important that the alternator is capable of delivering the correct voltage and
recommended regulated voltage will vary slightly between motor. It is equally
important that the system is neither under or over charging.
current available from the alternator will also vary depending on the type of
alternator fitted. The current seen will depend on the state of charge of the
battery and what loads are switched on.
the alternator has a specific problem that is reducing the current, such as a
faulty diode, this would not be seen by a drop in the regulated voltage,
however it would be found when the alternator waveform was monitored.
of the charging circuit is to provide a regulated voltage to charge the
battery and replenish the current consumed by the vehicle's electrical
circuits. The alternator is a fairly recent addition to the motor vehicle,
replacing the dynamo which was fitted until the 1970's.
from a dynamo was determined by engine speed and unlike the alternator, it had
negligible output when the engine was at idle. It was not unknown for the
charging warning light to flicker at idle and have regular dynamo brush
changes. These brushes were considerably larger than those found on alternator
as they carried the total current output unlike alternator brushes that carry
the field current, this current provides the energising of the electromagnet
to produce the output.
current is approximately six to eight amps.
The rating of
the alternator will tend to be vehicle specific, as a base model will have
less electrical demand than a vehicle with typical 'top of the range'
accessories such as electric front and rear heated screens, heated mirrors,
additional lighting, heated and electrical adjusted seats, etc.
alternator output, as the name implies, produces an Alternating Current (AC)
output , which is rectified to the Direct Current (DC), to provide the correct
type of voltage to replenish the battery, keeping it at full charge.
alternator has three internal windings wound 120 degrees between phases and
requires nine diodes in 'bridge' configuration to rectify the output. The
voltage is controlled by a solid state regulator that maintains the voltage at
a predetermined setting. The output current is determined by the requirement
seen at the time, for example, a battery that has just been subject to
prolonged cranking will see a higher output from the alternator than if the
battery was fully charged.
voltage can be measured on a multimeter, however this reading can be seen to
be correct even if the alternator has a diode fault which will reduced the
output by 33%. The only true way to monitor the alternator output is to
observe the resultant waveform on an oscilloscope.
shows an alternator wiring diagram with a nine diode system.
shows a typical alternator.
system employed on the Ford Focus is unlike any other charging system that is
currently in production. Ford utilise
what is termed a 'smart charge' system. With a conventional charging system
the battery is charged at a voltage that is determined by the voltage
regulator, with all the electrical load being drawn from the alternator fed
charging enables the voltage supply from the alternator to vary depending on
the temperature of the battery's electrolyte. It has been proven that a cold
battery will respond better to a higher voltage than a hot battery, which
responds better to a slightly lower voltage. The temperature of the
electrolyte is calculated by monitoring the air intake temperature when the
engine was last stopped and the current intake air temperature. From these two
datum points, the battery's temperature can now be calculated and the
appropriate charge sent to the battery.
alternator will have two connections to the Engine Management Module (ECM),
these are to monitor and control the output. This monitoring also allows the
Idle Speed Control Valve (ISCV) to be operated when high electrical demands
are seen when the engine is at idle. The ECM will also control the engine run
relay, which only allow circuits with a high current demand to be activated
when the alternator is charging, until which point the components remain
The ECM is
now responsible for switching off the dashboard mounted 'charging light'. When
starting the engine with a conventional alternator, the unit is activated as
soon as the ignition is switched on, a 'smart charging' system will only
initiate the alternator once the engine has started. This action avoids an
unnecessary waste of voltage on a vehicle with a discharged battery and also
avoids the extra effort involved in cranking an engine with an operational
is the block wiring diagram for the Ford Focus charging circuit.