See FigureS 1, 2 and 3
All vehicles covered in this guide are equipped with the Bosch Digital Motor Electronics (DME) system. The DME system operates the ignition and fuel injection systems with a single control unit. The DME is a very sophisticated computerized engine management system that works as well on a racing engine as it does on a street car.
For removal and installation procedures for the components of the DME system, refer to Fuel System . This section contains the description and testing procedures for the DME system.
The DME control unit accepts input signals from the intake air flow or air mass sensor, crankshaft pulse sensor, cylinder identifying sensor, coolant and air temperature sensors, an exhaust oxygen sensor, a throttle position switch and if so equipped, the automatic transmission control unit. Optimum fuel quantity and spark timing requirements are calculated based on sensor inputs and internal programs and are adjusted as required for each engine revolution. Output signals operate the idle control valve, fuel pump relay, fuel injectors and the ignition system. Fuel injectors are operated semi sequentially or sequentially depending on the engine. The air/fuel ratio, ignition timing and idle speed are not adjustable.
Most items of the DME system can be tested with a volt/ohm meter, however some in depth testing procedures that utilize the full power of the DME system can only be performed using the BMW Diagnostic Tester that only a dealer or specialty shop would own. This does not preclude the average person from working on the fuel injection as most problem can be diagnosed and solved using common tools and testing methods. A variety of test connectors is also required but some of these can be easily made up as needed. The control unit includes a self diagnostic program that will store fault codes.
The first step in repair or service to engine management systems is to gain as much information as possible, especially regarding intermittent problems. Also check any service records which may be available. Before checking fault codes, it is absolutely essential to check for any obvious mechanical faults or failures. Remember, a trouble code only indicates which sensor or circuit is effected by the problem. Simple mechanical faults such as a vacuum leak or poor electrical connection can cause a fault code.
Never disconnect any wiring with the ignition switch ON. Always turn the ignition switch OFF when using an ohmmeter. Any time the battery or main power terminal is disconnected, the control unit fault memory will be erased. Make sure all codes have been retrieved before disconnecting the battery or removing the control unit connector.
DME CONTROL UNIT AND RELAY
E30 3 Series
The DME unit is located above the glove compartment. The fuel pump relay is mounted on a bracket on the left inner fender in front of the shock tower. There is a grouping of 3 relays. The forward relay is the main DME relay. The middle relay is the fuel pump relay. The rear most relay is the oxygen sensor heater relay.M42 ENGINE
The DME unit is located above the glove compartment. The relays are mounted on the firewall above and to the right of the brake booster. The relay on the right is the main DME relay. The middle relay is the fuel pump relay. The leftmost relay is the oxygen sensor heater relay.S14 ENGINE
The DME unit is located above the glove compartment. The relays are located on the far right of the firewall. The relay on the right is the evaporative purge relay. The middle relay is the main DME relay. The leftmost relay is for both the oxygen sensor heater and the fuel pump.E36 3 Series
The DME unit is located behind a cover on the right side of the engine compartment firewall. The relays are located in the relay and fuse box on the left side of the engine compartment. The relays will be marked on the cover of the relay and fuse box. If the cover is not available, the relays are in the row of relays closest to the engine. The forward relay is the fuel pump relay. The middle relay is the main DME relay. The rear most relay is the oxygen sensor heater relay.E34 5 Series
The DME control unit is mounted in the electronic control box under the hood, on the right side of the firewall. With the cover removed, the DME control unit is closest to the firewall. The box also contains the cruise control module, the automatic transmission control module, the main DME relay, the fuel pump relay, a main power connection and the oxygen sensor heater relay. Some models have a fan below the DME control unit that runs when ever the ignition switch is on.
The main DME relay coil receives 12 volts at all times on terminal 86 and the control unit supplies ground to terminal 85 to operate the relay. When the relay is closed, voltage is supplied to the fuel injectors, idle speed actuator, carbon canister purge valve, air mass sensor, ABS control unit, ignition system and the fuel pump relay. On vehicles equipped with the factory installed anti-theft system, the control unit will not operate the relay when the system is armed.M20 ENGINE
The relays are located in the electronics box at the right, rear of the engine compartment. With the lid of the electronics box removed, visible is a group of 3 relays centered in the box. The relay on the left is the main DME relay. The center relay is the fuel pump relay. The right relay is the oxygen sensor heater relay.
Some vehicles may only have the main DME relay and the fuel pump relay in the electronics box. In this case the relay on the left is the main DME relay and the right relay is the fuel pump relay.M30 ENGINE
The relays are located in the electronics box at the right, rear of the engine compartment. With the lid of the electronics box removed, visible is a group of 3 relays centered in the box. The relay on the left is the main DME relay. The center relay is the fuel pump relay. The right relay is the oxygen sensor heater relay.M50 ENGINE
The relays are located in the electronics box at the right, rear of the engine compartment. With the lid of the electronics box removed, visible is a group of 3 relays centered in the box. The relay on the left is the main DME relay. The center relay is the fuel pump relay. The right relay is the oxygen sensor heater relay.S38 ENGINE
The relays are located in the electronics box at the right, rear of the engine compartment. With the lid of the electronics box removed, visible is a group of 3 relays centered in the box. The center relay is the fuel pump relay. In 1991, the relay on the left is the air pump relay and the right relay is the main DME relay. In 1992 and later, the left and right relay positions are reversed.
