Failure to start is the most common fault in engine faults. Oil circuit, circuit, gas circuit, machinery, etc. will lead to failure to start. It is impractical to generalize its investigation ideas in one sentence; The troubleshooting should follow a certain process according to the fault type, and step by step from outside to inside and from shallow to deep. Today, we will discuss the troubleshooting ideas for the failure types that cannot be started.
fault type
The diagnostic instrument cannot be connected and the engine cannot be started The diagnostic instrument cannot be connected. At this time, we don’t know the condition of the engine, whether the synchronization is correct and whether the rail pressure is enough. There is no clear direction to guide you to check which aspect. In case of such problems, don’t mess around. Our goal is to connect the diagnostic instrument and understand the information of the engine. If the diagnostic instrument cannot be connected, first confirm whether the ECU works normally. Check whether the corresponding pin of ECU is powered on, such as whether the sensor power supply, main relay power supply and communication line voltage are normal. If these are abnormal, check whether the key switch T15 is powered and whether the ECU power supply grounding is abnormal. If the above checks are normal, the next step is to check whether each harness has virtual connection.
The wiring harness layout on the whole vehicle was also introduced to you before. Compared with the engine pin diagram, it is quite complex. There are many fuses, relays, etc. in the middle of the wiring harness, and these components and the terminals of each connector are the high fault points of virtual connection. We must pay attention to it when troubleshooting. If the ECU output voltage is normal, the harness is not falsely connected, but the diagnostic instrument cannot be connected.
In case of such problems, it is recommended to try to solve them by flying wires: connect the power supply grounding wire of ECU and T15 harness of key switch to the power supply separately without any switch; Then pull the ECU communication line to enter the diagnostic instrument. As long as this is done, the diagnostic instrument can basically be connected.
The diagnostic instrument can be connected. The line drops as soon as you take a taxi, and the oil pump does not deliver oil
This is a new car with a distance of 0 km. In case of the above situation, the diagnostic instrument drops during the taxi, and the oil output of the oil pump is abnormal. Sometimes it is accompanied by some fault codes. Generally, the diagnostic instrument drops during the taxi, and these fault codes are basically caused by the virtual connection of ECU power supply and grounding or T15. When troubleshooting, focus on troubleshooting the harness.
The diagnostic instrument can be connected normally. When taking a taxi, the starter does not rotate and the engine cannot be started If the starter does not turn, the rail pressure and synchronization must be abnormal. When you see such problems, you should not pay attention to synchronization and rail pressure, but focus on why the starter does not turn? The common reasons why the starter does not run are as follows:
ECU does not receive start signal
If the ECU doesn’t know you want to start, it won’t control the starter to rotate. How does the ECU know you want the starter to rotate? This is relatively simple. When taking a taxi (the key is in St gear), we can read a data stream on the diagnostic instrument
Some diagnostic instruments are also translated as “start status” and so on. When taking a taxi, if this data flow changes, it indicates that the ECU has received the start information. If there is no change, it means that the key switch is in St gear, and there is a problem with this route bundle (some vehicles will connect the neutral switch line into T50 line). Focus on troubleshooting.
Abnormal startup information
When ECU receives the start signal, it will start. Does it control the starter to rotate? This is not true. ECU also has the ability to think. It needs to analyze some information to judge whether the starting conditions are met, such as neutral switch, protection switch and so on. So how do we know whether the startup information meets the startup requirements? At this time, there is another data flow for reference.
“Control signal of starting relay”, this data flow is used to judge whether ECU has pulled in the starting relay. If the starting information meets the starting conditions, the state of this data flow will change; If the conditions are not met, the state of the data flow will not change. This data stream is another watershed, guiding you to check which part.
The neutral switch and clutch switch are abnormal
The most common components that cause the engine to fail to meet the starting conditions are neutral switch and clutch switch. There are three strategies for neutral switch and clutch switch to participate in starting control: the first is to fully participate in starting control. It is not allowed to start when the gear is not in neutral. It is usually seen that the neutral switch is connected in the starting relay circuit or the circuit of key switch T50.
In this case, if the vehicle is forced to start when it is not in neutral gear, either the ECU cannot receive the start signal or the start signal is received, but the start relay cannot be engaged.
