Understanding of automotive diagnostic system

Some of  auto electrician meet some problem when they were using ECU Diagnosis Device,such as reading out the plurality of fault codes, or  the fault lights but no fault code,the system fault but not fault code is nothing,or display fault code but can not find an appropriate fault: those problem always make them so confused and nothing to do.then they began to complain about the quality or performance testing equipment in question.

In fact, the auto electrician should be know all of automotive electronic control system principle, the principle of the principle of self-diagnosis system, automotive electronic control system diagnostic equipment before using the auto diagnosis equipment


Summary :This Post describes briefly the use of the principles of automotive electronic control system detection equipment, principle and characteristics of automotive self-diagnostic systems, and automotive self-diagnostic system for fault acknowledgment range determination method, time domain determination method, a function judgment, logic Decision method four methods; highlighted car fault diagnosis system abnormality diagnosis causes and troubleshooting instance, rely on self-diagnosis system Finally troubleshooting tips and related considerations.

Keywords Auto diagnostic tool, Vehicle diagnosis system,Car Troubleshooting
1 The principle of auto self-diagnostic system
1.1 Vehicle Control System fault problem .
Automotive control systems during normal operation, the input and output signals of the electronic control unit ECU is in the range of a predetermined operation, when the signal control circuit is abnormal, ECU diagnostic system determines that the signal circuit failure. Exceptions circuit is divided into three kinds:
The first is a signal circuit out of range. For example: coolant temperature sensor (CTS) in normal operation, the output voltage is within 0.1V ~ 4.8V, such as outside this range, it is determined that the fault diagnosis system signal;
The Second is that ECU are not receiving the sensor signal over time or the received signal in the same period of time, the system will be judged as a fault diagnosis signal. For example: the oxygen sensor is in normal operation, the input voltage should be within 0.1V ~ 0.9V, volatility less than 8 times / 10 sec;
The third is the electronic control unit ECU diagnostic system once accidentally discovered an abnormal input signal does not diagnose the fault signal, not only the normal input signals appear repeatedly or persist for some time before it is determined fault signal. For example: the rotational speed signal (Ne) is a pulse signal, the engine speed at 100r / min or more, some loss of signal, the ECU is not determined to be failure. B A
1.2 Automotive self-diagnostic system for fault confirmation method
1.2.1 Range determination method
When the electronic control unit receives the input signal exceeds a predetermined value range, the self-diagnostic system to confirm that the input signal failure. For example: a car designed in the normal temperature sensor temperature range -30-120 ℃ (or bigger range), the output voltage is 0.30-4.70V, so when the electronic control unit detects the signal voltage is less than 0.15V or greater than 4.85v when it is determined that the water temperature sensor signal system short circuit or open circuit failure.
1.2.2 Time Domain Decision Method
When certain input signal is found within a certain period of time there is no change or no change in the number reaches a predetermined when the electronic control unit detects, self-diagnostic system determines that the signal failure. For example: an oxygen sensor in the engine has reached normal operating temperature, into the closed-loop control system, the electronic control unit detects the output signal of the oxygen sensor is less than a certain time, or in the case of the oxygen sensor signal has exceeded a certain 0.45V and down time, self-diagnostic system it is determined that the oxygen sensor signal system failure.
1.2.3 function determination method
When the electronic control unit to the actuator gives an operation instruction detection sensor corresponding output parameters change, if the sensor output signal is not in accordance with the procedure laid down parameters, on the confirmation of the actuator or circuit failure. For example: General Automotive EGR system equipped with EGR valve height sensor for detecting the EGR valve is working properly. However, some cars did not set the EGR valve height sensor, when the electronic control unit issues a command EGR valve is turned on, by detecting whether there is a corresponding change in the intake pressure MAP sensor output signal can also be determined whether the EGR valve operation, if there is no change, confirmation EGR valve and circuit failure.
1.2.4 logical decision method
Electronic control unit has two sensors interconnected to compare data, found that when the logical relationship between the two sensor signals violation of the set conditions, it must be concluded that its failure. For example: When the electronic control unit detects that the engine speed is greater than a certain speed, throttle position sensor output signal is less than a certain value, it is determined that the throttle position sensor failure.
