[Erich]

Fuel injectors
The fuel injector is a magnetically operated solenoid valve that is actuated by the ECU. Voltage to the injectors is applied from the main relay and the earth path is completed by the ECU for a period of time (called pulse duration) of between 1.5 and 10 milliseconds. The pulse duration is very much dependant upon engine temperature, load, speed and operating conditions. When the magnetic solenoid closes, a back EMF voltage of up to 60 volts is initiated. The fuel injectors are mounted in the inlet stubs to the engine inlet valves so that a finely atomised fuel spray is directed onto the back of each valve. Since the injectors are pulsed in two banks, fuel will briefly rest upon the back of a valve before being drawn into a cylinder.
Air Flow Sensor (AFS)
The AFS is located between the air filter and the throttle body. As air flows through the sensor it deflects a spring loaded vane (flap). The greater the volume of air, the more will the flap be deflected. The vane is connected to a wiper arm which wipes a potentiometer track and so varies the resistance of the track. This allows a variable voltage signal to be returned to the ECU. The voltage signal varies in proportion to the volume of air that flows through the vane. Three wires are used by the circuitry of this sensor and it is often referred to as a three wire sensor. A 5 volt reference voltage is applied to the resistance track with the other end connected to the AFS earth return circuit. The third wire is connected to the wiper arm.
From the voltage returned, the ECU is able to calculate the volume of air (load) entering the engine and this is used to calculate the main fuel injection duration. To smooth out inlet pulses, a damper is connected to the AFS vane. The AFS exerts a major influence on the amount of fuel injected.
ATS
The ATS is mounted in the AFS inlet tract and measures the air temperature before it enters the inlet manifold. Because the density of air varies in inverse proportion to the temperature, the ATS signal allows more accurate assessment of the volume of air entering the engine. However, the ATS has only a minor correcting effect on ECU output.
The open circuit supply to the sensor is at a 5.0 volt reference level and the earth path is through the AFS earth return circuit. The ATS operates on the NTC principle. A variable voltage signal is returned to the ECU based upon the air temperature. This signal is approximately 2.0 to 3.0 volts at an ambient temperature of 20° C and reduces to about 1.5 volt as the temperature rises to around 40° C.
CO pot
The CO pot mixture adjuster is a three wire potentiometer that allows small changes to be made to the idle CO. A 5.0 volt reference voltage is applied to the sensor and connected to the AFS earth return circuit. The third wire is the CO pot signal. As the CO pot adjustment screw is turned the change in resistance returns a voltage signal to the ECU that will result in a change in CO. The CO pot adjustment only affects idle CO. Datum position is usually 2.50 volts. On catalyst equipped models, the CO pot has no effect and the CO is thus non-adjustable.
CTS
The CTS is immersed in the coolant system and contains a variable resistance that operates on the NTC principle. When the engine is cold, the resistance is quite high. Once the engine is started and begins to warm-up, the coolant becomes hotter and this causes a change in the CTS resistance. As the CTS becomes hotter, the resistance of the CTS reduces (NTC principle) and this returns a variable voltage signal to the ECU based upon the coolant temperature. The open circuit supply to the sensor is at a 5.0 volt reference level and this voltage reduces to a value that depends upon the resistance of the CTS resistance. Normal operating temperature is usually from 80° to 100° C.
The ECU uses the CTS signal as a main correction factor when calculating ignition timing and injection duration.
Throttle switch
A throttle switch with dual contacts is provided to inform the ECU of idle position, deceleration, cruising and full-load (WOT) conditions. When the engine is at idle the idle contact is closed and the full-load contact is open. As the throttle is moved to the fully open position, the full-load contact closes and the idle contact becomes open. Under cruising conditions with a part-open throttle, both contacts are open. During full-load operation, the ECU provides additional enrichment. During closed throttle operation above a certain rpm (deceleration), the ECU will cut-off fuel injection. Injection will be reintroduced once the rpm returns to idle or the throttle is opened.
Electronic throttle control
Some models are equipped with Electronic throttle control, the so-called 'drive by wire' system whereby the mechanical control using a throttle cable is discontinued. A TS is not used on vehicles equipped with Electronic throttle control and the position of the throttle valve is signalled to the ECU electronically.
ISCV (2 wire type)
The ISCV is a solenoid controlled actuator that the Motronic ECU uses to automatically control idle speed during normal idle and during engine warm-up. Motronic detects the engine idle situation from the position of the TS or TPS. The ISCV is located in a hose that connects the inlet manifold to the air filter side of the throttle plate. A voltage supply is applied from the main relay and the ISCV earth is actuated by the ECU according to load.
