if the battery cable needs to be disconnected, the battery can be disconnected from the circuit only if the fault code and relevant information are transferred out by the self-diagnostic system and the cause of the fault is diagnosed.


ECU power is usually not controlled by ignition switch. Closing the ignition switch will not cut off the ECU power supply. When the cause of the fault is diagnosed, the ignition switch should be switched off and the accumulator should be removed by wire. If only the electrical control system is checked, the ignition switch can only be turned off.


It is not possible to disconnect the power supply line that is the same as the battery voltage. When the ignition switch is in the ON position, no matter whether the engine is running or not, the battery cable or fuse must not be removed. Because the sudden power failure will cause the coil in the circuit to produce self-induced electromotive force and high instantaneous voltage (sometimes up to nearly 10,000 volts), so that ECU and sensors and other microelectronic devices are seriously damaged.


In addition, it must be noted that all other power supply lines (such as DC motors, fuel pumps, etc.) with the same battery voltage can not be removed when the ignition switch is in the on-line position, otherwise, the relevant coil will produce self-inductance and burn out the ECU or sensor.


The power must be cut off if necessary. Because the electric fuel pump is often used in the fuel system of the electronic control engine, if the power supply is not cut off during the maintenance of the fuel pump system, it is possible to unintentionally connect the electric fuel pump circuit during the maintenance process, so that the electric fuel pump works, high-pressure fuel will be ejected from the dismantled fuel pipeline, causing personal injury or fire. Therefore, before overhauling the fuel system of the electronic-controlled engine, the power supply should be cut off. The method is to turn off the ignition switch, or disconnect the battery cable, or pull out the main fuse.


It is not allowed to use the method of cutting off the ECU power supply to clear the fault code. After the engine is repaired, the original fault code stored in the ECU must be cleared. Otherwise, although the engine fault has been cleared, the fault code is still stored in the ECU. The fault indicator on the cab dashboard will be thrown on and lit. The driver can not be sure whether there is a new fault or the old fault code is not clear. There is no solution.


For most electronically controlled engines, the fault code can be made clear by disconnecting the battery cable or the main fuse so that the ECU is powered off for more than 30 seconds. But it must be noted that the car anti-theft password, audio password, quartz clock and other information are also stored in the ECU RAM, the use of power-off to remove the fault code method, the above temporary information stored in the ECU will be removed together, resulting in sound lock code. Generally speaking, the fault code should be clear according to the method required by the maintenance manual, do not know whether there are burglar-proof passwords and audio passwords, or do not know what the password is, do not cut off the ECU power supply at will.


Do not appear overvoltage or polarity reversal. In vehicle maintenance, it is not allowed to start the engine directly with other power sources (such as startup power supply for startup only) other than the battery. When installing the battery, pay attention to its positive and negative polarity can not be connected inversely. Otherwise, if the supply voltage is too high and the reverse power is turned on, the ECU or other electronic control components will be damaged.


In the test of the electronic control system, except for the special instructions in the test process, the ECU and sensor can not be tested by the pointer mode multimeter, and the high impedance digital multimeter can be used for testing. It is strictly forbidden to use test lamp to test the electrical components connected with ECU. It is forbidden to test the circuit by means of fire test with iron to avoid damaging the ECU or other electronic control components.


We should not blindly conduct dismantling. Electrical control system has high reliability and low probability of failure in use. Most of the faults are caused by bad contact of wiring harness connector. But it must be noted that "high reliability" does not mean "absolute reliability", "low probability of failure" does not mean "absolute failure", "most faults are caused by bad contact of wiring harness connector". "Bad connection" does not mean that "all failures are caused by poor connection". Some maintenance personnel, especially drivers, because of the one-sided understanding of the above "correct language description", when the engine failure indicator lights on, according to their own supervisor's assumption, in the ignition switch on, and even in the engine operation process, some of the electronic control components of the harness connector to open, plug in the test. Every time a sensor harness connector is disconnected, the ECU will record a fault code, which will cause confusion between the artificial fault code and the actual fault code, and bring unnecessary confusion to the fault diagnosis. Especially those who lack the knowledge or experience of electronic control engine maintenance can't start the engine because of blind operation. When the engine is repaired by professional staff, several or even dozens of fault codes can only be removed one by one according to the read fault codes, which is time-consuming and laborious. Therefore, non professional maintenance personnel should not blindly check.


We can not blindly use replacement method to diagnose faults. When an electronic control component is suspected to be faulty, a new component (or the same component of a barrier-free vehicle) is used to replace the old one to verify whether the electronic control component is faulty. This is the method most maintenance personnel have adopted in the maintenance of electronic control system. But it must be noted that the replacement method is based on the validation method adopted after the preliminary diagnosis has been obtained. Otherwise, if a pile of parts are replaced, even if the fault is repaired, the exact location of the fault is not known.


In addition, the replacement method is very effective in diagnosing the faults of sensors and actuators, but the faults of the electronic control system are mostly caused by the damage of external components or lines. If the replacement method is used in this case, it is more dangerous, and it is very easy to cause the damage of new electronic control components and increase the loss due to the peripheral faults of the faulty vehicle. Therefore, if the method of changing parts is used to diagnose the faults of the electronic control system, the faulty electronic control components can only be replaced by other similar types of fault-free vehicles to test, and other fault-free electronic control components can not be installed on the fault vehicle to test.


When necessary, disconnect fuel injection pump or fuel injector wiring harness connector. In maintenance, when the engine is running but does not want to start the engine (such as testing cylinder pressure, etc.), it is necessary to remove the fuel injection pump or the fuel injection wiring harness connector, so as to avoid engine Misstart or fuel injector mis-injection accident.


Pay attention to the cleanliness of the fuel system. Before disassembling the fuel system, the relevant parts and adjacent areas must be cleaned. The disassembled parts of the fuel system must be placed on a clean surface and covered with non-plush cloth. Before installation, the parts must be kept clean. During maintenance, if there is a diesel drip, the parts should be wiped off in time. After the fuel system is disassembled, the fuel system should be cleaned. Try not to use compressed air as far as possible, do not move the car as far as possible, so as to avoid dirt entering the fuel system.


ECU and sensor must prevent dampness, do not allow the ECU or sensor sealing device damage, let alone water wash ECU and sensor, ECU must prevent violent vibration. Do not open the ECU cover plate, because the ECU can not be repaired even if broken, if good, open the ECU is likely to damage, or damage its sealing.