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HOW THE STARTING SYSTEM WORKS
To make an engine start, it must be turned at some speed so that it draws fuel and
air into the cylinders and compresses it. The powerful electric starter motor does
the turning. Its shaft carries a small ,pinion (gear wheel) which engages with a
large gear ring around the rim of the engine flywheel. In a front-engine layout,
the starter is mounted low down near the back of the engine. The starter needs a
heavy electric current, which it draws through thick cables from the battery. No
ordinary hand-operated switch could switch it on: it needs a large switch to cope
with the high current. The switch has to be turned 'On and off very quickly to avoid
dangerous, damaging sparking. For this reason, a solenoid is used - an arrangement
in which a small switch activates a larger electromagnetic switch to complete the
circuit. The ignition switch is usually worked by the ignition key. The driver turns
the key beyond the 'ignition on' position to feed current to the solenoid.
The ignition switch has a return spring which makes the switch spring back and turn
the starter switch off as soon as the ignition key is released. When the switch feeds
current to the solenoid, the electromagnet attracts an iron rod. The movement of
the rod closes two heavy contacts, completing the circuit from the battery to the
starter. The rod also has a return spring - when the ignition switch stops feeding
current to the solenoid, the contacts open and the starter motor stops. The return
springs are needed because the starter motor must not turn more than it has to in
order to start the engine. The reason for this is partly that the starter uses a
lot of electricity, which quickly runs down the battery. Also, if the engine starts
and the starter motor stays engaged, the engine will spin the starter so fast that
it may be badly damaged. The starter motor itself has a device, called a Bendix gear,
which engages its pinion with the gear ring on the flywheel only while the starter
is turning the engine.
The engine does not crank and the starter does not work. Tracing why the starter
motor does not work is simple and straightforward. Turn the headlights on. If they
are dull the battery is discharged.If the headlights are bright - but dim noticeably
as the starter switch is engaged - observe whether, and how quickly, they regain
their full brightness once the starter switch is released. A slow recovery, or no
recovery, indicates a defective battery. if the recovery is immediate, a poor connection
on the main battery cables is indicated. With the vehicle gearbox put in neutral,
have a helper engage the starter switch while you jam the blade of a screwdriver
between a . battery post and the terminal on the battery cable - do each post in
turn. The starter will respond immediately the poor connection is found. Remove the
battery cable and clean it thoroughly. A jammed starter motor may also cause the
head lamps to dim.
Usually this will be accompanied by an"audible click - especially on older, inertia-type
starter motor systems - as the solenoid engages. With pre-engaged starter motors,
the solenoid may be relatively silent but it will be noted that the return spring
for the actuating arm is almost fully compressed. A further clue in deciding on whether
the starter motor is probably jammed is to observe the fan blades as the solenoid
clicks in.
If they do not move even slightly, the starter motor is probably jammed - if it is,
check whether there is a square stub situated on the end of the armature shaft. If
so, turn it with a spanner to free the pinion. If not - and the car has manual transmission
- place the vehicle in one of its lower gears.
With the ignition off, release the handbrake and rock the car repeatedly forward
and back until the pinion frees itself. If the car has automatic transmission, loosen
the starter mounting bolts and shake the starter pinion free. On very rare occasions,
it may be found that although the solenoid is clicking, the heavy-duty contacts within
the solenoid itself are burnt and not providing electrical contact. Or, alternatively,
there is poor contact between the brushes in the starter motor and the commutator.
If the battery is in order but the solenoid does not click when the starter switch
is engaged, either the solenoid, the wiring to the solenoid or the starter switch
is faulty. In such instances the headlamps will not dim. Check the solenoid switch
by tracing the low-tension LTI wire,which leads back to the ignition switch. To activate
the solenoid (depending on its type) this, contact has to have current applied to
it, or - on some older' vehicles - it may have to be earthed. Begin by earthing this
contact and then bridging it to the nearby battery power lead. If the solenoid does
not respond, it is faulty. Alternatively the fault will either be found in the ignition
switch or the wire itself. (Note: the solenoid switch can be by- passed by temporarily
bridging its main terminals with a well- insulated screwdriver.
 I Remember that a test lamp is useful not only to check whether particular component
is, or becomes, live - but can also be used instead of an ohmmeter to find high resistance
points and poor connections. For example, if a test lamp that is made from a headlamp
or similar globe were to be connected in series with any other circuit, the test
lamp would light up if there was a high resistance in the circuit, since current
always tries to find the path of least resistance.
Checking and replacing the starter motor
 If testing the starter circuit in-dicates a fault in the starter motor, disconnect
the battery and remove the motor from the car - complete with its attached solenoid
if it is a pre-engaged type. To trace the fault and carry out repairs, you will nearly
always have to strip down the motor, especially if the fault is in the commutator
or brush gear. However, the Bendix gear of an inertia- type starter can be repaired
without stripping; removing the motor from the car gives sufficient access. Some
faults, such as a badly worn commutator, are beyond repair and you have to buy a
new armature. However, you can renew worn bearings repair a solenoid ; fit new brushes
in the commutator to replace worn ones and repair minor damage to the commutator.
Damage to the electrical windings is usually too difficult to deal with at home.
An auto-electrician can cure armature short circuit in the field coils, but anything
else calls for fitting new components. A single fault in the motor is probably worth
repairing, but if you find more than one fault, check the cost of all the spares
you need. You may find it cheaper - and quicker - to replace the motor with another
one.
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