Mercedes S-Class W-140
1991-1999 of release
Repair and operation of the car
+ Mercedes-Benz cars of the class S (W-140)
+ Operation manual
+ Routine maintenance
+ Cooling systems, heating
+ Power supply system and release
- Engine electric equipment
- Systems of ignition and engine management - the general information
Checks of a control system of ignition and injection
Search of malfunctions - the general information and preliminary checks
Check and adjustment of a corner of an advancing of ignition
Self-diagnostics of systems of electronic control of OBD
Diagnostics of electronic system of the automatic air conditioner (AC)
Application of an oscillograph for observation of signals of a control system
Ignition coils - installation details
Removal, installation and adjustment of the sensor of position of the camshaft (CMP)
The distributor of ignition (M119.97 engines) - installation details
+ Systems of a charge and start
+ Gear shifting box
+ Automatic transmission
+ Brake system
+ Suspension bracket and steering
+ Onboard electric equipment
Self-diagnostics of systems of electronic control of OBD
ECM/PCM is a brain of a control system of the engine. Basic data arrive on the module from various information sensors and other electronic components (switches, the relay, etc.). On the basis of the analysis of the data arriving from information sensors, and according to the basic parameters put in the processor memory, ECM/PCM develops commands for operation of various operating relays and actuation mechanisms, carrying out thereby adjustment of working parameters of the engine, and providing maximum efficiency of its return at the minimum fuel consumption.
Data on diagnostic unitsCheck of serviceability of functioning of components of systems of injection and decrease in toxicity of the fulfilled gases is made by means of the universal digital measuring instrument (multimeter)
Connection of a multimeter to sockets of the control unit of the engine by means of an auxiliary splitter
Use of the digital measuring instrument is preferable for several reasons. First, on analog devices it is rather difficult (sometimes, it is impossible), to define result of the indication to within the 100-th and thousand shares while at inspection of the contours including electronic components in the structure, such accuracy is of particular importance. The second, not less important, the fact that the internal contour of a digital multimeter, has rather high impedance is the reason (internal resistance of the device makes 10 megohms). As the voltmeter is connected to the checked chain in parallel, the accuracy of measurement of subjects is higher, than smaller current will pass through actually device. This factor is not essential at measurement of rather high values of tension (9 ÷ 12 V), however becomes defining at diagnostics of the elements giving low-voltage signals, such as, for example, l-probe where it is about measurement of shares of volt.
Some scanners, besides usual operations of diagnostics, allow to print out, at connection with the personal computer, the schematic diagrams of electric equipment which are stored in memory of the control unit (if are put), to program anticreeping system, to observe signals in car chains in real time.
Diagnostics of electronic control systems of the engine, injection and ignition, automatic conditioner of air and ABS/ASR/ETS/ESP
Scheme of an arrangement and design of diagnostic socketsArrangement of diagnostic sockets
The 9-contact socket for diagnostics of a control system on value of porosity of an impulse, by means of the device for measurement of so-called duration of the closed condition of contacts of the breaker (dwell-meter)
Purpose of contacts of the 38-contact diagnostic socketThe 38-contact diagnostic socket for extraction of the blinking codes
Plugs of the socket have the following appointment:
Arrangement of the 16-contact diagnostic socket (on the USA models)
Plugs of the socket have the following appointment:
Measurement of porosity of an impulse
Reading and removal of the blinking codes
The controller of interface of the personal computer to onboard system of self-diagnostics of OBD II under protocols of the SAE standards (PWM and VPW) and ISO 9141-2
General dataScheme of the organization of the controller of interface to onboard system of self-diagnostics of OBD II
The considered device represents the microcontroller executed on KMOP (CMOS) technology. The device plays a role of the elementary scanner and is intended for reading of diagnostic codes and data of the OBD II system (engine turns, temperature of cooling liquid and the soaked-up air, load characteristics, a consumption of the air coming to the engine, etc.) within the SAE J1979 standard via the tire of any execution (PWM, VPW and ISO 9141-2).
Main destinationFor connection to the computer of rather 3-wire wire, connection to the diagnostic socket is carried out by a 6-wire wire. Supply voltage moves on the adapter via the 16-contact diagnostic OBD socket.
Recommendations about applicationFor connection of the device to the car the unscreened cable, no more than 1.2 m long can be used that has special value when using the PWM protocol. When using a cable of bigger length it is necessary to reduce resistance of resistors on an entrance of the device (R8 and R9 or R15). When using a screened cable, the screen should be switched-off for the purpose of decrease in capacity.
The cable for connection to a serial port of the computer can also be unscreened. The device steadily works with a cable up to 9 m long. With much bigger length of a cable it is necessary to use more powerful communicator of RS 232.
The topology of electric connections is any. At the increased humidity use the additional shunting condensers.
The free software (browser) for reading of codes and data can be downloaded from the websites of producers, or the website of our arus.spb.ru publishing house and is intended for use under DOS. The insignificant size of a software application in option "under DOS" allows to accomodate it on the loading diskette DOS and to use even on the computers equipped with the software, incompatible with DOS. An optional condition is even existence in the computer of the hard drive.
