Control Unit (DME)

System Diagram:

PURPOSE
The Digital Motronic Engine control module (1.7) provides full control of the engine management system.

LOCATION
Above the glove box.

OPERATION
Inputs signals include:
^ Engine Speed
^ Reference Mark Signal
^ Airflow Volume
^ Intake Air Temperature
^ Engine Temperature
^ Cylinder Identification
^ Throttle Position
^ Lambda Signal
^ Battery Voltage
^ Vehicle Speed
^ Gear Position (Auto Trans. only)
^ A/C Signal
^ On-board Computer (Drive Away Protection)

Output Signals include:
^ Fuel Injection Control
^ Ignition Control
^ Idle Speed Control
^ Fuel Pump Control
^ Evaporative Control

FUEL INJECTION CONTROL
Basic fuel injection time is calculated using the airflow volume and engine speed signal. Fuel injector opening time is then corrected by other input signals depending on engine operating conditions. The DME control unit directs the fuel injectors to operate in two groups. The groups consist of cylinders 1 & 3 and cylinders 2 & 4. Each group injects fuel once every 720° of crankshaft rotation (semi-sequential injection). This operation can only take place if the signal from the cylinder recognition sensor is present.

Cold Start
During cold start operation a coolant temperature related quantity of fuel is injected; two times per crankshaft revolution for up to five crankshaft revolutions. The quantity of fuel is in excess of the normal injected quantity.

After completion of five revolutions, the high initial fuel quantity is reduced on a rpm-related basis. This is accomplished in the control unit by start-up recognition. Start-up recognition is permanently programmed in the DME control unit and is temperature and speed-dependent. After the start-up recognition threshold has been reached (600-800 rpm), injection will take place only once per crankshaft revolution.

Warm-up
The DME control unit will use the after-start warm-up function, which is mapped and controlled up to a coolant temperature of 158°F, to determine the fuel injection quantity during the warm-up phase. The control unit will compare and evaluate input signals from the coolant temperature sensor and the engine speed signal against fixed values in it's program. When these values are combined the proper fuel quantity will be injected. The system also recognizes a repeat start and reduces the injected quantity of fuel to avoid an over-rich condition in the event of a restart.

Normal Operation
When the engine reaches normal operating temperature (above 158°F), basic fuel injection time will be calculated from the airflow volume signal, engine speed signal and the oxygen sensor signal. The oxygen sensor signal is used as a correction factor to obtain the optimum air/fuel mixture for reduction of pollutants and maximum engine performance.

FUEL DELIVERY CONTROL
Operation of the electric fuel pump is controlled through a relay which is energized by the Motronic control unit. The relay is energized only during engine cranking or while the engine is running. The DME control unit uses the engine speed signal as an input for fuel pump control.

Ignition Timing Program Map:

IGNITION CONTROL
The primary circuits of the ignition coils are switched by the power output stage of the DME control unit. The uncorrected basic ignition time is used as a load signal for the ignition. Based upon these load conditions in relation to various engine speeds a three dimensional ignition performance characteristic "Basic Ignition Map" is used to optimize the ignition to achieve the most favorable exhaust gas and fuel consumption performance for any given load or speed condition.

When the throttle valve is in the closed position, the bottom line of the basic ignition map is selected as the idle/coast characteristic. For engine speeds below the nominal values, the ignition timing is advanced to obtain idle stabilization. For coasting, ignition timing is programmed to obtain minimum exhaust gas pollutants and optimum driveability performance. For the wide open throttle position, the top line of the basic ignition map is selected. This full-load characteristic is programmed with engine knock limits taken into consideration. For very high ambient temperatures the ignition is retarded as a function of coolant and intake air temperature.

IDLE SPEED CONTROL
Idle speed is electronically controlled by the DME control unit with relation to the respective engine coolant temperature and load signals.

EVAP EMISSION CONTROL
The DME control unit controls operation of the evaporative emission system by switching the electric purge valve off and on based on engine speed, load, and temperature characteristics.

ON-BOARD DIAGNOSTICS
In the event of a DME system malfunction, circuit information will be stored in the internal memory of the DME control unit. Intermittent failures are stored with a record of the failure frequency. The DME control unit is able to store up to thirty malfunctions, and will give priority to those items which can cause the vehicle to exceed exhaust emission standards. In addition to being stored in memory, an emissions-related failure will cause the "Check Engine" light to illuminate on the instrument cluster. Furthermore, the "Check Engine" light is also used to display faults which are stored in the control unit's memory when the self-diagnostic procedure has been initiated. Faults may also be retrieved with the BMW service tester if available.

ADAPTIVE CONTROL
The DME control unit has adaptive idle speed, air/fuel mixture and charcoal canister purge capabilities. The control unit continuously compares the programmed data for these functions, with the input values. If a deviation occurs, new input values are superimposed and stored in the programmed data map. This allows the DME control unit to correct for break-in wear or slight air leaks which might occur in the systems. If power is interrupted to the control unit, new adaptation takes place within several minutes of engine operation.