When the electronic control unit ECU the diagnostic system detects a fault signal, they immediately fault information in the form of fault codes stored in the reservoir, fault warning light lit simultaneously to display fault information. Electronic control system to improve vehicle performance, but also make the car fault diagnosis become complicated. Vehicle maintenance personnel by reading the trouble code, in most cases can diagnose the cause and location of faults and fault may occur. When the car repair, if blindly rely on fault diagnostic trouble codes, often mistakes in judgment resulting in unnecessary losses. DTC is only whether the electronic control unit (ECU) programming system to define “normal” conclusion, in the case of complex, electronic control unit (ECU) may not be able to ascertain the real part of the fault lies. Therefore, when electric cars a comprehensive analysis of maintenance should be judged, combined automobile self-diagnosis results, the phenomenon of car trouble to find the fault location.
Abnormality diagnosis 2 Car fault diagnosis system
Car fault diagnosis system to diagnose fault codes stored in random access memory (RAM), a fault code can be stored for a long, clear DTCs need to disconnect the dedicated random access memory circuit is connected directly or disconnect the battery. Car fault diagnosis system error record and store fault codes for electric vehicle maintenance inconvenient. In the following three cases, the fault code prone to errors.
2.1 Cars Run fault apparent faulty sensor without self-diagnostic system to monitor
Automotive electronic control unit (ECU) when the sensor signal is detected, the sensor can only accept abnormal signal within the setting range, thereby determining the quality of the sensor, recording or not recorded fault code, once the interpretation of fault code, as long as the corresponding sensors, wire connectors, wire check, find and eliminate the short circuit fault can be. However, if the issues causing some reason the sensor sensitivity decreased, unresponsive, the output characteristics of offset (That is, when the sensor does not fail completely) self-diagnostic system can not be detected. Although the car does have fault phenomena manifested, but the car has self-diagnostic system output of the “normal system” code (the fault indicator is flashing). Maintenance personnel or equipment would detect suspicious car in this case. Maintenance personnel should be based on fault symptoms automotive analysis and judgment, then the sensor to detect specific monomer (data flow, etc.), in order to find and eliminate the sensor failure. For example, when the engine speed and stall the engine idle speed instability associated with, but the self-diagnostic system and no fault code output, the first suspect is worthy of consideration and air flow sensor or an intake air pressure sensor failure, because both by performance will directly affect the basic fuel injection quantity of the engine ECU controls. Although at this time does not display the corresponding fault code, they should also be checked. For example, when the air flow shell cracks leak, it will cause air flow sensor measurement are not allowed, the engine speed disorders, and self-diagnostic system and electronic control unit ECU can not detect this failure phenomenon, no fault code output .
2.2 Engine failure phenomenon similar, ECU monitoring errors, self-diagnostic system may display incorrect fault codes
Volkswagen throttle sensor sensitivity decreased, unresponsive and so cause the engine air-fuel ratio imbalance and decreased sensitivity caused by the air flow meter air-fuel ratio imbalance Symptoms similar to self-diagnostic system will display “throttle sensor” or “air flow meter” fault code. For electric cars fitted with catalytic converters, once used leaded gasoline, sometimes more obvious characteristics of this type of failure. In the car for testing, often you find fault code display is the “temperature sensor open or short circuit” fault, and not the engine speed. Apparently these little to fault with the temperature sensor in the water temperature sensor measurements did not find any fault. However, when the three-way catalytic converter removed from the car and found open, three-way catalytic converter inside congested, it can be concluded that the engine failure was caused thereby. So when the self-diagnosis system fault codes appear later, should the actual engine failure symptoms were analyzed and compared in order to get the right sound judgment, should not be used as the sole basis for the fault code troubleshooting.
