BMW Coupes and Sedans 1970-1988 Repair Guide

Digital Motor Electronics (DME) System

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OPERATION



Some BMW models are equipped with the Bosch Digital Motor Electronics (DME) system. This system incorporates various data sensors that monitor the air intake volume, engine speed, crankshaft position, coolant temperature, intake air temperature and throttle position. Signals from these sensors, as well as the oxygen sensor signal and the start sensor signal, are sent to the electronic control unit. The electronic control unit is a microcomputer and the brain of the DME system. It uses the information obtained from the data sensors in order to determine the correct amount of fuel and the optimum ignition timing.

The DME system also has the capability of switching from an open loop system to a closed loop system when the coolant temperature is above 113°F (45°C) and when the oxygen sensor temperature is above 480°F (249°C). The digital engine control system consists of 4 main sub-systems: spark timing, fuel control, electronic control unit, and various data sensors.

SUB-SYSTEMS



Spark Control

The spark control system allows the electronic control unit to determine the exact instant that ignition is required to operate the vehicle properly, based upon the information provided from the data sensors. At the proper time the electronic control unit breaks the primary circuit of the ignition coil and this in turn produces a high voltage spark at the coil center tower. This voltage surge fires the spark plug at the proper time for most efficient combustion, eliminating the need for vacuum and/or centrifugal advance.

Fuel Control

The vehicle is equipped with the Bosch air flow controlled fuel injection system. The system is electronically controlled by the electronic control unit, which is programmed to regulate fuel injection based upon data received from the various data sensors. The electronic control unit generates control signals for the fuel pump relay, auxiliary air valve, cold start injector coil and the cylinder port injector coils. These components control the curb idle speed and mixture, cold idle, air/fuel ratio and the fuel supply.

Electronic Control Unit

The electronic control unit monitors and controls all digital engine control functions. The electronic control unit consists of input and output devices, a central processing unit, a power supply and various memory banks. The input and output devices of the electronic control unit convert electrical signals received by the data sensors and switches to the digital signals that are used by the central processing unit. The central processing unit receives digital signals that are used to perform all mathematical computations and logic functions necessary to deliver proper air/fuel mixture. The central processing unit is also responsible for calculating spark timing information. The main source of power that allows the electronic control unit to function is generated from the battery of the vehicle and transported through the ignition system. The memory bank of the electronic control unit is programmed with exact information that is used by the electronic control unit during the open loop mode. This data is also used when a sensor of other component fails, allowing the vehicle to be driven to a repair facility.

Data Sensors

The digital engine control system consists of 6 data sensors. They are an oxygen sensor, reference mark sensor, speed sensor, coolant temperature sensor, air intake temperature sensor and the air flow sensor. Each sensor supplies electronic data to the electronic control unit, which in turn computes spark timing and the correct amount of fuel that is necessary to maintain proper engine operation. The system also uses a throttle switch, a high altitude switch, an auxiliary air valve, a fuel pressure regulator and a pressure damper.

The oxygen sensor is mounted in line with the exhaust system directly in front of the catalytic converter. The oxygen sensor supplies voltage under one half volt when the fuel mixture is lean and up to one volt when the fuel mixture is rich. The sensor must be hot to function properly and to allow the electronic control unit to accept its power signals. The function of the oxygen sensor measures the amount of oxygen. Most vehicles are equipped with a special electrically heated oxygen sensor which aids the system so that it will begin to function earlier. The heated oxygen sensor has 3 wires, 2 for the heater element and one for the sensor signal. The heating function begins as soon as the ignition is turned on. The plugs from the sensor to the wiring harness are located near the flywheel sensor plugs.

No attempt should be made to measure oxygen sensor voltage output. Current drain on the voltmeter could permanently damage the sensor, shift sensor calibration range and/or render sensor unusable. Do not connect jumper wire, test leads or other electrical connectors to sensor. Use these devices only on the electronic control unit side of the harness after disconnecting sensor.