The second is to partially participate in the start-up control. When the ECU is not in neutral gear, it will be delayed to start, and if the ECU is not in neutral gear, the power will not be released. This type is common in crane systems, such as Jiefang j6p and edc17cv44
The neutral switch is neither connected to T50 line nor starting relay line. At the software level, it realizes the control of starting.
Third, do not participate in start-up control. As long as the key is turned to the st gear, the starter starts to rotate. It is common for the trolley or masters to change the line by themselves and connect the T50 harness directly to the starting relay or starter.
Abnormal starter circuit for engine
If the starting signal meets the starting conditions, turn the key to st gear, and the state of “control signal of starting relay” changes, but the starter does not turn. At this time, we should check the following points:
a) Whether the starting relay circuit is abnormal;
b) Whether the starting relay is engaged;
c) Check whether the starter and its circuit are abnormal.
There is another watershed in the middle, which is “whether the starting relay is pulled in”? How to judge whether the relay is closed? The simplest is to listen to the sound. The relay will ring at the moment of closing.
After the key is turned to st gear, listen to whether the starting relay has the sound of closing. If the contents of some items are relatively complete, you can use the action test of the diagnostic instrument to judge whether the starting relay works normally. If the starting relay can be pulled in normally, check the starter and its circuit, otherwise check the starting relay circuit.
The rail pressure is insufficient and the engine cannot be started
1) For such problems, first confirm whether the low-pressure oil circuit is blocked or leaking;
2) If there is any fault in the oil transfer pump, the pressure gauge can be used to test the outlet pressure of the oil transfer pump;
3) Whether the overflow valve on the oil pump is stuck and fully opened, resulting in all fuel flowing back through the overflow valve;
4) Whether the metering unit on the oil pump is stuck in the state of small opening or even closed, so that the fuel cannot enter the high-pressure plunger cavity through the metering unit.
5) Whether there is leakage in the high-pressure plunger part The above points are all routine methods, which are easier to check. There are often such problems reported by the master: when the metering unit is normally open, pull out the harness, the oil pump will discharge oil, and the oil will not be discharged when it is plugged in; Or the normally closed metering unit, remove the rubber ring for oil discharge, and install it without oil discharge.
Let’s analyze the above problems. For the normally open metering unit, under the condition of no power on, the metering unit is normally open and the oil pump outputs oil; If there is no oil after plugging in, it means that the metering unit is closed.
How to turn off the normally open metering unit? Power it up. The common cause of such problems is the short circuit and grounding of the harness of the metering unit, resulting in the abnormal shutdown of the metering unit. The normally closed metering unit cannot discharge oil when it is inserted into the wire bundle, and the oil can be discharged when the rubber ring of the metering unit is removed.
After removing the rubber ring of the metering unit, the fuel does not pass through the metering unit, and the oil output of the oil pump is normal; If the rubber ring is installed without oil, it means that the metering unit is not opened. The fault of the metering unit itself will not be discussed for the time being.
From the perspective of circuit, it is that the metering unit is not powered on. It is common that the harness is open or the power supply of the metering unit is falsely connected.
For such problems, we can check the trigger current and duty cycle of the metering unit on the data flow. These two values can indicate whether the ECU controls the opening of the metering unit; At the same time, the voltage of the two lines of the lower metering unit can also be measured during a taxi to assist in judgment.
The engine cannot be started due to abnormal synchronization
The engine obtains the current position of the engine through software calculation according to the signals of the crankshaft position sensor and camshaft position sensor. The ECU monitors the correct synchronization signal to control the fuel injection of the fuel injector. The small rail will not be described in detail here.
If the fuel injector is not powered on or the actual fuel injection quantity is 0, the engine cannot be started
One item in the data flow is called actual fuel injection quantity. This data flow does not represent the actual fuel injected by the injector. The most important reference significance for us is whether the ECU powers up the injector and allows the injector to inject.
Several possible fault points that cause the fuel injector not to be powered on or the actual fuel injection quantity is 0:
1) Insufficient rail pressure;
2) Incorrect synchronization;
3) Anti theft fault;
4) Remote locking of arrears;
5) Injector harness short circuit;
6) ECU fault and abnormal power supply and grounding;
When you encounter the problem that the fuel injector is not powered on in the future, you should pay attention to the above points, especially remember to ask the driver if he owes money. Whether the ECU energizes the fuel injector can be seen from the actual fuel injection quantity. At the same time, it can also be detected with a test lamp. Connect a diode test lamp to the fuel injector harness. If the test lamp flashes, it indicates that the fuel injector has been powered on, otherwise it is not powered on.