2.3 automotive electronic control system may also lead to improper maintenance error DTCs
When electric cars on the implementation of maintenance, improper maintenance personnel for maintenance or operational errors, will result in automatic changes to the system output error fault code. For example, during engine operation, the sensor has no intention Unplug, unplug every time the sensor plug, self-diagnostic system will record a fault code. In addition, if at a car repair, improper operation could not be completely removed the old fault code, then the electronic control unit also saves the original old fault code therein, and therefore when electric vehicle maintenance it should be noted It should not result in unnecessary human error codes, maintenance work to bring confusion and difficulties.
ECU diagnostic instruments for use limited to reading, clear code, data flow detection ignore this most important detection method. In fact, the failure diagnosis of the vehicle, not necessarily a failure sometimes occurs a fault code, as described above. Then we can use data flow analysis method to judge. In this case you will need to repair the flexible use of automotive professionals and theoretical knowledge.
Through the analysis of the data stream, it will be very easily judge where component failure.
Example 3 car fault diagnosis system abnormality diagnosis
3.1 First example: Symptom Santana Jetta AHP 2000Gsi or cold engine the car is not easy, after starting idle instability, poor acceleration hot car
3.1.1 Fault Diagnosis and Treatment:
Hot car poor acceleration, speed and difficulty. Normal maintenance, replacement spark plugs. Use decoder reads fault codes in the following fault codes: 00561- mixture over adaptive adaptive limit; 00553 —— air flow meter sensor (G70).
3.1.2 to read the data stream is shown below:
1) 000 Group: Mixed gas composition measurement is 109, lower than the standard 115-141 (equivalent -0.64ms – + 0.64ms);
2) 001 760-800 1.9-2.00 group 3 8-9.0;
3) 4.42 3.1 002 Group 760-800 1.9-1.95;
4) 003 group 760-8001310080;
5) 005 Group 790-820800 0.0-0.8 3.3;
6) 006 800-810 -0.8-0.0 0.8-0.16 group 10;
7) 007 9% -0.8-0.0 group 01;
8) 023 82.7 72.2 30.6 Group 0100-0000;
9) 098 4.4 3.7 Idle group match errors
3.1.3 Data flow analysis is as follows:
The 1) 00 Group mixed gas composition measurement is 109, lower than the standard 115-141, indicates the mixing ratio is not normal, the control unit records the fault code 00561 echoes; mixed gas exceeds a predetermined value.
The 2) group 001 engine load, throttle angle, ignition advance angle value within the specified value.
. 3) group 002 engine load, air quality measurement value is within a predetermined value.
4) 003 groups battery voltage, water temperature, intake air temperature value is within the predetermined value.
Of 5) 005 Group idling control value, the magnitude of the air intake within the specified value.
6) 006 Group mixture excess air ratio control value within the specified value.
The 7) 007 group oxygen sensor voltage within the specified value.
8) state the value of 023 represents the group matches completed the throttle and adjust properly.
9) 098 sets of data flow shows throttle matching error; if the basic setup is completed normally, the fourth set of 098-bit data should be “OK”. This data does not comply with the 023 group.
3.1.4 Analysis prepared mixed gas of gasoline: complete combustion of 1 kg of gasoline takes about 14.6㎏ air. Excess air ratio make this a standard air-fuel ratio of λ = 1. Lean burn as (λ = 1.1) when the air sucked more; and less rich mixture (eg λ = 0.9) when the inhaled air. Maximum engine power and better working conditions in the rich gas area; starting from the reduced fuel consumption, engine hopes to work in lean burn zone. The engine controller calculates the control voltage signal based on the throttle opening and engine speed, oxygen sensor is also required to be tightly regulated. Thus, the air than normal or not, rely on to judge the value of the oxygen sensor. The following conditions ECU relies throttle signal, air signals, temperature, speed, knocking and other signals to implement. gas mixture enriched: the role of cold start, warm-up operation, idling and full load conditions, as described below:
a) Cold Start: cold start, when the electronic control unit in accordance with the water temperature at the start of additional instruction nozzle spray fuel into the intake manifold so that the cold start.
b) warm-up operation: After a cold start, warm-up must be supplied to the rich gas. Warmup valve adjusting device with temperature and time change control pressure. Under conditions of equal air flow control pressure reduction valve sheet has a larger stroke and get proper enriched mixture.
c) idling: electronic control unit of the throttle valve occurs slight deflection. Produce a desired idle idle air flow supplied by the tiny deflection.
d) full load: In order to full load of the engine maximum power output, the electronic control unit in accordance with a throttle sensor signal, speed signal to send out a signal so enriched mixture thickens.