General principles of data exchange
Data exchange goes on three-wire consecutive connection without application of initsializatsionny exchange of office messages (handshaking). The device wiretaps the channel on existence of messages, executes the accepted commands and transfers results to the personal computer (PC) then immediately comes back to the listening mode. The data entering the controller and proceeding from it are organized in the form of a chain of the bytes which are consistently going one after another, first of which is control.
Initialization of the controller and onboard system of self-diagnosticsTo start data exchange of PC has to make installation of connection with the controller, then initialize the controller and the channel of data of OBD II.
Connection installationAfter connection of the controller to PC and the diagnostic OBD socket its initialization for the purpose of prevention of the "lags" connected with noise in consecutive lines has to be made in case their connection was made before inclusion of power supply of the controller. The simplest check of activity of the interface is at the same time made. First of all the one-byte signal of 20 hex perceived by the controller as the command for connection installation is sent. In reply the controller instead of control sends the only byte of FF hex (255 dec) and passes into a waiting mode of reception of data. Now PC can pass to initialization of the channel of data.
InitializationAt this stage initialization of the protocol under which data exchange will be made, and in case of the ISO protocol – initialization of onboard system is made. Data exchange is made on one of three protocols: VPW (General Motors), PWM (Ford) and ISO 9141-02 (Asian/European producers).
The choice of the protocol is made by transfer of the combination consisting of control byte of 41 hex and the byte defining protocol type following directly it: 0 = VPW, 1 = PWM, 2 = ISO 9141. So, for example, at the command of 41 02 hex initialization of the ISO 9141 protocol is made.
After installation of connection and initialization of the protocol regular data exchange, consisting of the inquiries arriving from PC and the answers issued by the adapter begins.
Data exchange orderFunctioning of the controller when using protocols of the ISO 9141-2 and SAE family (VPW and PWM) happens according to a little various scenarios.
Exchange under the SAE protocols (VPW and PWM)At data exchange under these protocols there is a buffering of only one shot of data that means need of a specification subject to capture or return of a shot. The onboard processor can transfer the packages consisting more than of one shot in some (rare) cases. In such situation the inquiry has to repeat until all shots of a package are not accepted.
The inquiry is always formed as follows: [Control byte], [Inquiry according to the SAE standard], [Number of a shot]. As it was already mentioned above, the control byte usually represents the number equal to the full number of the following bytes behind it. The inquiry is made out according to the SAE J1950 and J1979 specifications and consists of heading (3 bytes), the sequence of information bytes and byte of control of a mistake (CRC). Let's notice that while information on inquiry is formed in strict compliance with the SAE specifications, a consumer of control byte and number of a shot is the interface controller.
At a successful completion of the procedure the response message always has the following format: [Control byte], [The answer according to the SAE standard]. The control byte, as well as earlier, defines the number of the information bytes following it. The answer according to requirements of the SAE standard consists of heading (3 bytes), a chain of information bytes and byte of CRC.
At failure the 2-byte response message is sent: [Control byte], [State byte]. At the same time in control byte the MSB installation is made. Four younger bits form number 001 demonstrating that control is followed by the only byte - state byte. This situation can arise rather often as Specifications allow a possibility of a lack of distribution the onboard processor of data, and also transfer of incorrect data in case the inquiry does not conform to the standard supported by producers of the car. Also the situation when required data are absent in random access memory of the processor of time at the moment is possible. When the controller does not receive the expected answer, or obtains the damaged data, the MSB installation of control byte is made, and after control the state byte is given.
At collisions in the tire the interface develops the only byte of 40 hex which is control byte with the nullified younger bit. The similar situation can arise rather often when loading a car tire messages of higher than at diagnostic data of a priority, - the computer has to repeat initial inquiry.
Exchange under the ISO 9141-2 protocolsISO 9141-2 is used by most of Asian and European producers of automotive vehicles. The structure of the formed PC of inquiry a little in what differs from used in the SAE standards, with only that difference that the adapter does not need information on number of a shot and the relevant data should not be present at a package. Thus, the inquiry always consists of control byte and the chain of the information bytes including checksum following it. As the response message the controller just relays the signals created by the onboard processor. The control byte in the response message is absent therefore PC perceives the arriving information a continuous stream until the chain is not interrupted by the pause in 55 milliseconds reporting about end of information package. Thus, the response message can consist of one or more shots according to requirements of the SAE J1979 specifications. The controller does not make the analysis of shots, does not reject not diagnostic shots, etc. PC has to make by own forces processing of the arriving data with the purpose of exarticulation of separate shots by the analysis of heading bytes.
Modifications of controllers of the latests version
The main differences of process of data transmission under the SAE and ISO 9141 protocols, characteristic of the interface controllers of the latests version, and also a data transmission order under the ISO 14230 protocol are given below:
Connection installationThe order of installation of connection did not change:
Choice of the protocolThe protocol is chosen in as follows:
Typical affirmative answer looks as follows:
Remark and commentsIf use of the controller for data transmission only on any to one is planned or to two of protocols, excess components can be excluded.
For example, at the organization of the scheme under the VPW (GM) protocol in a wire of connection of the controller to the car only three veins of an electrical wiring (plug 16, 5 and 2) will be required.