Air Flow Sensor
See Figure 4
Two types of air flow sensor are used, an electronic air mass sensor and an electro-mechanical vane type air flow meter. On the vane type, when air enters the housing and moves the air vane, the vane moves a potentiometer. The control unit sends a voltage to the potentiometer and the return signal to the control unit is calculated as air flow. The air flow meter housing also contains a temperature sensor at the inlet side. If either the flow meter or the sensor fails, the control unit will substitute pre-programmed values to allow "limp home" operation of the engine. On previous models, the CO and idle speed adjustments were on the air flow meter. These adjustments have been eliminated, CO and idle speed can no longer be adjusted. The air flow meter and temperature sensor cannot be repaired.Air Mass Sensor
See Figures 5 and 6
This unit is a fully electronic hot wire air mass sensor with no moving parts. It consists of a platinum resistor wire that is part of a bridge circuit that includes a measuring resistor. An amplifier mounted on the side of the housing controls and monitors this circuit. The temperature of the wire is maintained at a constant 311°F (155°C). As air flow changes, the current required to maintain the temperature changes. As the circuit adjusts the current, the measuring resistor reports the change in voltage to the control unit, which then calculates the mass of air flowing through the sensor housing. This type of air measurement system automatically compensates for changes in air temperature, humidity and altitude. When the engine is stopped and after a delay of approximately 4 seconds, the bridge circuit is altered for approximately 1 second. The hot wire is heated to a temperature of approximately 1832°F (1000°C) to burn free any deposits which could influence the resistance or measuring results.
When testing or removing the air mass meter, do not touch the hot wire element or any other electronic parts inside. Protect the element from dirt and fluids. The screen in front of the element can be cleaned or replaced but there are no other serviceable parts.Intake Air Temperature Sensor
See Figure 7
On engines with a vane type air flow sensor, the M20, M30, M42 and S14 engines, the air temperature sensor is inside the housing and must be tested as part of the air flow meter circuit. The air temperature sensor is located in the intake manifold next to the throttle body and switch on the M50 engine. The air temperature sensor on the S38 engine is located in the air cleaner housing next to the air mass sensor.Coolant Temperature Sensor
See Figure 8
On the M20 and M30 engines, 2 sensors are mounted on the thermostat housing. The blue sensor is for the DME control unit, the brown sensor is for the temperature gauge.
On the M42 engine, the coolant temperature sensors are located under the intake manifold on the engine. The forward most sensor is the DME sensor and the rear sensor is for the temperature gauge.
On the M50 engine, the coolant temperature sensors are located under the intake manifold near the front of the engine. The forward sensor is for the DME and the rear sensor is for the temperature gauge.
The coolant temperature sensors on the S14 engine are located in the coolant pipe above the exhaust side of the engine. The rear sensor services the DME and the forward services the temperature gauge.
The coolant temperature sensors on the S38 engine are located in the coolant pipe above the exhaust manifold and in the thermostat housing. The senor in the coolant pipe is for the DME and the sensor in the thermostat housing is for the temperature gauge.
The sensors are an NTC (negative temperature coefficient) type, meaning as the temperature increases, electrical resistance of the sensor decreases. The control unit reads the voltage change in the circuit as temperature. They are the same type as the air temperature sensors and can be tested the same way using the same temperature/resistance chart.Throttle Position Switches
See Figure 9
The throttle position switches are used to tell the control unit when the throttle is fully closed or fully open. They are mounted in a single housing on the end of the throttle assembly. If either switch fails, the control unit will use air meter signals to calculate idle and full load control and a fault code will be set in memory.Accelerator Pedal Sensor
See Figure 10
The E34 5 Series with the M30 engine and traction control is equipped with the EML (throttle by wire) throttle control system. A pedal sensor is used to send pedal position and speed of movement to the EML control unit. This unit is mounted in the box with the DME control unit and has its own diagnostic program and fault codes. The sensor is attached to a bracket above the accelerator pedal and mechanically connected to the pedal. When the sensor is removed or replaced, it must be adjusted and tested for correct output signal. Any time the EML system is worked on, the external safety path check must be made.Speed/Position Reference Sensor
See Figures 11 and 12
Engine rpm and crankshaft position are detected with a sensor mounted on the front of the engine. If this sensor fails, the engine will not run. A special wheel is mounted on the vibration on the front of the crankshaft. As the teeth in the wheel pass the sensor, a magnetic pulse is generated and sent to the control unit, which reads the pulse frequency as engine rpm. A wide gap in the teeth indicates TDC of number 1 cylinder but the gap will not be at the same place on the wheel as the 0/T timing mark stamped on the wheel used to manually set the engine to TDC.