3.1.5 Analysis ideal conditions:
The main reference signal above conditions are: speed, air amount, the knock signal. Speed signal provides basic reference injection and ignition parameters knock signal correction ignition signal. Suppose the engine cylinder pressure is no problem, empty the fuel injection quantity meter provides correct (the theoretical air-fuel ratio), the ignition advance angle speed signal is provided without a problem and timely ignition energy to ignite the combustible mixture; combustible gas mixture can in theory complete combustion, the oxygen sensor detects an oxygen content in the exhaust gas is zero.
3.1.6 Analysis of abnormal conditions:
Suppose abnormal air supply, fuel supply abnormal, abnormal ignition supply, can not fully burn, the oxygen sensor detects excessive levels of oxygen in the exhaust gas. Electronic control unit according to the oxygen sensor signal to adjust the amount of oil, if you reduce or increase the amount of oil still can not get the engine combustion of the combustible mixture, the electronic control unit relies on the oxygen sensor signal trying to get information from the change in the oxygen content, participation in the mixture adjustment again, but adjust the mixture over adaptive value is too high or too low. MAF value is relatively large, ECU adjust A / F value based on the signal value, which can not be adjusted to the desired A / F value, it generates adjusted to limit the fault code. Check the ignition system and should change MAF. Only the principle of the electronic control unit of a deep understanding and analysis, before arrive at a correct diagnostic conclusions.
3.1.7 Troubleshooting:
Check battery start-up voltage, temperature sensors; there is a problem after replacing the Qing code as a group setting. And then detecting the air flow meter and replace the oxygen sensor. Start the engine and keep it idle state. 007 display groups, observe the oxygen sensor G39 voltage feedback signal, the signal voltage can fluctuate between 0.1V-1.0V, but the frequency changes very slowly. Conduct a hydraulic test, idling hydraulic table shows 0.25MPa. When the fuel pressure gauge pointer door between 0.28-0.30MPa swing. After closing the ignition switch 10min, keeping the fuel system pressure is 0.16MPa. Oil pressure values are standard, you can determine good performance fuel pump, pressure regulator properly.
1) cleaning the fuel system, the basic setting of the throttle body, but still not paid off.
2) Check and clean the air flow meter, replace the oxygen sensor fault persists.
3) Check the ignition system and found that l, 4-cylinder spark plug sparks weak. After considering this car, 4-cylinder share the same ignition coil, replace the ignition coil N152, fault completely ruled out.
3.1.8 Exclude Results:
Whereby that: bad ignition module caused by lack of ignition energy 1,4 cylinder, resulting in mixed gas combustion conditions deteriorate is the root cause of the failure. Cause similar fault elements include: oil pressure, air flow meter, throttle position sensor, an oxygen sensor. Therefore, to correctly understand the automobile self-diagnostic system, you should set the conditions for a correct understanding of the diagnostic system: From the initial simple output, the input voltage monitoring circuit components, such as when monitoring low voltage potential at the short circuit condition, line voltage and high potential mutations in the off state when the voltage exceeds the gate internal self-diagnostic system set limit, the self-diagnostic system corresponding to the pre-set in a read only memory ROM code number setting appropriate fault code according to monitor voltage corresponding to the line ports and fault symptoms. Early self-diagnostic system can identify only a small amount or a set of fault codes, fault code and content is also limited to the loss of the line open, short, signal failure, abnormal changes in the current element of the type of work execution. Since the control accuracy requirements and the various monitoring and diagnosis system diagnostic requirements, the content is no longer fault code is not limited to high voltage, low voltage or signal is absent and other simple expressions, and the emergence of new fuel trim long-term system is too thick, MAP performance degradation, EGR system positional deviation, etc. glance, makes it difficult to read the fault code. And these failures are often set up with the specific requirements of the monitoring system has its own specific conditions. After retrieval of fault diagnostic code, carefully read the maintenance manual, it is very important to adopt strategies to find the corresponding error message, set the fault condition, the fault setting.
