Some vehicles are equipped with either the Bosch® Digital Motor Electronics (DME) system or the Siemens MS system. Both engine management systems utilize an Engine Control Module (ECM) allowing the fuel metering and ignition system to adapt to the engine's operating parameters.
The engine management systems used in 1989-95 BMW models meet the On Board Diagnostic version one (OBD I) requirements. These systems have the ability to store fault codes which can be displayed via a blink code using the check engine light or accessed using the BMW diagnostic testers. The check engine light is also used as a warning light to indicate a failure of an engine management component. If the ECM control unit identifies a problem in the engine management system, the check engine light is turned on, and one or more fault codes are stored within the control unit.
Beginning with model year 1996, as mandated by federal emissions standards, the BMW engine management systems became compliant with the On Board Diagnostic version two (OBD II) requirements. The OBD II standards require that the engine management components that could affect the emission performance be electronically monitored, and should a failure occur an associated fault code stored in the ECM/DME control unit fault memory. If the fault is detected in more than one drive cycle, the check engine light is activated.
The ECM accepts input signals from the intake air flow or air mass sensor, crankshaft pulse sensor, cylinder identifying sensor, coolant and air temperature sensors, one or more oxygen sensors, a throttle position switch and if so equipped, the automatic transmission control unit known as the Transmission Control Module (TCM). Optimum fuel metering and ignition timing requirements are calculated based on sensor inputs and internal maps and are adjusted as required for the engine's operating parameters. Output signals from the ECM operate the idle control valve, fuel pump relay, fuel injectors and the ignition system. Each cylinder's fuel injector is located very near to the opening of the cylinder head's intake port and are operated semi sequentially or sequentially depending on the specific fuel management system. The air/fuel ratio, ignition timing and idle speed are not adjustable. The fuel mixture is controlled by the length of time the injector is opened. The adaptive capabilities of the engine management system occurs in a matter of milliseconds.
Some of the components of the ECM/DME system can be tested with affordable test equipment such as a volt/ohmmeter, or an oscilloscope. The OBD I engine management system uses blink codes which can be read using the check engine light, however to access the fault memory of the EMC/DME on OBD II systems requires using the BMW Diagnostic Tester or a suitable Data Scan Tool (DST) that only a dealer or a specialty shop would own. This does not preclude the average person from working on the fuel injection system as most problems can be diagnosed and solved using common tools and logical testing methods. To properly probe the electrical connections of the wiring or the components, a variety of test connectors are needed to provide accurate measurements and to avoid the risk of damaging a component's terminals or electrical connectors.
In addition, just because a fault code is stored does not mean that the related component has definitely failed. The ECM/DME can only evaluate the signal it receives for a particular sensor's circuit. The ECM/DME has no way of knowing if the sensor is disconnected, if the sensor wiring is damaged, or the electrical connector(s) corroded or making poor contact. Any of these conditions could affect a sensor's electronic value and cause a fault to be stored.
Because of the cost of specialty diagnostic equipment, it is often not a justifiable expense for the average consumer. The advantage this equipment offers is that these diagnostic testers can be connected to the vehicle without disconnecting or removing any components, and the vehicle can be tested dynamically while the engine is running or while the vehicle is test driven. This can be considerable time savings as many of the ECM/DME components are not readily accessible and the replacement cost of most components is relatively substantial. Furthermore, most parts vendors are not willing to accept returns on electrical components that have been previously installed, and proper diagnosis avoids replacing components unnecessarily.
Without the use of sophisticated diagnostic equipment, the first step in repair or service to engine management systems is to document the circumstances and operating parameters as to when the problem occurs to gain as much information as possible, especially regarding an intermittent problem. Also, check any service records which may be available. Before checking fault codes, it is essential to check for any obvious loose wires, poor connections, and mechanical faults or failures. Remember, a trouble code only indicates which sensor or circuit is effected by the problem. Poor grounds, loose wires and simple mechanical faults such as a disconnected vacuum hose or leak or poor electrical connection can cause a fault to be stored.
For more information pertaining to the sensors and operating parameters of the engine management system, please refer to Section 4.