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Alternator FAQ list
What does the Alternator do ?
The vehicle battery provides reserve electrical power when the engine is stopped to enable it to be rotated by the starter
motor to start the engine.
As power is used from the battery it will gradually discharge until very little happens when you turn the key to start.
The automotive alternator is an electricity generation plant which provides power to recharge the vehicle starter battery
and also supply electricity for other vehicle electrical systems such as lighting, wipers, heater motor, car audio ,sat nav,
etc whilst the vehicle engine is running.
Where is my alternator located ?
The engine driven alternator will be located in the engine bay of the vehicle and normally secured to the engine block. Not
always accessible. Sometimes other components have to be removed before the alternator can be accessed.
How is the Alternator driven
Normally belt driven from a drive belt connected to the crankshaft pulley . In a few instances the alternator may be shaft
driven. The alternator must be mounted securely to enable it to be driven efficiently.
How long does an alternator last ?
How long does the alternator last?
Usual life expectancy of a motor car alternator is 100,000 to 200,000 miles, though this depends on the operating conditions
of the vehicle such as environment, temperature, driving cycle, maintenance, etc.
I have fitted extra electrical accessories such as amplifiers and driving lamps do I need a bigger alternator?
The vehicle manufacturer calculates the power output of the alternator required for each vehicle with regard to original equipment
fitment. Increasing the load on a vehicles electrical system may require an alternator system upgrade including cable. Some
special purpose vehicles may be fitted with an additional alternator or battery charging management system.
How does an Alternator work ?
How does the alternator work?
The alternator generates electricity by "electromagnetic induction". When a conductor such as a wire moves through
the lines of force of a magnetic field a voltage is induced in the conductor. Whether the conductor moves in relation to the
magnetic field or vice versa the principle is the same.
The basic vehicle alternator consists of a ROTOR with magnetic poles and an excitation winding which spins as it is driven
from the engine . The housing of the Alternator consists of Stationary wires forming a "winding" of coils of wire
known as a STATOR.
The stator normally consists of a soft iron laminated core around which reasonably heavy guage copper wire is wound
to form usually three windings connected in Star or delta formation.
The ROTOR is an electromagnet which when energised creates a magnetic field.
Power is supplied to EXCITE the rotor so the magnetic field is created. (Without excitation current there will not be
a magnetic field) some alternators are SELF-EXCITED, this means after the rotor is initially excited a wire connected to the
stator taps some of the power generated to keep the rotor excited and power generation in progress whilst the rotor is spinning.
The brush pack or brushes form the means of connecting the spinning rotor to the stationary excitation power source.
By controlling the excitation current of the rotor the magnetic field strength is proportionally controlled. This excitation
control is used to regulate the alternator output by means of the Voltage regulator.
With ECU regulated systems, the duty cycle of the excitation current can be reflected as a feedback signal to measure
the amount of alternator capacity being used via the DFM signal terminal (DF Monitor).
The voltage produced by the alternator is as its name suggests alternating. That is AC.
Unfortunately AC cannot be used to recharge the vehicle battery so has to be converted to DC . The process of converting
AC power to DC power is known as rectification. The rectification is done by the Diode pack (a diode acts like a one way
valve in an electrical circuit) which is built to form a "Bridge Rectifier".
Most car alternators are Three Phase AC generators with rectification to DC.
The DC voltage level produced by the alternator must be slightly higher than battery voltage to allow the battery to charge.
The Diode pack has a secondary function when the ignition key is off to prevent the power draining from the battery via
the alternator windings causing the battery to discharge.
How will I know if the Alternator is faulty ?
The first sign of an alternator being faulty is usually the battery warning light on the dash illuminating followed by loss
of electrical power which may result in a discharged battery and failure of the vehicle to start.
 Have the vehicle battery tested.
A visual check with the engine stopped should be carried out to ensure the alternator is secure and drive belt correctly
tensioned. Also check for oil ,fuel or coolant leaks which may have caused the alternator to fail.