3.2 Second example: Symptom or Jetta Santana 2000Gsi AHP engine idle speed instability, acceleration black smoke.
3.2.1 Troubleshooting:
1) read fault codes: 00561 and 00522 overrun mixture adaptive temperature sensor open / short circuit to positive; sensor break / short circuit to ground. After a note of the fault code clear code, re-reading, only “00522” temperature sensor. The investigation of the temperature sensor is 0Ω, after replacing find fault as usual. Thus, the effect of DTCs done.
2) Do not read data stream Diagnosis: idle instability, after cleaning the throttle body redo basic settings; black smoke, check oil pressure, normal. Cleaning fuel injectors, fuel filter change. Re-launch and found black smoke still, but has changed a smooth idle. Because also smoke, replace the oxygen sensor, but in vain. Check spark plugs and high-voltage, high voltage is normal, the spark plug gap is large and black. Replace the spark plugs, test failure phenomenon weakened, but acceleration is still black smoke. ECU suspected damage; or the ignition coil is damaged, improper valve timing, air flow meter or damage. In the simple to complex, never to change a program to replace parts, check valve timing, good; replace the ignition coil, invalid; after the replacement of the air flow meter, failure to eliminate.
3) read data stream diagnostic methods (Santana 2000GSi as an example):
For exhaust black smoke and engine idle speed instability, 01,02-readable data set and the stream of Group 07.
a) from Group 07 to read: λ mixture control 23% (normal is -10% ~ 10%), λ sensor voltage 0.6V ~ 0.8V (normal is 0.1v ~ 1.0V). This shows that the mixture is too thick indeed, far beyond the capacity of λ control.
b) from 02 to read the group: engine load 2.8ms (normal is 0 ~ 2.5ms); engine cycle, the injection flow rate of 5.8g / s (normally 2.0 ~ 4.0g / s).
c) read from the 01 group throttle opening angle is 4 ° ~ 5 ° (normal is 0 ° ~ 5 °), although not overrun, it is too large.
3.2.2 Troubleshooting:
When idle, since the throttle is located in the idle position, ECU and will try to adjust the idle speed according to engine speed, so the λ control overrun. The excessive intake flow, ECU believe the engine load is large, it will not reduce the amount of fuel injection (ie, injection duration), Idle is fluctuated. Since the idle fuel injection amount, the fuel injection quantity during acceleration is even greater, leading to the exhaust pipe black smoke. Throttle body cleaning, replace the air flow meter fault eliminated. Read data stream, made a quantitative analysis to detect the destination can replace the relevant components. Read data stream using a method of changing the spark plugs and ignition coils, reducing troubleshooting time, labor and material.
4 self-diagnostic system with claudication system
Automotive normal operation, the input voltage value of the output signal of the electronic control unit ECU has a certain range. When the voltage value of the signal exceeds a certain range, and this phenomenon will not disappear over a period of time, the ECU determines whether or failure of this part. ECU put this fault code is stored in the form of internal random access memory (RAM), while lights troubleshooting lights. When a circuit generates a fault, the signal can not be used as a control parameter of the engine to use.
In order to maintain operation of the engine, ECU begins its program memory (ROM), and call up a fixed value for the engine emergency parameters, ensure that the engine can continue to operate. When the ECU electronic control unit fails, the backup control circuit ECU automatically enables simple control of the engine, making the car can start to go home or to a nearby garage for repairs, this function is trouble-free operation, also known as ” limp “mode. On the other hand, when a faulty actuator ECU detects, for safety reasons, to take some security measures. This function is called failsafe.
ECU fault diagnosis for sensor systems, electronic control units and actuators conducted. When the sensor and the electronic control unit fails, often taking trouble-free operation mode; and when the actuator fails, often take failsafe measures.
