Paper Keywords automobile fault comprehensive judgment
Analyze the causes and parts of automobile failure (all parts of the vehicle, machinery, electricity, oil, gas, etc.). ), and put forward scientific and reasonable judgment methods, that is, fault probability sequence arrangement method and auxiliary judgment method, which can quickly determine the fault location. The results show that the possibility of automobile failure mainly depends on the quality of products, and the fault parts of products with high reliability can often be thought of with normal thinking. For models with defective product quality, the fault parts often appear in areas that people's normal thinking can't judge. The application of comprehensive analysis method has important guiding value for judging vehicle faults.
First, the reason analysis
A Mercedes-Benz 560SEL car experienced serious idle jitter after replacing a new rocker arm due to abnormal valve sound. According to the general thinking process, only the rocker arm and camshaft are removed, and these parts should be considered first to find the fault. If you follow this clue, you may be able to troubleshoot soon. But unfortunately, the repairman found that at least three cylinders were not working properly in the fire fighting test. Of course, he thinks that replacing the rocker arm of only one cylinder, even if there is a fault, will not lead to all three cylinders not working. The fault may be caused by other aspects. According to experience, the possible reasons are as follows:
(1) The exhaust gas recirculation (EGR) system is out of order, and a large amount of exhaust gas enters the cylinder (which is most likely).
(2) The air intake system is leaking, the mixture is too thin, and the idling work is not good (this possibility is in the middle).
(3) The model (quality) of the replacement rocker arm is defective (this possibility is the least).
Second, the fault judgment method
(1) Check the exhaust gas recirculation system. Remove the vacuum tube on EGR valve, and the fault still exists; When the EGR valve was removed from the engine, it was found that the valve was seriously corroded, the exhaust passage and the intake passage were not connected at all, and the exhaust gas did not enter the cylinder, so the most likely one became impossible. Next, the air intake system was checked, and no air leakage was found, which ruled out the second possibility. As for the third possibility, even if the problem of the rocker arm is finally found, it can't be regarded as a complete success of this maintenance, because the maintenance work has been going on for a long time at this time, and the owner is already dissatisfied with it. Of course, it was finally found that the rocker arm was indeed the wrong model, and the contact surface with the valve was about 2mm higher than that of the old rocker arm. After grinding for a period of time, it is corrected to the standard value. After reassembly, the idle speed of the engine is stable and the fault is eliminated. If the vehicle is overhauled after disassembly and adjustment, this part should be inspected first, instead of following the routine steps. Soon after, a new Toyota (CARMY 2.2) car came into the factory for repair because of engine damage caused by cylinder collision during driving. After replacing the new cylinder block and other parts, the engine was started, but it never started. When the starter drives the engine to run, the engine is either backfired or caught fire, which seems to be an ignition obstacle. Check that the high-voltage line is not plugged in incorrectly (the engine is directly ignited). Drawing lessons from the last time, we should not blindly check. First, ask the repairman if there is anything unusual when disassembling the engine. The repairman replied that there was a gear-like sensor on the crankshaft, which could not be removed from the crankshaft because of rust when disassembling the engine. After forcibly prying it off, I found a cracked tooth, glued it with 502, and then reinstalled it. As a result, the above failure occurred. According to the information obtained, the repairman removed the crankshaft position sensor rotor from the car. After careful inspection, no obvious abnormality was found, and almost no trace was found at the bonding place. But for the faults and parts defects that people can't see, the computer may not be able to monitor them, because the conditions in the repair shop are limited, and the output waveform of the sensor can't be observed with an oscilloscope. However, from the perspective of probability, I still think that the crankshaft position sensor rotor is most likely to be damaged.
(2) The computer is damaged. But as we all know, even if the computer can output normal code, it can't be absolutely believed that the computer must be normal, but this possibility is very small.
(3) Insufficient cylinder pressure. However, if the valve timing is correct, it is less likely that the pressure of four cylinders is insufficient at the same time. After the above analysis, it is suggested that the repair shop buy a new crankshaft position sensor rotor, and the new parts will arrive the next day and be loaded as soon as they are tried.
Third, the fault probability analysis method
An Audi car produced by FAW has battery loss. After three or four days in the garage, the battery is almost completely exhausted. At first, the repairman thought that the battery failed and self-discharged and cut off power. After replacing the new battery, the fault still exists. The repairman checked almost all the parts and still couldn't find the fault. Finally, the conclusion is that when the fourth fuse is pulled out, the battery will no longer lose power. The fourth type of insurance is indoor lights, reading lights, lighters, clocks, radios, luggage compartment lights and air conditioning indicator lights. First determine whether the fault exists, turn off the ignition switch, disconnect and reconnect the negative electrode of the battery, and you can see the blue electric spark, which proves that there is indeed a large current discharge. Next, don't rush to check the fault location, but analyze the fault probability of the fourth fuse.
