The engine management systems used on the 1989-98 BMW 3 series vehicles can be broken down into two basic systems, On Board Diagnostic version 1 (OBD I) and On Board Diagnostic version 2 (OBD II). Both systems are controlled by an Engine Control Module (ECM) which relies on input devices to monitor the engine's operating parameters. The ECM monitors the input signals and activates the emission devices, and controls the ignition and fuel systems to maximize the efficiency of the engine thereby reducing the emissions.
One of the regulations for both OBD I and OBD II equipped vehicles is that the system must be able to monitor it's own operation and warn the driver of an emission related failure. The Check Engine Light also known as the Malfunction Indicator (MIL) Light is illuminated if an emissions related problem is detected by the ECM.
Both the OBD I and OBD II ECM's have the ability to store fault codes. These codes can be read by the BMW dealer's diagnostic equipment or displayed by a blink code on OBD I systems, or a compatible Data Scan Tool (DST) on OBD II systems.
Accessing the fault codes can save valuable diagnostic time, although some systematic diagnostic trouble shooting is necessary. The OBD II systems will store faults that can be retrieved, and, if using a BMW diagnostic tester or a suitable DST tool such as Baum Tools' model CS-2000, the system can be checked dynamically and the component values read while the engine is running.
Using a suitable Data Scan Tool (DST) also allows the system to be checked without the chance of damaging the component, wire connections or the insulation. Many of the input sensor electrical connections are very difficult to access and will require using compatible electrical connectors in order to obtain an accurate reading. A scan tool that can monitor the system with the engine running allows the system to be checked during initial start-up, monitored during the warm up period and at normal operating temperatures. Once a repair is completed, the DST can clear all of the stored Diagnostic Trouble Codes (DTC's).
The following systems can be checked individually, and most of them can be checked using suitable diagnostic equipment. Beginning with model year 1996 all passenger vehicles are On Board Diagnostic version II (OBD II) compliant. Using a suitable OBD II Data Scan Tool (DST) can save precious diagnostic time. Depending on the capabilities of the scan tool, the systems can also be checked in a live data mode, while the engine is running without chance of damaging the component, wire connections or the insulation.
Another advantage of using a suitable live data OBD II DST such as the BMW tester or the Baum Tools CS 2000 DST, is that the systems can be checked how they interact with one another, and checked during initial start-up, monitored during the warm up period and at normal operating temperatures.
A suitable OBD II DST also allows for any stored Diagnostic Trouble Codes (DTCs) faults to be accessed and cleared.
The BMW models both use an Engine Control Module (ECM) to control the fuel, ignition and emission systems of the engine. The ECM relies on a variety of sensors to evaluate the engine's operating condition. The ECM also has the ability to recognize when a sensor's value or input is beyond the normal operating range for that component.
If a sensor's wire is damaged, disconnected, or the sensor fails, the ECM receives an invalid signal for that sensor. The ECM has no idea what the cause of the problem is, however it does recognize that the signal is not within the acceptable operating range for that particular component. In order for the engine to run, the ECM will substitute a default value for the sensor that allows the engine to continue running, however the engine's performance and efficiency may be compromised. When this condition occurs, the ECM stores Diagnostic Trouble Codes (DTCs) into its fault memory.
If the condition is severe enough to cause potential damage to another component, the CHECK ENGINE LIGHT/MALFUNCTION INDICATOR LIGHT (MIL) is turned on and remains on after the vehicle is started.
Diagnostic testing is done by checking the inputs and the outputs for the ECM control unit, and by accessing the fault memory of the ECM. The ECM used in the OBD I equipped models has on board diagnostic capabilities that are decoded by using the blink code method. The 1996-98 BMW models are OBD II compliant and the PCM fault memory is accessed by using a BMW tester or a suitable OBD II capable Data Scan Tool (DST), and the fault codes are displayed on the DST screen.