The sensor is a magnetic pick-up mounted in a bracket that holds the gap between the sensor and the wheel. Power to the sensor and the sensor resistance can be checked with a volt/ohm meter. Sensor function can only be checked with an oscilloscope, though a resistance measurement can be made as a quick test.Cylinder Identifying Sender
See Figure 13
This is an inductive pulse sensor similar to the crankshaft sensor. It consists of a coil attached to an ignition wire, supplied with 5 volts from the control unit or camshaft sensor similar to the crankshaft sensor. When that spark plug is fired, the fluctuation in the return signal is used as a real time reference for sequential fuel injection and ignition control functions. If the sender fails while the engine is running, the engine will be operated in parallel fuel injection control instead of sequential.
There are 2 types of sensors. One type of sensor is attached to spark plug wire No. 6 near the distributor. The sender can be replaced by cutting the distributor end off the ignition wire and sliding the sender off the end. The other type of sensor reads off the camshaft and is similar to the speed reference sensor. It is bolted into the cam cover and is used on the M42 and M50 engines.Oxygen Sensor
See Figures 14 and 15
The oxygen sensor consists of a ceramic tube coated with a thin layer of platinum. The closed end of the tube is in the exhaust stream and the reference end is open to the air. When the sensor is at operating temperature, oxygen in the exhaust will cause the unit to generate a voltage. The control unit reads the signal and adjusts the fuel injection to control the amount of oxygen in the exhaust. This is called "Closed Loop" control strategy. Under normal conditions, the control unit will constantly oscillate between rich and lean to create an average ideal air/fuel ratio. This means the output of the oxygen sensor also oscillates between 0 and 1 volt.
The sensor is equipped with an internal resistance heater to bring it to operating temperature quickly. Power for the heater comes from a separate relay mounted near the control unit. If the heater fails, the engine will take longer to switch into Closed Loop operation. If the sensor fails completely, the engine will be operated in Open Loop control and slightly lean to protect the catalytic converter from overheating.
Vehicles with traction control utilize the EML system and do not have a throttle cable or idle speed actuator. The throttle is operated with a motor and controlled by the accelerator pedal sender and the EML control unit. Inputs are accepted from the accelerator pedal sensor, vehicle speed sensor, coolant temperature sensor, throttle motor feedback potentiometers, air conditioner relay, the clutch switch on manual transmissions and the control units for anti-lock braking, anti-skid, traction control, door locks, body and instrument control units and the automatic transmission control unit. Output signals are sent to all of the control units as well as to the throttle motors.
The motors are a single winding DC type that is driven in both directions. The motor unit includes a position feedback potentiometer to tell the control unit throttle position and speed of movement. This potentiometer also includes the throttle position switches whose state (open or closed) is compared with the state of the switches in the pedal sensor. If the information provided from the switches does not match the information provided by the potentiometer, the engine will be operated at reduced power and a warning light is displayed on the instrument panel. If this condition exists, check the pedal sensor adjustment first.
A feature of the EML system is a fail-safe mode that will cut the fuel to the injectors. The EML control unit is constantly monitoring the operation of the system and if a situation occurs that denotes a fault, the fail-safe mode will activated. As an example, if the system sees a steady throttle position that would be typical of a stretch of steady speed driving, the EML unit will see this a typical. A steady throttle position can also indicate a stuck throttle and if the brake pedal is applied while the EML unit is seeing a steady state throttle position, the fail-safe will activate and reduce power.
If a fault occurs that could affect the engine speed, the EML will reduce the throttle opening to 20 degrees or to idle position. If the throttle plate is sensed not to move, the EML control unit sends a signal to the DME to reduce power by adjusting the fuel injection.
Since this system communicates with so many of the other control units in the vehicle, the complete cannot be tested without the BMW Diagnostic Tester or equivalent. Some basic tests and position adjustments can be made, but complete testing should be left to a qualified BMW technician.
As this system relies strictly on computer controls, with no direct mechanical control of the throttle plates, the integrity of the system is of utmost importance. Any time any work has been performed on the EML system, a check of the safety system incorporated into the system has to be made. This will examine the safety controls of the EML system. This is called the external safety path.
Idle Speed Actuator
See Figure 16
On vehicles with a throttle cable, idle speed is controlled with a motor operated rotary valve. The motor is double wound and can be driven in both directions. Power is supplied from the main DME relay anytime the ignition switch is ON . The control unit selects which of 2 ground circuits to complete to open or close the valve as required to maintain the programmed idle speed. Further idle speed control is accomplished with very fine ignition timing adjustments. This allows very fast idle speed control so that turning on the air conditioner compressor at idle will produce almost no momentary drop in idle speed.
Idle speed and idle CO cannot be adjusted or changed. If the either is incorrect and the valve works properly, look for a vacuum leak, ignition or fuel injection problems.