Fault diagnosis and fault 4.1 Sensor operation
Since the sensor itself generates an electrical signal, and therefore, the fault diagnosis of the sensor does not require special wiring, but only need to develop a sensor input signal recognition program can be realized in the sensor fault diagnosis software. Normal input voltage value of the water temperature sensor 0.3-4.7V, corresponding to the engine cooling water temperature of -30 ℃ -120 ℃. So, when the ECU detects the voltage signal outside this range, if it is once in a while, diagnostics ECU is not a malfunction. However, if the signal is not normal for a period of time, i.e., the diagnostic program determined that the cooling water temperature sensor or circuit malfunction. ECU In this case the code (this code is designed to have a good agreement on behalf of the water temperature sensor fault signal abnormal digital code) is stored in the form of random access memory. At the same time, by checking the fault warning light to inform the driver and maintenance personnel in the engine control system malfunction. When ECU found the water temperature sensor is not functioning properly, you use a pre-set constant value as an alternative water temperature signal, the system is running.
4.2 self-diagnostic electronic control unit and back-up circuit
ECU If a failure occurs, the control program can not be normal operation, the electronic control unit is in an abnormal state. And this will make the car due to engine control system failure can not travel. In order to ensure the car could continue running when the electronic control unit fails, the control system engineering, design reserve circuit (IC backup system). When the ECU electronic control unit fails, ECU automatically call the reserve loop control tasks to complete, easy control operation into the state, with the fixed control signal, the vehicle continued to travel. Since the system only has a simple function to maintain the operation of the engine but can not replace the work of all the electronic control unit, so this back-up circuit work, also known as “limp” mode. When using backup systems work, fault indicator light. Whether the electronic control unit is working properly carried out by the monitoring circuit is called a circuit monitor. Monitoring circuit is installed separate from the electronic control unit counters. Electronic control unit normal operation, the program run by the electronic control unit on the timing counter clearing processing. Thus, monitoring the value of the counter circuit is never an overflow phenomenon. When the electronic control unit that there is no running phenomenon, the electronic control unit can not be timed counter is cleared, so that this monitoring counter overflow occurs. Monitor counter overflow output level from low to high (This output typically carry flag counter. When the counter reaches its maximum value, and then add a count pulse, the counter will overflow occurs. In this case, counter overflow level by the end of the low to high; at the same time, the counter is cleared). This change counter output level will directly trigger the alternate circuit. Spare circuit only in accordance with the start signal and the idle contact closed state, a constant injection duration of the fuel injection and ignition timing and ignition control.
4.3 Actuator Fault Diagnosis and Fault Insurance
Automotive electronic control systems, the actuator is to determine the operation of the engine and the main cars with safety device, when the actuator fails, often driving the car will have been affected. Therefore, treatment is usually actuator failures: When the actuator is recognized as failure by the ECU to take appropriate security measures implemented in accordance with the severity of the failure in the control system, and designed a fail-safe system.
Since the ECU of the actuators is to control operation, the control signal is the output signal. Therefore, in order for the work of each actuator diagnosis, generally adding circuit fault diagnosis, namely ECU issues a control signal, the actuator must be a dedicated circuit to feedback on its implementation to the ECU to the actuator. Electronic engine control system, a typical member of the actuator fault diagnosis is ignition. Under normal circumstances, when the ECU to control ignition, ignition once for each ignition, the igniter will be confirmed by the ignition circuit within the ignition performance in the form of an electrical signal back to the ECU. When the ignition circuit or faulty ignition, ignition control ECU command is issued, get feedback signal; at this time that the ignition has been ECU will not work properly. Engine operation, if the ignition system failure, will make the unburned gas mixture into the exhaust system and the exhaust pipes. The exhaust gas purification device in the catalyst temperature will greatly exceed the allowable value. Meanwhile, the unburned gas mixture in the exhaust pipe gathering too much can also cause explosion of the exhaust system. For this reason, the use of fail-safe system, when the ECU does not receive an acknowledgment signal fire, immediately cut off the fuel injection system the power to stop the fuel injection.
In short, the use of automotive electronic control system diagnostic, maintenance personnel can quickly, easily and accurately locate the fault, thereby successfully troubleshooting. But no matter how powerful the instrument functions, the effectiveness of how to produce or rely on the maintenance staff initiative.

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