Crankshaft sensor
Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ADU 7340
Distributor
Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NJC 10034
Direction of rotor arm rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anti-clockwise
Firing order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3-4-2 (No 1 cylinder next to radiator)
Timing mark locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scale on timing cover, notch on pulley
Ignition timing
Nominal value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15º BTDC at idle*
*This value will constantly vary under control of the ECU idle function (by as much as -11º to +10º) - see text for further information.
Ignition coil
Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AUU 1326 or ADU 8779
Primary resistance at 20ºC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.71 to 0.81 ohms
Spark plugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . See Chapter 1 Specifications
Torque wrench settings Nm lbf ft
Crankshaft sensor bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4
Distributor clamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.8 5
5B•4 Ignition system
1 General information
Contact breaker point ignition
system - carburettor models
In order that the engine can run correctly, it
is necessary for an electrical spark to ignite
the fuel/air mixture in the combustion
chamber at exactly the right moment in
relation to engine speed and load. 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 and that all adjustments are
correct.
The ignition system is divided into two
circuits: the low tension circuit and the high
tension circuit.
The low tension (sometimes known as the
primary) circuit consists of the battery, lead to
the starter solenoid, lead to the ignition
switch, lead from the ignition switch to the low
tension or primary coil windings (coil terminal
+), and the lead from the low tension coil
windings (coil terminal -) to the contact
breaker points and condenser in the
distributor.
The high tension circuit consists of the high
tension or secondary coil windings, the
ignition king lead from the centre of the coil to
the centre of the distributor cap, the rotor arm,
the spark plug HT leads and the spark plugs.
The system functions in the following
manner. Low tension voltage is changed in the
coil into high tension voltage by the opening
and closing of the contact breaker points in
the low tension circuit. High tension voltage is
then fed, via the carbon brush in the centre of
the distributor cap, to the rotor arm of the
distributor. The rotor arm revolves inside the
distributor cap, and each time it comes in line
with one of the four metal segments in the cap,
which are connected to the spark plug leads,
the opening and closing of the contact breaker
points causes the high tension voltage to build
up and jump the gap from the rotor arm to the
appropriate metal segment. The voltage then
passes, via the spark plug lead, to the spark
plug, where it finally jumps the spark plug gap,
before going to earth.
A ballasted ignition system is used on
certain models to improve ignition
performance, particularly when starting the
engine. Ballast is provided by a low resistance
lead incorporated in the supply from the
ignition switch to the ignition coil + terminal.
The starter solenoid circuit is wired so that
upon operation of the starter, the ballast
resistance is bypassed. This has the effect of
slightly increasing coil primary voltage, which
in turn temporarily increases HT output to
improve starting.
The ignition is advanced and retarded
automatically, to ensure that the spark occurs
at just the right instant for the particular load
at the prevailing engine speed. The ignition
advance is controlled both mechanically and
by a vacuum operated system on all but
Cooper S models. Automatic advance is
mechanical only on Cooper S models. The
mechanical governor mechanism comprises
two lead weights, which move out from the
distributor shaft, due to centrifugal force, as
the engine speed rises. As they move
outwards they rotate the cam 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 tube, to the carburettor, and
the other side to the contact breaker plate.
Depression in the inlet manifold and
carburettor, which varies with engine speed
and throttle opening, causes the diaphragm to
move, so moving the contact breaker plate,
and advancing or retarding the spark.
Certain models are also fitted with a
transmission-controlled ignition advance
system. The system consists of a vacuum line
connected to the distributor-to-inlet manifold
vacuum hose, which runs to an inhibitor
switch located behind the gearchange remote
control housing. A solenoid valve, operated by
the inhibitor switch, is mounted in this vacuum
line. When fourth gear is selected, the inhibitor
switch energises the solenoid valve, and
increased vacuum is applied to the advance
mechanism of the distributor. When anything
other than fourth gear is selected, the system
is de-energised; vacuum is vented to
atmosphere, the vacuum line to the distributor
is sealed, and the system reverts to normal
operation.
Electronic ignition system -
carburettor models
The Lucas electronic ignition system used
on later 1275 cc engines consists of a
distributor, an amplifier module and a coil.
Externally, the distributor resembles a
conventional type, but internally, a reluctor
and a pick-up unit take the place of the cam
and contact breaker points.
Each time one of the reluctor arms passes
through the magnetic field of the pick-up coil,
an electrical signal is sent to the amplifier
module, which then triggers the coil in the
same way as the opening of the points in a
conventional system. Both centrifugal and
vacuum advances are used in the
accustomed manner.
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