8. The fuel injector is powered on and the engine cannot be started If the fuel injector has been powered on, but the engine still cannot be started, what needs to be checked?
a. First, check whether there are problems with the fuel injector itself. How to evaluate whether the performance of an injector is normal? How many criteria are there?
1) Whether the injection quantity matches the design value. The injection quantity includes static injection point, dynamic injection curve and injection repetition performance (i.e. standard deviation);
2) Oil return at full load point: leakage + functional oil return;
3) whether the spray delay is normal.
4) Whether the atomization effect of fuel injection is good; We usually measure whether the fuel injector is normal on the test bench, and only measure 5 points. These points meet the relevant requirements, which does not mean that the fuel injector must meet the design requirements. We must be aware of this.
b. Is the fresh air volume, intake air temperature and engine temperature normal If the exhaust gas in the cylinder is too much and the temperature is too low, it will not work even if the fuel is injected.
c. Cylinder itself
If the cylinder leaks air, or the valve is not matched correctly, it will also cause the engine to fail to start. One of the functional tests in the diagnostic instrument is called compression test. Compression test is an engine test function integrated in the electronic control unit, which is mainly used to detect the air tightness of each cylinder of the engine. This function can only work after being triggered by the diagnostic instrument.
When the compression test is run through the diagnostic instrument, turn the key to st gear to enable the starter to drag the engine to run. The electronic control unit will automatically measure the running time of each cylinder in an angle interval after the top dead center, so as to calculate the instantaneous speed.
During this process, the electronic control unit will prohibit fuel injection and the engine will not start. After the test, the diagnostic instrument will display the speed of each cylinder. The air tightness of each cylinder can be evaluated by calculating the deviation between the speed of each cylinder and the average speed of other cylinders.
When the deviation is too large, it is recommended to check the air tightness of the cylinder, including valve clearance, piston ring and other mechanical parts. Having said that, let’s go back and consider the first type of fault. If the diagnostic instrument is not connected, how can we judge whether the rail pressure and synchronization of the engine are normal?
First of all, when the vehicle is under certain rail pressure, you can collect the voltage signal when it is under certain rail pressure. When it is under certain rail pressure, you can simulate the signal when it is under certain rail pressure.
Whether the synchronization is correct or not, the voltage cannot be measured. However, we can use other methods to roughly infer whether the synchronization is normal. For example, use the test lamp to measure whether the harness of the fuel injector is powered on. If the test lamp flashes, it indicates that the synchronization has at least reached the allowable injection state.
Recommended data flow for engine
Having said so much, what data streams can we refer to when the engine fails to start
1. System voltage Too high or too low system voltage will lead to abnormal flameout or failure to start the engine, especially the abnormal flameout of high-speed engine. The system voltage should also be paid attention to.
2. Rail pressure Check the actual rail pressure and rated rail pressure, and whether the rail pressure is above 200bar; Whether the rail pressure is too high. For the system without pressure relief valve, the rail pressure is too high, which will also lead to the failure of landing.
3. Synchronization What is the synchronization status, whether it is synchronized or not, and whether it meets the power on conditions of the fuel injector.
4. Actual injection quantity This data flow is used to determine whether the ECU powers up the fuel injector.
5. Neutral switch and clutch switch status Some of these switches are abnormal, which will lead to failure to start, while others will lead to delayed start.
6. T15 status T15 is the on gear of the key switch. This problem may cause the ECU to fail to power on or the fuel injector does not power on.
7. Auxiliary start switch and auxiliary stop switch In particular, the abnormal stop switch under the vehicle will cause the engine to shut down abnormally and even fail to start the vehicle.
8. T50. Starting relay control signal Judge whether the ECU receives the start signal and allows the engine to start.
9. Water temperature, intake air temperature, oil temperature Judge whether the temperature is too low to start, or the temperature is too high, and the engine overheat protection stops abnormally.
10. Preheating state Especially when the ambient temperature is low and cold start is difficult, we must pay attention to these.
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