When an electrical load, such as headlights, A/C or heater fan etc are switched on, the idle speed would tend to drop. The ECU will sense the load and rotate the ISCV against spring tension to increase the air flow through the valve and thus increase the idle speed. When the load is removed, the ECU will pulse the valve so that the air flow is reduced. Normal idle speed should be maintained under all cold and hot operating conditions. If the ISCV fails it will fail in a fail-safe position with the aperture almost closed. This will provide a basic idle speed.
ISCV (3 wire type)
The ISCV is a solenoid controlled actuator that the Motronic ECU uses to automatically control idle speed during normal idle and during engine warm-up. The ISCV is located in a hose that connects the inlet manifold to the air filter side of the throttle plate. Unlike the two pin ISCV which is controlled by a separate Idle speed ECU, the 3 pin ISCV is controlled by the Motronic ECU via ECU pins 33 and 34. The ISCV is a DC motor that the ECU can rotate either clockwise or anti-clockwise. Rotating in one direction will open the valve and rotating in the opposite direction will cause it to close. A voltage supply is applied to the ISCV from the battery and the earth for the motor is made through two connections to the ECU.
Rotation of the motor in the appropriate direction is accomplished by actuating the motor through one or the other of the earth circuits. In reality the two circuits are opposed. This prevents the valve from being fully opened of closed in one particular direction. The valve will thus take up an average position that reflects circuit bias to be open or closed. Normally, this bias would be towards the open position. A duty cycle can be measured on each earth circuit to determine the opening or closing time period as a percentage of the total time available. When an electrical load, such as headlights or heater fan etc are switched on, the idle speed would tend to drop. The idle ECU will sense the load and rotate the ISCV to increase the air flow through the valve and thus increase the idle speed. When the load is removed, the ECU will pulse the valve so that the air flow is reduced. Normal idle speed should be maintained under all cold and hot operating conditions. If the ISCV fails it will fail in a fail-safe position with the aperture almost closed. This will provide a basic idle speed.
Relays
The Motronic electrical system is controlled by a main fuel injection relay and a fuel pump relay. A permanent voltage supply is made to the main relay terminals 30 and 86 from the battery positive terminal. When the ignition is switched on, the ECU earths terminal 85 through ECU terminal number 36 which energises the first relay winding (on the 1987 M1.1 system, this connection was made directly to earth). This causes the first relay contacts to close and terminal 30 is connected to the output circuit at terminal 87. A voltage supply is thus output at terminal 87. Terminal 87 supplies voltage to the injectors, ECU terminal 37, ISCV and the CFSV when fitted. In addition voltage is supplied to the fuel pump relay terminal 86. When the ignition is switched on. the ECU briefly earths fuel pump relay contact 85 at ECU terminal 3. This energises the relay winding, which closes the relay contact and connects voltage from terminal 30 to terminal 87, thereby providing voltage to the fuel pump circuit. After approximately one second, the ECU opens the circuit and the pump stops. This brief running of the fuel pump allows pressure to build within the fuel pressure lines, and provides for an easier start.
The fuel pump relay circuit will then remain open until the engine is cranked or run. Once the ECU receives a speed signal from the CAS, the relay winding will again be energised by the ECU, and the fuel pump will run until 3 seconds after the engine is stopped. The 3 seconds delay in switching off the pump relay allows the fuel pump to maintain pressure on engine shut-off to avoid engine run-on.
In addition, on some vehicles control of the OS heater is made through a separate OS relay. Once the ignition is switched on, a voltage supply is made to the OS heater relay terminals 30 and 86. When the engine is started, the ECU connects relay terminal 85 to earth through ECU pin 23. The relay actuates and the output voltage at terminal 87 provides voltage to the OS heater. The ECU switches off the relay under certain conditions of speed and load.
Fuel pressure system
From build date 3/1988 the BMW vehicles in the Motronic 1.3 range were mounted internally in the fuel tank. Prior to that date the pump was mounted externally and in some instances an additional internal transfer pump was fitted to aid the pumping of fuel from the fuel tank.
internal pump operation (where fitted)
The fuel pump assembly comprises an outer and inner gear assembly termed a gerotor. Once the pump motor becomes energised, the gerotor rotates and as the fuel passes through the individual teeth of the gerotor, a pressure differential is created. Fuel is drawn through the pump inlet, to be pressurised between the rotating gerotor teeth and discharged from the pump outlet into the fuel supply line. When both an internal and external pump are fitted, the internal pump is ancillary to the main external pump and is provided to aid the pumping of fuel from the fuel tank.
external pump operation (where fitted)
A roller type fuel pump, driven by a permanent magnet electric motor mounted close to the fuel tank draws fuel from the tank and pumps it to the fuel rail via a fuel filter. The pump is of the 'wet' variety in that fuel actually flows through the pump and the electric motor. There is no actual fire risk because the fuel drawn through the pump is not in a combustible condition.
