Control Unit (DME)

System Overview:

PURPOSE
The Digital Motor Electronic (DME) control module (3.3.) provides precise control of fuel injection and ignition systems to ensure optimum performance and strict emissions compliance. Various engine sensors and other systems (i.e.. air conditioning, auto transmission etc.) supply the DME control unit with operating information. The DME control unit processes the sensor input and sends output signals to adjust injector pulse width, injector timing, and ignition timing.

System Diagram:

OPERATION
Inputs signals include:
^ Engine Speed
^ Reference Mark Signal
^ Air Mass Sensor (Hot Film)
^ Engine Temperature
^ Auto Trans Control Unit
^ Cylinder Identification
^ Throttle Position
^ Lambda Signal
^ Battery Voltage
^ Knock Sensors
^ Vehicle Speed
^ Gear Position (Auto Trans. only)
^ A/C Signal
^ On-board Computer (Drive Away Protection)

Output Signals include:
^ Fuel Injection Control
^ Ignition Controls
^ Idle Speed Control
^ Fuel Pump Control
^ O2 Heating Control
^ Evaporative Control

Injection Comparisons:

FUEL INJECTION CONTROL
The DME control unit operates each injector separately, (sequential injection). The basic fuel injection time is calculated from the air mass meter signal and engine speed signal input. The injector opening time and duration is then corrected from other input signals depending on the engine's operating conditions.

Cold Start
During a cold start operation increased fuel is injected, and the system operates with parallel injection. The quantity depends on engine speed and temperature. If starting is repeated within one minute, full cold-start injection quantity is not injected.

Warm-up
During the warm-up phase, the initial injection quantity is enriched dependent on temperature, load and engine speed.

Normal Operation
When the engine reaches normal operating temperature (above 158°F), the basic fuel injection time will be calculated from the air mass meter signal, the 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. The system will operate with full sequential injection unless the cylinder identification sensor malfunctions.

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 side of the eight DIS coils are connected with the positive battery terminal via the ignition switch. During the primary current-flow time (coil saturation) the coils are connected to ground through the power output transistors of the DME control unit. Based upon load conditions received by the DME control unit 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. For very high ambient temperatures, the ignition is retarded as a function of engine and intake air temperature.

Knock Control
Knock control allows ignition advance right up to the knock limit without the need for a safety margin. Ignition is retarded only when the actual knock point is detected, and then only the cylinder(s) identified as knocking. The best ignition timing point can be selected for optimum fuel consumption and operating efficiency.

Signals produced by the knock sensors are compared to set values in the DME programming. When these predetermined values are reached, that combustion process is viewed as "knocking" and the timing is immediately retarded (e.g. 3°). This can occur for one or more cylinders as needed. The ignition retard occurs well before the knock poses a hazard for the engine. Following knock retardation, the ignition is advanced step by step until the optimum timing point is reached, or until knock is again detected.

Secondary Ignition Monitor Diagram:

Secondary Ignition Monitoring
The DME control module continuously checks secondary ignition system integrity.

Monitoring detects misfires on a cylinder-selective basis via a shunt (240 ohm resistor), and monitoring circuit in the DME control module. If the primary ignition circuit monitoring is recognized as OK, but secondary ignition voltage is not reached, the coresponding fuel injector output stage is switched off.

IDLE SPEED CONTROL
The 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, throttle position, and air/fuel mixture capabilities. The control unit continuously compares the programmed data, for idle control and air fuel mixture, with the input values. If a deviation occurs, the new input values are superimposed and stored in the programmed data map. As a result, new output signals are sent to the idle control valve, and or to the injectors. This allows the DME control unit to correct the idle speed, or injector pulse width 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.