MEMS is controlled by the MEMS control unit (ECM) which is located in the RH front corner of the engine compartment or on the RH inner wing valance, according to model.
The ECM is an adaptive unit, which means that, over a period of time, it can 'learn' the load and wear characteristics of the engine. Because no two engines have identical characteristics, this information is needed by the ECM to determine the amount of stepper motor movement required to achieve the specified idle speed.
The features of MEMS are:
MEMS incorporates a programmed ignition system, the timing being controlled using digital techniques instead of the conventional mechanical and vacuum advance mechanisms.
The ECM determines the correct ignition timing by receiving signals from the following:
Timing is controlled by the ECM which is energised by the main relay, located within the relay module. Spark distribution is achieved by a direct ignition system which consists of an ignition coil driven directly by the ECM. The twin-ignition coil is mounted on the front of the engine and has a primary winding resistance of 0.63 to 0.77 ohms at 20°C, which allows full h.t. output to be attained sooner thus making coil operation more consistent throughout the engine speed range.
The ignition sense of the ECM receives battery voltage from fuse A4 in the passenger compartment fuse box on a W wire provided the starter switch is in position II. As a result the ECM supplies an earth from the main relay control on a WK wire to one side of the main relay coil (within the MEMS relay module). Since the other side of the main relay receives battery voltage on an N wire from link 4 in the engine compartment fuse box, the relay will energise switching battery voltage via the relay contacts on an NK wire to the injector(s), purge valve, stepper motor, ignition coil, manifold heater (Japan only) and positive feed to the ECM.
The speed and position of the engine are detected by the crankshaft position (CKP) sensor which projects through the engine adapter plate. A critical air gap exists between the CKP sensor and the flywheel, which is essential to allow correct engine operation.
The flywheel incorporates a reluctor ring which consists of 32 poles spaced at 10° intervals, with 4 missing poles at 30° , 60° , 210° and 250° . The missing poles inform the ECM when to operate the injectors, with the remaining poles providing a continual update of crankshaft position and engine speed. As the flywheel rotates, each pole that passes the CKP sensor disturbs the magnetic field created by the sensor inducing a voltage pulse in the coil. The CKP sensor is monitored by the ECM on UP and WU wires.
The camshaft position sensor has two functions; the first is to enable the ECM to run a sequential fuelling mode; the second is to measure the actual cam period, this measurement is achieved using teeth on the camshaft to indicate when the valve opens and closes. The CMP sensor is monitored by the ECM on BU and RY wires.
The manifold absolute pressure sensor is mounted directly on the inlet manifold and provides the ECM with an accurate representation of the load placed on the engine. This allows the ECM to adjust the quantity of fuel being injected together with the ignition timing, to achieve optimum fuelling of the engine. The MAP sensor achieves this by detecting pressure variations inside the manifold, then converting these variations into graduated electrical signals which are monitored by the ECM on RG and YP wires to determine engine load. Earth supply is on a KB wire.
The engine coolant temperature (ECT) sensor is located on the coolant outlet elbow. It is a temperature dependent resistor (thermistor), the voltage output of which varies in inverse proportion to temperature, in that the output increases as temperature decreases or the reverse. The change in resistance is monitored by the ECM on a KG wire and as a result the ECM can adjust the length of injector opening time required. The ECM supplies the coolant temperature sensor with an earth path on a KB wire.
One side of the manifold heater relay coil receives battery voltage from link 4 in the engine compartment fuse box via the closed contacts of the main relay (providing it is energised). The other side of the relay coil receives a control earth on a BK wire from the link 4 in the engine compartment fuse box via the closed contacts of the relay to the manifold heater.
The throttle position (TP) sensor is a potentiometer attached to the throttle housing and is directly coupled to the throttle disc. Closed throttle is detected by the TP sensor which initiates idle speed control via the idle air control valve (IACV).
The TP sensor receives a 5 volt supply from the ECM on a YP wire and is supplied with an earth on a KB wire. The sensor then provides a signal which is proportional to throttle disc position on a YG wire to the ECM.
With the throttle pedal released and the engine at idle, the ECM maintains stable idling performance by using the fast response of the engine to changes in ignition timing. According to the loads placed on or removed from the engine, the ECM responds to changes in engine speed and, together with adjustments to the idle air control valve (IACV), advances or retards the ignition timing to achieve a constant idling speed. When load is removed from the engine, the IACV returns to its original position and the ignition timing reverts to the idle setting.