The reference mark sensor is located on the engine crankcase flange. Its function is to detect crankshaft position in relation to top dead center and then to send the proper signal to the electronic control unit. It is triggered by a bolt which is fastened to the engine flywheel.

The speed sensor is mounted on an adjustable bracket along with the reference mark sensor. This sensor measures engine speed by counting the teeth on the starter ring gear. The speed sensor sends voltage surges to the electronic control unit for each tooth that passes.

The coolant temperature sensor is located on the intake manifold. Its function is to supply coolant temperature information to the electronic control unit. This generated data effects the air/fuel ratio, the spark timing and the engine temperature light.

The air intake temperature sensor is located in the air stream of the air flow meter. The main function of this sensor is to supply incoming air temperature information to the electronic control unit. The electronic control unit uses this data along with other important data to regulate the fuel injection rate.

The air flow sensor functions the same as the air intake temperature sensor, except that the air flow sensor incorporates a measuring flap that opens against the pressure of a spiral spring which is connected to a potentiometer. The potentiometer transmits an electrical signal which is determined by a position on the measuring flap to tell the electronic control unit the vehicle engine load.

A contact type throttle link switch, which is located on the throttle body, convert throttle position into electrical signals. These signals are used to inform the electronic control unit of throttle position. The potentiometer within the air flow meter prevents the loss of engine power during sudden acceleration or deceleration by signaling the electronic control unit for the necessary fuel requirements.

The high altitude switch is mounted under the dashboard on the driver's side of the vehicle. In altitudes higher than 3300 feet the switch closes, signaling the electronic control unit to lean out the fuel mixture so that the vehicle will continue to function properly.

The function of the auxiliary air valve is to provide additional air during engine warm up. The valve is located next to the throttle body. It consists of an electrically heated bi-metallic strip, a movable disc and an air by-pass channel. The heating coil on the bi-metallic strip is energized by the fuel pump relay.

Control of the auxiliary valve is governed by engine temperature. The air by-pass channel is open when the engine is cold and gradually closes as the engine warms up. At predetermined temperature the air by-pass channel in the valve is blocked and additional air flow is cut off.

The fuel pressure regulator is located at the end of the fuel injection collection line. The function of the pressure regulator is to maintain constant fuel pressure to the fuel injectors.

The pressure damper is located at the inlet of the fuel injection collector tube. The pressure damper absorbs fuel pressure oscillation caused by the fuel injection cycle.

TESTING



DME Control Unit
  1. Check the electrical power supply. Turn the ignition switch ON and disconnect the DME control unit electrical connector.
  2.  
  3. With a suitable voltmeter, check connection 18 and 35 of the DME control unit. There should be approximately 12 volts.
  4.  
  5. Connections 5, 16, 17 and 19are all connected in with the ground.
  6.  
  7. Pull off relay number 2 and jump terminals 87 and 30. This will supply voltage to the control unit.
  8.  
  9. If necessary, check activation or replace relay number 2. Turn on the ignition and check the voltage on terminals 85 and 86. There should be 12 volts present.
  10.  
  11. To be sure that the source of defect is only in the control unit, it is recommended to replace the DME unit for comparison.
  12.  
  13. In addition, carry out the various L-Jetronic test with a suitable BMW service test unit first, depending on the type of complaint.
  14.  

Adaptive Pilot Control

The adaptive pilot control has been integrated in the DME control unit since 1985.