(1) The cigarette lighter can't pop up automatically: pull out the front and rear cigarette lighters, and the fault remains, so rule out this possibility.
(2) Indoor lights, reading lights, clocks, radios and air conditioning indicator lights can all work normally, but it is uncertain whether the current consumption is normal after the ignition switch is turned off. This possibility is in the middle.
(3) The line involved in the fourth fuse is short-circuited and grounded, which consumes current. This possibility is the same as above. The current consumed by the fourth fuse (ignition switch turned off) is 0.3l A measured by a digital multimeter, and its power is roughly estimated to be 12VX 0.3A = 3.6W, which is close to the luggage compartment lamp, but the luggage compartment key was taken away by the driver and cannot be opened for inspection. The repairman suggested disassembling the instrument to check the clock, radio and related circuits, but the author decided that the trunk light was most likely to be damaged, and asked the repair shop to check the trunk light first. The next day, I got the news from the repair shop: the luggage compartment lamp really burned out: the switch seat of the lamp sank, and even if the luggage compartment lid was closed, the light bulb could not be extinguished. The light bulb burned out and turned white, but the filament was not broken, so it kept consuming current. Replace the new light bulb, repair the switch seat and eliminate the fault. Fourth, the auxiliary diagnosis method
(1) eye view. Observation instrument: observe the current, oil pressure gauge, water temperature gauge and gasoline indicator light to indicate the working conditions of relevant parts of the vehicle. If the displayed figures are not normal, there is something wrong with this part. Look at the appearance: if the engine emits too much smoke, the color of the smoke is abnormal; Some parts have water leakage, air leakage, oil leakage, electric leakage and other phenomena; The frame is deformed, and the gap between components is too large or too small. Inspection of oil: routine inspection of oil, liquid and medium can not be ignored. Check oil, automatic transmission oil, steering power oil, gear oil, brake fluid, coolant, glass water, coolant and other oils. Ensure the normal operation of the vehicle, and the relevant indicator lights are on, or supplement it in time if it is found missing. Check the color: judge the fault by checking the liquid quality of vehicle parts. If the color of a car's automatic transmission oil turns purple and there is a little turbidity, it can be judged that the automatic transmission fault is not the lack of engine power.
(2) listening. Engine: Because the throttle is constantly changing, the noise emitted by the engine is also different. Listen carefully for any abnormality in the engine sound. Chassis: When the driving speed changes constantly, the noise of the transmission system will generally increase with the increase of the speed, but when the speed increases to a certain extent, some noise will weaken or even disappear. Distinguish the types of sound: such as continuous sound and intermittent sound; Crisp and low; There are regular rings and irregular rings. Confirm which ones are normal and which ones are abnormal.
(3) nasal odor. Burnt smell: It is caused by brake delay and clutch slipping. Burning engine oil and brake fluid will produce a special smell. When electrical appliances are working, burning wires will give off a burning smell. Proper use of odor method can provide guidance for fault diagnosis.
(4) Touch by hand. Touch the brake drum, rear axle housing and transmission housing by hand to determine the temperature of these parts: if it is hot by hand, the temperature is about 40℃. It feels hot, but it can last for a few minutes, and the temperature is about 50℃~60℃. Hands can't stand it at all, and the temperature is at least above 80℃.
(5) isolation. Part of the interval, or cut off the work of some systems and some components to determine the fault range. If the fault disappears after isolating a component, it means that the fault occurred on this component; If the fault still exists, the fault is not here. Engine: Broken cylinder (flameout or oil cut off). If the smoke exhaust disappears or decreases, the cylinder is faulty. Chassis: If abnormal noise of the chassis is diagnosed, you can put the gear lever in neutral position and constantly clutch, and analyze whether the abnormal noise occurs in the clutch or the transmission according to the change of abnormal noise. Electrical: If the light is not on, you can connect the battery directly. If the light is on, it means that the wire connecting the light is out of order.
(6) testing. Such as diagnosing abnormal valve sounds. If you suspect that the valve clearance is too large, you can check and adjust the specified value with a thickness gauge. If the abnormal sound disappears, the judgment is correct. If the noise still exists, continue to look for other parts.
(7) comparison. When the cylinder is not working, if the spark plug problem is suspected, the spark plug can be replaced with a normal spark plug. If the fault fails, the fault is caused by the original spark plug.
Verb (abbreviation of verb) conclusion
In automobile fault diagnosis, it is often encountered that it takes a long time to check the parts involved in the fault, but the fault can not be found. Even if the fault can be found, the time and energy may not be worth the loss. If the probability analysis rules are adopted, the fault can be determined quickly and accurately, which will save customers' time and improve their reputation. In automobile maintenance, besides using instruments, overhaul instruments and tools to diagnose the automobile, simple manual diagnosis should also be combined, which is of great value to automobile fault diagnosis.
References:
[1] automobile engineering manual [M]. Beijing: People's Communications Press, 200 1.