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ABS Speed Sensor
Air Flow Meter / Sensor
Air Intake Pressure Sensor
Alternator Current and Voltage
Amplifier Earth
Camshaft Sensor
Carbon Canister Solenoid Valve
Crankshaft sensor
Coolant Temperature sensor
Diesel Glow Plugs
Digital ECM to Ignition Amplifier Signal
Distributor Pickup
Dual Trace
Electronic Fuel Pump
Exhaust Gas Recirculation
Idle Speed Control Valve
Knock Sensor
Lambda Sensor
MAP Sensor
Relative Compression
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Alternator Current and Voltage Diagnostics.
It is important that the alternator is capable of delivering the correct voltage and current output.

The recommended regulated voltage will vary slightly between motor. It is equally important that the system is neither under or over charging.
The 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.

If 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.
Technical Information

The objective 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.

The output 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.

The field 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.

The 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.

The 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.

The regulated 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.

This shows an alternator wiring diagram with a nine diode system.

This shows a typical alternator.  

Ford Focus: 'Smart' Alternator

The charging 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 battery.

Smart 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.

The 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 inactive.

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 alternator.

This is the  block wiring diagram for the Ford Focus charging circuit.