Diagnostics of electronic control systems and diagnostic equipment (Mercedes-Benz M-Class W163)

The OBD system includes several diagnostic devices that monitor individual parameters of the toxicity reduction systems and record detected failures in the on-board processor memory in the form of individual fault codes. The system also checks sensors and actuators, monitors vehicle maintenance cycles, and provides the ability to memorize even short-term failures during operation and clear the memory block.

All models described in this Manual are equipped with a second-generation on-board diagnostics system (OBD II), the main element of which is an on-board processor, more often called an electronic control module (ECM - next), or (less commonly) the powertrain control module (PCM).



The ECM is the brain of the engine management system. Input data is supplied to the module from various information sensors and other electronic components (switches, relays, etc.). Based on the analysis of data received from information sensors, and in accordance with the basic parameters stored in the processor memory, the ECM generates commands to operate various control relays and actuators, thereby adjusting the engine operating parameters and ensuring maximum efficiency with minimum fuel consumption.



Reading the OBD II processor memory (ECM) is done with a special scanner connected to the 16-pin diagnostic connector for reading the database (DLC). The same scanner is used to clear the processor memory. It would be reasonable to entrust the procedures for reading DTC codes and clearing the ECM memory to car service specialists.

Note: On some models, reading of fault codes (DTC) stored in the ECM memory can in principle be performed using the auxiliary LED, as well as by codes displayed on the automatic air conditioning display.

Information about diagnostic devices

The proper functioning of injection system components and exhaust gas toxicity reduction systems is checked using a universal digital meter (multimeter). Using a digital meter is preferable for several reasons. Firstly, it is quite difficult to measure the value of an analog meter (sometimes - impossible), determine the result of the reading with an accuracy of up to hundredths and thousandths, while when examining circuits that include electronic components, such accuracy is of particular importance. The second, no less important, reason is the fact that the internal circuit of the digital multimeter has a fairly high impedance (the internal resistance of the device is 10 mOhm). Since the voltmeter is connected to the circuit being tested in parallel, the accuracy of the measurement is higher, the smaller the current that passes through the device itself. This factor is not significant when measuring relatively high voltage values (9÷12 V), but it becomes decisive when diagnosing elements that produce low-voltage signals, such as, for example, a lambda probe, where we are talking about measuring fractions of a volt.





Parallel monitoring of signal parameters, resistances and voltages in all control circuits is possible using a splitter connected in series to the engine control unit connector. In this case, with the engine off, running or while the car is moving, the signal parameters are measured at the terminals of the splitter, from which a conclusion is made about possible defects.

Special diagnostic scanners or testers with a specific cartridge can be used to diagnose electronic systems of the engine, automatic transmission, ABS, SRS and others (if provided), a universal cable and connector. In addition, for this purpose, you can use an expensive specialized automotive diagnostic computer, specially designed for full diagnostics of most systems of modern cars (for example, ADC2000 from Launch HiTech). Also, for this purpose, you can use scanners and specialized diagnostic analyzers, for example FDS 2000, Bosch FSA 560 (bosch.de), KTS500 (0 684 400 500) or a regular personal computer with a special adapter, cable (for example, set 1 687 001 439) and the installed OBD II browser program.

Some scanners, in addition to the usual diagnostic operations, allow, when connected to a personal computer, to print out the basic diagrams of electrical equipment stored in the control unit memory (if any are included), program the anti-theft system, observe signals in the car circuits in real time.