  1. Remove the adaptive pilot control screw from the exhaust manifold and mount exhaust tester 13 0 090 or equivalent with adapter 13 0 100 or equivalent into the exhaust manifold.
  2.  
  3. Connect the BMW service test unit or equivalent. Remove the anti-tamper lock.
  4.  
  5. Remove the air cleaner assembly and the air flow sensor.
  6.  
  7. Drill a hole in the anti-tamper lock with special tool 13 1 092 or equivalent.
  8.  
  9. Knock the tool with the anti-tamper lock out of the air flow sensor with suitable impact. Re-install the air flow sensor and air cleaner assembly.
  10.  
  11. Start the engine and let it run at idle to reach normal operating temperature. Pull off and plug the vacuum hose on the fuel pressure regulator.
  12.  
  13. The oxygen sensor must regulate the CO level back to nominal valve after a brief rise.
  14.  
  15. Tighten the air control screw in the air flow sensor completely with special tool 13 1 100 or equivalent for a richer mixture.
  16.  
  17. Run the engine at idle speed and the CO level will be regulated back to its nominal value.
  18.  
  19. Disconnect the oxygen sensor plug. The CO level will rise approximately 2.0% by volume. Also note the instantaneous actual value.
  20.  
  21. Stop the engine. Disconnect the negative battery so as to cancel the value stored in the memory of the DME control unit.
  22.  
  23. Start the engine. If the actual CO level value is considerably higher, the adaptive control pilot is working. Reconnect the vacuum hose.
  24.  
  25. Adjust the CO level to its nominal value (0.2-2.0%) with tool 13 1 100 or equivalent.
  26.  
  27. Connect the oxygen sensor plug and remove the exhaust tester. Remove the air cleaner and the air flow sensor (if necessary). Install a new anti-tamper lock in the air-flow sensor.
  28.  
  29. Reinstall the air flow sensor and air cleaner assembly. Remove all test equipment.
  30.  

DME CHIP REPLACEMENT



See Figures 1, 2, 3, 4, 5 and 6

Inside the DME control unit is a removable computer chip which governs many of the function of the unit. In the event the control unit is found to be defective, this chip may well be the source of the problem.

In recent years, many companies have offered replacement control unit chips which can be installed to improve engine performance and driveability. Although these chips are not original BMW parts, many work with excellent results and reliability.

If you are replacing a DME chip with a non-BMW chip, make sure the new chip conforms to all state and federal emission regulations. Most chips available will clearly state on the package whether the product meets these government standards.

To replace a DME chip, proceed as follows:

  1. Access the DME control unit. Depending on the model, this unit can be found above the glove box, or under the rear seat.
  2.  
  3. Disconnect the negative battery cable.
  4.  
  5. Remove the DME control unit from the vehicle.
  6.  
  7. Unplug the harness from the DME unit, and place on a clean, well lit surface.
  8.  


WARNING
Before working on any electronic control unit, make sure your body is properly grounded to prevent the risk of any static electricity from your body damaging the control unit. Many electronic parts stores offer ground straps which attach to your wrist and neutralize any static electricity on your body.

  1. With your body properly grounded, remove the screws from the DME control unit cover, and place aside.
  2.  
  3. Using a prytool with the end taped to protect the control unit, carefully pry off the cover.
  4.  
  5. Remove the screws from the back of the circuit board.
  6.  
  7. Lift the circuit board up to access the chip.
  8.  
  9. Locate the chip with a sticker on its top. Before attempting to remove the chip, make sure it is installed into holes and is not permanently installed with solder. In the event the chip is soldered in place, you have picked the wrong chip.
  10.  
  11. Once the correct chip has been located, carefully unplug the chip.
  12.  

To install:
  1. Install the chip in the same direction as the chip just removed. Do not attempt to force a chip into place. Make sure that each contact spider on the chip aligns with a hole in the circuit board.
  2.  
  3. Install the circuit board and secure with the screws.
  4.  
  5. Install the control unit cover making sure the seal is positioned correctly. Install the retaining screws.
  6.  
  7. Plug in the control unit and install it into the vehicle.
  8.  
  9. Connect the negative battery cable.
  10.  



Click image to see an enlarged view

Fig. Fig. 1: Remove the DME control unit; it is mounted above the glove box on 3 Series models such as this



Click image to see an enlarged view

Fig. Fig. 2: Unplug the harness from the control unit



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Fig. Fig. 3: Carefully pry off the control unit cover



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Fig. Fig. 4: Unfasten the screws securing the circuit board in place



Click image to see an enlarged view

Fig. Fig. 5: Lift the circuit board up to access the DME chip



Click image to see an enlarged view

Fig. Fig. 6: Carefully lift out the DME chip

 
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