NOTE: Due to the sensitivity of this system the ignition timing will be constantly changing at idle speed.
MEMS injection system incorporates one (on SPI version) or two (on MPI version) injectors which are located between the pressurised fuel rail and the inlet manifold. The injectors are solenoid operated and direct a spray of fuel into the inlet manifold onto the back of the inlet valves.
The amount of fuel injected is determined by how long the injector is held open (known as the injector pulse width). To achieve the required air fuel ratio the ECM receives signals from the following inputs:
The idle air control valve (IACV) is mounted on the inlet manifold. It receives battery voltage on an NK wire via the main contacts and is controlled by the ECM on OS (phase 1), KU (phase 2), OG (phase 3), and OU (phase 4) wires and receives an earth supply on a KB wire. During cold starting, the ECM provides a fast idle by sending a signal to the IACV which opens a pintle valve situated inside an air passage within the throttle housing. This allows air to bypass the throttle disc and flow directly into the inlet manifold. As the engine coolant temperature rises the fast idle is reduced until normal idle speed is attained.
The intake air temperature (IAT) sensor is located on the side of the inlet manifold. The IAT sensor is of the negative temperature coefficient (NTC) type, reducing its resistance with increases in air temperature. The ECM monitors the IAT sensor on a GB wire and is supplied with an earth on a B wire. When the ECM receives a signal from the sensor, it uses the signal along with that from the MAP sensor to calculate the volume of oxygen in the air and carry out fine adjustments of the injected fuel to attain the optimum mixture strength.
The heated oxygen sensor (HO2S) is part of a closed loop-type exhaust emission system. The sensor is fitted in the exhaust manifold and is designed to monitor the exhaust gases. In weak air/fuel mixtures, oxygen content in the exhaust gas increases, decreasing the voltage output to the ECM. As the air/fuel mixture becomes richer so oxygen content decreases, increasing the voltage output to the ECM.
One side of the oxygen sensor relay coil receives an earth from the ECM on a BG wire providing the starter switch is in position II. The other side of the relay coil receives battery voltage from link 4 in the engine compartment fuse box on an N wire via the contacts of the main relay (providing it is energised). As a result the oxygen sensor relay energises and battery voltage is switched from link 4 in the engine compartment fuse box on an N wire via the closed contacts of the oxygen sensor relay to the oxygen sensor on a UR wire.
Since an earth is provided on a B wire, the integral heating element will quickly reach an efficient operating temperature from cold.
The resultant output voltage on the LGS and S wires is used by the ECM to determine what correction to fuel delivery is necessary.
CAUTION: An oxygen sensor will not operate if its power supply is removed, if it has been dropped, subjected to any impact or if cleaning materials are used on it.
The purge valve receives battery voltage on an NK wire from link 4 in the engine compartment fuse box on an N wire from the closed contacts of the main relay (providing it is energised) and a control signal from the ECM on a BW wire. The valve remains closed when the engine is cold and at idling speed to protect engine tune and catalyst performance. When the purge valve is open, fuel vapour from the charcoal canister is drawn into the throttle housing for combustion.
The switch fitted to the gearbox inhibits operation of the starter motor unless the selector lever is in position P or N.
With the selector lever in position P or N the inhibitor switch will provide an earth path for the starter relay on a BLG wire which will allow the engine to be started.
If the selector lever is in any other position than P or N, the inhibitor switch will not provide an earth path, so preventing the vehicle starting.
The inhibitor switch also provides the ECM with an input so that idle speed is automatically adjusted when a drive position is selected.
One side of the fuel pump relay coil receives battery voltage from fuse A4 in the passenger compartment fuse box on a W wire provided the starter switch is in position II. The other side of the relay coil receives an earth on a BP wire from the MEMS control unit provided the correct operating conditions exist, (see Engine Management System) causing the relay to energise.
The energised relay switches battery voltage from link 4 in the engine compartment fuse box on an N wire, via fuse C7 in the passenger compartment fuse box to the inertia fuel shutoff switch on an NS wire. Provided the inertia fuel shutoff switch remains closed, battery voltage is fed on a WP wire to the fuel pump. Since the pump has a permanent earth on a B wire it will commence operation.