Mounted upon the armature shaft is an eccentric rotor holding a number of pockets arranged around the circumference - each pocket containing a metal roller. As the pump is actuated, the rollers are flung outwards by centrifugal force to act as seals. The fuel between the rollers is forced to the pump pressure outlet.
Fuel pressure in the fuel rail is maintained at a constant 2.5 bar by a fuel pressure regulator. The fuel pump normally provides much more fuel than is required, and surplus fuel is thus returned to the fuel tank via a return pipe. In fact, a maximum fuel pressure in excess of 5 bar is possible in this system. To prevent pressure loss in the supply system, a non-return valve is provided in the fuel pump outlet. When the ignition is switched off, and the fuel pump ceases operation, pressure is thus maintained for some time.
Fuel pressure regulator
The pressure regulator is fitted on the outlet side of the fuel rail and maintains an even pressure of 2.5 bar in the fuel rail. The pressure regulator consists of two chambers separated by a diaphragm. The upper chamber contains a spring which exerts pressure upon the lower chamber and closes off the outlet diaphragm. Pressurised fuel flows into the lower chamber and this exerts pressure upon the diaphragm. Once the pressure exceeds 2.5 bar, the outlet diaphragm is opened and excess fuel flows
back to the fuel tank via a return line.
A vacuum hose connects the upper chamber to the inlet manifold so that variations in inlet manifold pressure will not affect the amount of fuel injected. This means that the pressure in the rail is always at a constant pressure above the pressure in the inlet manifold. The quantity of injected fuel thus depends solely on injector opening time, as determined by the ECU, and not on a variable fuel pressure.
At idle speed with the vacuum pipe disconnected, or with the engine stopped and the pump running, or at WOT the system fuel pressure will be approximately 2.5 bar. At idle speed (vacuum pipe connected), the fuel pressure will be approximately 0.5 bar under the system pressure.
Catalytic Converter and emission control
The Motronic injection system fitted to BMW vehicles equipped with a Catalytic Converter implements a closed loop control system so that exhaust emissions may be reduced. Closed loop systems are equipped with an oxygen sensor which monitors the exhaust gas for oxygen content. A low oxygen level in the exhaust signifies a rich mixture. A high oxygen level in the exhaust signifies a weak mixture. The Oxygen Sensor closed loop voltage is quite low and switches between 100 mVolts (weak) to 1.0
volt (rich).
The signal actually takes the form of a switch and switches from weak to rich at the rate of approximately 1 HZ. A digital voltmeter connected to the signal wire, would display.an average voltage of approximately 0.45 volts. In the event of OS circuit failure, the ECU substitutes a constant voltage of 0.45 volts and this should not be confused with the average voltage of 0.45 which occurs during switching from approximately 1.0 volt to 0.1 volt. When the engine is operating under closed loop control, the OS signal causes the ECU to modify the injector pulse so that the AFR is maintained close to the stoichiometric ratio. By controlling the injection pulse, during most operating conditions, so that the air/ fuel ratio is always in a small window around the Lambda point (ie Lambda = 0.98 to 1.04), almost perfect combustion is achieved. Thus the Catalyst has less work to do and it will last longer with fewer emissions at the tail pipe. The closed loop control is implemented during engine operation at engine normal operating temperature. When the coolant temperatures is below 70° C, or the engine is at full load or is on the overrun the ECU will operate in open loop. When operating
in open loop, the ECU allows a richer or leaner AFR than the stoichiometric ratio. This prevents engine hesitation, for example, during acceleration with a wide open throttle.
The OS only produces a signal when the exhaust gas, has reached a minimum temperature of approximately 300° C. In order that the OS will reach optimum operating temperature as quickly as possible after the engine has started, the OS contains a heating element. The OS heater is controlled by the ECU through an OS relay or from the fuel pump relay depending on vehicle.
The ECU switches off the OS relay under certain conditions of speed and load.
CFSV
A CFSV and activated carbon canister is employed to aid evaporative emission control. The carbon canister stores fuel vapours until the CFSV is opened by Motronic under certain operating conditions. Once the CFSV is actuated by the EMS, fuel vapours are drawn into the inlet manifold to be burnt by the engine during normal combustion.