2 Controlfunctions
Rover MEMS
- MPi/SPi 14-3
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!.:gnal processing
The MEMS ECM is designed with three
main areas of control. These are the ignition,
fuel system and idle speed. The correct
ignition dwell and timing for all engine
operating conditions are calculated from data
provided by the CAS (crankshaft position and
speed), and the MAP sensor (engine load).
Basic ignition timing is stored in a three-
dimensional map, and the engine load and
speed signals determine the ignition timing.
The main engine load sensor is the MAP
sensor, and engine speed is determined from
the CAS signal.
Correction factors are then applied for
starting, idle, deceleration, and part- and full-
load operation. The main correction factor is
engine temperature (CTS). Minor corrections
to timing and AFR are made with reference to
the air temperature sensor (ATS) and throttle
potentiometer sensor (TPS)signals.
The basic AFR is also stored in a three-
dimensional map, and the engine load and
speed signals determine the basic injection
pulse value. Using the speed/density method,
MEMS calculates the AFR from the pressure
in the inlet manifold (MAP) and the speed of
the engine (CAS).
This method relies on the theory that the
engine will draw in a fixed volume of air per
revolution. The AFR and the pulse duration
are then corrected on reference to ATS, CTS,
battery voltage and rate of throttle opening
(TPS). Other controlling factors are
determined by operating conditions such as
cold start and warm-up, idle condition,
acceleration and deceleration. During
acceleration, additional injection pulses are
provided at 80° crankshaft intervals.
MEMS accesses a different map for idle
running conditions, and this map is
implemented whenever the idle switch is
closed and the engine speed is at idle. Idle
speed during all warm-up and normal hot
running conditions is maintained by the idle
speed stepper motor. However, MEMS makes
small adjustments to the idle speed by
advancing or retarding the timing, and this
results in an ignition timing that is forever
changing during engine idle.
Basic ECMoperation
Oncethe ignition is switched on, a voltage
supply to ECM pin 11 is made from the
ignition switch. This causes the ECM to
connect pin 4 to earth, so actuating the main
fuel injection relay. A relay switched voltage
supply is thus made to ECM pin 28, from
terminal 87 of the main fuel injection relay.
Depending on model, the coil is supplied with
voltage from either the main relay or from the
ignition switch direct.
The majority of sensors (other than those
that generate a voltage such the CAS, KS and
CID sensor), are now provided with a 5.0-volt
reference supply from a relevant pin on the
ECM. When the engine is cranked or run, a
speed signal from the CAS causes the ECM to
earth pin 20 so that the fuel pump will run.
Ignition and injection functions are also
activated. All actuators (Injectors, ISCV, FTVV
etc), are supplied with nbv from the main
relay, and the ECM completes the circuit by
pulsing the relevant actuator wire to earth.
Self-diagnostic function
MEMS provides a serial port for diagnostic
and system tuning purposes. The port allows
two-way communication, so that certain
parameters can be changed (ie CO value) and
actuation of various output components.
In addition, a self-test capability regularly
examines signals from the engine sensors, and
internally logs a code in the event of a fault
being present. This code can be extracted
from the MEMS serial port by a suitable FCR.If
the fault clears, the code will remain logged
until the FCR is used to erase it from memory.
LOS (limp-home mode)
MEMS has a limited operating strategy
(LOS)or limp-home facility, and in the event of
a serious fault in one or more of the sensors,
the EMS will substitute a fixed default value in
place of the defective sensor.
For example, in limp-home mode the
coolant temperature sensor (CTS)value is set
to 60°C, the ATS is set to 35°C, and engine
load is based on rpm. The engine may
actually run quite well with failure of one or
more minor sensors. However, since the
substituted values are those of a hot engine,
cold starting and running during the warm-up
period are likely to be less than satisfactory.
Also, failure of a major sensor, ie the MAP
sensor, will lead to a considerable reduction in
performance.
Adaptive and
non-volatile memory
Over a period of time, the ECM will learn the
best idle position for a particular engine -
irrespective of age, engine condition and load,
so that the correct idle speed is always
maintained. The adaptive idle settings are
stored in non-volatile memory. Consequently,
a replacement ECM will need some time to re-
learn the system parameters before proper
idle control is restored. A tune-up with a
suitable FCR is recommended whenever a
new ECM is fitted.
Faults identified by the self-diagnostic
function will also be stored in non-volatile
memory, and will remainthere untilerased by a
suitable FCR. This allows the self-diagnostic
function to retain data of an intermittent nature.
Adaptive idle measurements and fault
codes retained in non-volatile memory cannot
be lost - even if the vehicle battery is
removed. If the ECM from one vehicle is
transferred to another vehicle, the contents of
non-volatile memory will also be transferred,
unless a FCR is used to erase the codes and
tune the engine to the new set-up.
Reference voltage
Voltagesupply from the ECM to the engine
sensors is made at a 5.0-volt reference level.
This ensures a stable working voltage,
unaffected by variations in system voltage.
The earth return connection for most engine
sensors is made through ECM pin number 30,
and this pin is not directly connected to earth.
The ECM internally connects pin number 30 to
earth via the ECM earth pin that is directly
connected to earth.
Signal shielding
To reduce interference (RFI), a number of
sensors (eg the crank angle sensor, knock
sensor and oxygen sensor) use a shielded
cable. The shielded cable is connected to the
main ECM earth wire at terminal 29 to reduce
interference to a minimum.
3 Primarytrigger
Crank angle sensor (CAS)
The primary signal to initiate both ignition
and fuelling emanates from a CAS mounted
next to the flywheel. The CAS consists of an
inductive magnet that radiates a magnetic field.
The flywheel incorporates a reluctor disk
containing 34 steel pins set at 10° intervals.As
the flywheel spins, and the pins are rotated in
the magnetic field, an AC voltage signal is
generated to indicate speed of rotation. The
two missing pins (set at 180° intervals) are a
reference to TDC, and indicate crankshaft
position by varying the signal as the flywheel
spins. One missing pin indicates TDC for
cylinders 1 and 4, and the other missing pin
indicates TDC for cylinders 2 and 3.
The peak-to-peak voltage of the speed signal
can vary from 5 volts at idle to over 100 volts at
6000 rpm. The ECM microprocessor contains
an analogue-to-digital converter to transform
the AC pulse into a digital signal.
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4 Ignition
14
Data on engine load (MAP) and engine
speed (CAS)are collected by the ECM, which
then refers to a three-dimensional digital
ignition map stored within its microprocessor.
This map contains an advance angle for basic
load and speed operating conditions. The
advance angle is corrected after reference to
engine temperature (CTS), so that the best
ignition advance angle for a particular
operating condition can be determined.
