8 With no reading from the moving point and
earth, test between the coil negative terminal
(marked CB or -) and earth. A reading here
indicates a broken wire between the coil and
distributor which must be repaired. No reading
confirms that the coil has failed and must be
renewed. If the coil is to be renewed, ensure
that the correct type is obtained. The coils
used on ballasted and non-ballasted systems
are not interchangeable (see Section 2 for
further details).
9 Where a ballasted ignition system is fitted, if
the engine starts when the starter is operated,
buts stops as soon as the ignition key is
returned to the normal running position, the
resistive wire may be open circuit. Connect a
temporary lead between the coil positive (SW
or +) terminal and the battery positive terminal.
If the engine now runs correctly, renew the
resistive wire. Note: Bypassing the resistive
wire in this way must only be done briefly for
the purpose of this test. The wire must not be
permanently bypassed, otherwise the coil will
overheat and be irreparably damaged.
Engine misfires
10 Uneven running and misfiring should first
be checked by seeing that all leads,
particularly HT, are dry and connected
properly. Ensure that they are not shorting to
earth through broken or cracked insulation. If
they are you should be able to see and hear it.
If not, then check the plugs, contact breaker
points and condenser just as you would in a
case of total failure to start. A regular misfire
can be isolated by removing each HT lead
from its spark plug in turn, (taking precautions
against electric shock) while the engine is
idling. Removing a good lead will accentuate
the misfire, whilst removing the defective lead
will make no difference.
11 Once the defective cylinder is located,
switch off the engine, remove the lead from
the spark plug again and hold the end about 5
mm away from the cylinder block. If the plug
caps are not detachable, insert a nail or piece
of stiff wire into the cap. Restart the engine. If
the sparking is fairly strong and regular, the
fault must lie in the spark plug. Remove and
examine its condition as described in Chap-
ter 1. If the spark plug is proved to be
satisfactory, there may be an internal engine
problem such as loss of compression on the
defective cylinder. Carry out a compression
test, as described in Chapter 2A to determine
the condition of the engine internally.
12 If there is no spark at the end of the HT
lead, or if it is weak and intermittent, check
the entire length of the lead from the
distributor cap to the spark plug. If the
insulation is cracked or perished, renew the
lead. Check the connections at the distributor
cap at the same time.
13 If there is still no spark, carefully examine
the condition of the distributor cap as
described in Chapter 1, looking closely for
signs of cracks or tracking between the
internal segments.
14 Apart from the ignition timing being
incorrect, other causes of misfiring have
already been dealt with under the sub-Section
dealing with the failure of the engine to start.
To recap, these are:
a) The coil may be faulty giving an
intermittent misfire.
b) There may be a damaged wire or loose
connection in the low tension circuit.
c) The condenser may be short-circuited.
d) There may be a mechanical fault in the
distributor (incorrectly assembled contact
breaker points or broken contact breaker
spring).
15 If the ignition timing is too far retarded it
should be noted that the engine will tend to
overheat and there will be a quite noticeable
drop in power. If the engine is overheating and
the power is down, and if the ignition timing is
correct, then the carburettor should be checked
as it is likely that this is where the fault lies.
Electronic ignition system
Note: Refer to the warning given in Section 1 of
Part A of this Chapter before starting work.
Always switch off the ignition before
disconnecting or connecting any component and
when using a multi-meter to check resistances.
Warning: Voltages produced by
an electronic ignition system are
considerably higher than those
produced by conventional
ignition systems. Extreme care must be
taken when working on the system with the
ignition switched on. Persons with
surgically-implanted cardiac pacemaker
devices should keep well clear of the ignition
circuits, components and test equipment.
General
16 Electronic ignition is normally very
reliable; if it does fail, such failure tends to be
complete. If a fault does occur, or if
performance is suspect, the following test
procedure should be used to isolate the
problem area.
17 An electrical multi-meter which can
measure voltage and resistance (ohms) will be
required for testing purposes.
Electronic ignition system test
procedure
Refer to the accompanying table
18 If, after carrying out the test procedures
the ignition amplifier module is diagnosed as
being faulty, make sure that the wiring is
intact and secure.
5B•6 Ignition system
Test Remedy
1 Is the battery voltage greater than 11.7 volts? Yes: Proceed to Test 2
No: Recharge the battery
2 Is the voltage at the coil “+” terminal within
1 volt of battery voltage? Yes: Proceed to Test 3
No: Faulty wiring or connector between
ignition switch and coil, or faulty ignition
switch
3 Is the resistance between the ignition coil “+”
and “-” terminals between 0.4 and 0.9 ohms? Yes: Proceed to Test 4
No: Renew the ignition coil
4 Is the resistance between the ignition coil “+”
and HT terminals between 5.0 and 15.0 kohms? Yes: Proceed to Test 5
No: Renew the ignition coil
5 Connect a low-wattage bulb across the ignition
coil “+” and “-” terminals, and spin the engine
on the starter. Does the bulb flash? Yes: Proceed to Test 6
No: Proceed to Test 10
6 Is the resistance of any HT lead greater than
20 kohms? Yes: Renew the HT lead
No: Proceed to Test 7
7 Are there any signs of tracking on the ignition
coil, distributor cap or rotor arm? Yes: Renew the component as necessary
No: Proceed to Test 8
8 Is the ignition timing correct? Yes: Proceed to Test 9
No: Adjust ignition timing
9 Are the spark plugs in good condition? Yes: Check carburettor settings and engine
mechanical condition
No: Renew the spark plugs
10 Are the ignition amplifier module connections
sound? Yes: Proceed to Test 11
No: See paragraphs 18 to 21 of this
Section
11 With the module removed, is the resistance
of the distributor pick-up coil between 950
and 1150 ohms? Yes: See paragraphs 18 to 21 of this Section
No: Renew the distributor pick-up coil
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