1993 FORD SIERRA change time

General information
The electrical system is of the 12 volt
negative earth type, and consists of a 12 volt
battery, alternator with integral voltage
regulator, starter motor and related electrical
accessories, components and wiring. The
battery is of the low maintenance or
maintenance-free “sealed for life” type and is
charged by an alternator which is belt-driven
from the crankshaft pulley. The starter motor
is of the pre-engaged type, incorporating an
integral solenoid. On starting the solenoid
moves the drive pinion into engagement with
the flywheel ring gear before the starter motor
is energised. Once the engine has started, a
one-way clutch prevents the motor armature
being driven by the engine until the pinion
disengages from the flywheel.
The ignition system is responsible for
igniting the air/fuel mixture in each cylinder at
the correct moment in relation to engine
speed and load. A number of different ignition
systems are fitted to models within the
Sierra/P100 range, ranging from a basic
breakerless electronic system to a fully
integrated engine management system
controlling ignition and fuel injection systems.
The ignition system is based on feeding low
tension voltage from the battery to the coil
where it is converted to high tension voltage.
The high tension voltage is powerful enough
to jump the spark plug gap in the cylinders
many times a second under high compression
pressures, providing that the system is in
good condition. The low tension (or primary)
circuit consists of the battery, the lead to the
ignition switch, the lead from the ignition
switch to the low tension coil windings
(terminal + /15) and also to the supply terminal
on the electronic module, and the lead from
the low tension coil windings (terminal -/1) to
the control terminal on the electronic module.
The high tension (or secondary) circuit
consists of the high tension coil windings, the
HT (high tension) lead from the coil to the
distributor cap, the rotor arm, the HT leads to
the spark plugs, and the spark plugs.
The system functions in the following
manner. Current flowing through the low
tension coil windings produces a magnetic
field around the high tension windings. As the
engine rotates, a sensor produces an
electrical impulse which is amplified in the
electronic module and used to switch off the
low tension circuit.
The subsequent collapse of the magnetic
field over the high tension windings produces
high tension voltage which is then fed to the
relevant spark plug via the distributor cap and
rotor arm. The low tension circuit is
automatically switched on again by the
electronic module, to allow the magnetic field
to build up again before the firing of the next
spark plug. The ignition is advanced and
retarded automatically to ensure that thespark occurs at the correct instant in relation
to the engine speed and load.
To improve driveability during warm-up
conditions and to reduce exhaust emission
levels, a vacuum-operated,
temperature-sensitive spark control system is
fitted to certain vehicles.Inductive discharge system
This is the least sophisticated system fitted
to the Sierra/P100 range, and comprises a
breakerless distributor and an electronic
switching/amplifier module in addition to the
coil and spark plugs.
The electrical impulse which is required to
switch off the low tension circuit is generated
by a magnetic trigger coil in the distributor. A
trigger wheel rotates within a magnetic stator,
the magnetic field being provided by a
permanent magnet. The magnetic field across
the two poles (stator arm and trigger wheel) is
dependent on the air gap between the two
poles. When the air gap is at its minimum, the
trigger wheel arm is directly opposite the
stator arm, and this is the trigger point. As the
magnetic flux between the stator arm and
trigger wheel varies, a voltage is induced in
the trigger coil mounted below the trigger
wheel, and this voltage is sensed and then
amplified by the electronic module and used
to switch off the low tension circuit. There is
one trigger wheel arm and one stator arm for
each cylinder (4).
The ignition advance is a function of the
distributor and is controlled both mechanically
and by a vacuum operated system. The
mechanical governor mechanism consists of
two weights which move out from the
distributor shaft as the engine speed rises due
to centrifugal force. As they move outwards,
they rotate the trigger wheel relative to the
distributor shaft and so advance the spark.
The weights are held in position by two light
springs and it is the tension of the springs
which is largely responsible for correct spark
advancement.
The vacuum control consists of a
diaphragm, one side of which is connected
via a small bore hose to the carburettor or
inlet manifold and the other side to the
distributor. Depression in the inlet manifold
and/or carburettor, which varies with engine
speed and throttle position, causes the
diaphragm to move, so moving the baseplate
and advancing or retarding the spark. A fine
degree of control is achieved by a spring in
the diaphragm assembly.
ESC (Electronic Spark Control) system
This system is only fitted to early
“Economy” models, and comprises a “Hall
effect” distributor, and an ESC module, in
addition to the coil and spark plugs.
The electrical impulse which is required to
switch off the low tension circuit is generated
by a sensor in the distributor. A trigger vane
rotates in the gap between a permanent
magnet and the sensor. The trigger vane has
four cut-outs, one for each cylinder. When
one of the trigger vane cut-outs is in line with
the sensor, magnetic flux can pass betweenthe magnet and the sensor. When a trigger
vane segment is in line with the sensor, the
magnetic flux is diverted through the trigger
vane away from the sensor. The sensor
senses the change in magnetic flux and sends
an impulse to the ESC module, which
switches off the low tension circuit.
The ignition advance is a function of the
ESC module and is controlled by vacuum. The
module is connected to the inlet manifold by a
vacuum pipe, and a transducer in the module
translates the vacuum signal into electrical
voltage. From the vacuum signal, the ESC
module determines engine load, and engine
speed is determined from the interval
between impulses supplied by the distributor
sensor. The module has a range of spark
advance settings stored in its memory, and a
suitable setting is selected for the relevant
engine speed and load. The degree of
advance can thus be constantly varied to suit
the prevailing engine speed and load
conditions.
ESC II (Electronic Spark Control II)
system
1.8 and 2.0 litre SOHC carburettor models
This system is a development of the ESC
system described previously in this Section,
but it enables more accurate control of engine
operation due to the inclusion of additional
monitoring features and control outputs.
Vehicles fitted with the ESC II system have an
electric inlet manifold heater which warms the
air/fuel mixture when the engine is cold, thus
reducing the amount of fuel enrichment
required, lowering fuel consumption and
improving driveability when the engine is cold.
The heater is operated by the ESC II module
receiving information on the engine temperature
from an engine coolant temperature sensor
mounted in the inlet manifold.
On 2.0 litre SOHC models, the ESC II
module operates a carburettor stepper motor
to control the engine idle speed. Using
information on engine speed, load,
temperature and throttle position (supplied by
a switch on the carburettor), the module
operates the stepper motor to maintain a
constant idle speed. On models equipped
with automatic transmission and/or air
conditioning, additional inputs are supplied to
the module to allow it to operate the stepper
motor to compensate for the additional engine
load imposed by the automatic
transmission/air conditioning. The ESC II
module also operates a “power hold” relay
which allows the stepper motor to function
briefly after the ignition has been switched off
in order to perform an anti-run-on and
manifold ventilation cycle.
2.0 litre DOHC carburettor models
A development of the ESC II system is used
to control the operation of the engine. The
module receives information from a
crankshaft speed/position sensor (similar to
that described for the ESC Hybrid system),
except that the sensor is activated by a
toothed disc on the rear of the crankshaft,
inside the cylinder block), and an engine
coolant temperature sensor.
1General information and
precautions
Engine electrical systems 5•3
5