Check all connections are tight and wires are in good condition.
Check all heat shields are in place .
A quick check for a failed alternator is to measure battery voltage with the engine off using a voltmeter. Start the
engine and run at around 2000rpm whilst monitoring battery voltage which should have increased slightly if the alternator
is charging the battery.
Some modern vehicles have alternators with dynamic electronically controlled output which is affected by the
embedded software in the vehicle engine control computer. These require special equipment to test and verify as output
varies according to demand.
What causes an Alternator to fail ?
Common causes of alternator failure are:
Broken or defective wiring to the alternator.
Wear and tear ; Worn out brushes , bearing failure, diode pack failure.
Excessive drive belt tension causing premature bearing failure.
Insecure mounting bolts / bracket causing insufficient drive belt tension.
Poor electrical connection causing high resistance and leading to overheating.
Electrical overload due to a defective battery or fitment of vehicle accessories, or vehicle wiring faults.
Inadequate ventilation and cooling possibly caused by fitment of vehicle body kits.
Voltage surge caused by boost starting using jump leads without surge protection. (Diode failure)
Oil,fuel or coolant leaks onto the alternator.
On modern alternators controlled by an electronic control unit an incorrect battery type may cause problems.
We have facilities to test and fault find vehicle alternators on site or remove from site for bench testing and repairs.
We can also supply outright brand new or service exchange alternators for most makes of vehicle.
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* unit warranty is subject to original suppliers standard terms & conditions.
Alternator Repair , Supply, Fitting.
We can supply & fit many makes of vehicle alternator to suit your automotive charging system including replacements for
Alfa romeo,AMC,Audi,AVF,BMW,Bosch,Butec,Cajavec,Carrier,Caterpillar,
CAV,Chrysler,Citroen,Cummins,Daihatsu,Delco remy,Ducellier,Efel,Elmot,Femsa,Fer,Fiat,Ford,GM,Halla,Hella,Hitachi,
Honda,Hyundai,Iskra,Isuzu,Iveco,John Deere,Kia,Komatsu,Kubota,Lada,Lucas,Magneti Marelli,A127,Magneton,
MAN,Mando,Massey Ferguson,Mazda,Mercedes Benz,MGF,Mitsuba,Mitsubishi,Monark,Motorola,Nikko,Nippondenso,
Nissan,OMC,Opel,Paris Rhone,Perkins,Peugeot,Poong sung,Prestolite,Real,Renault,Rover,214,Saab,Scania,SEV,SJCE,Skoda,Subaru,
Suzuki,Thermoking,Toyota,Unitedtechnologies,Valeo,Vauxhall,Astra,Corsa,Vectra,Zafira,Volvo,VW,Yanmar.
Please quote any reference numbers to help identify parts you may require.
Alternators built for specialist applications
Certain applications may require specialist alternator design:
Marinised Alternator
Marine grade alternators are often built to include insulated earth return to prevent galvanic corrosion of a ship's hull.
The marine grade alternator will normally include features to prevent excessive corrosion, and also enhanced insulation.
Race Car Alternator
Alternators for racing applications may be high output compact alternators specially designed to fit into small areas,
and can feature extra cooling.
Race grade alternators may have upgraded diodes capable of handling larger current , with specially machined rectifier
cages and heatsinks to provide better heat dissipation.
Race car alternators may also incorporate specially wound and insulated stator and rotor to generate larger current
Larger bearings may be fitted to support the rotor
Electronic control of field current may be introduced so that alternator output is cut at wide open throttle (WOT) to
increase vehicle acceleration.
Common Alternator Wiring Terminal Letter Designations
B+ , 30, Battery Positive
S, M, 2, Battery Voltage Sense
B- , 31, D-,GRD Battery Negative
F, DF , 67 , FLD , EXC, Field/Rotor
D+, 61 , L , IND, Charge Warning Lamp
DFM , FR, SIG, C, M, Digital Field Monitor output for computer ECU control
IG, 15 , R , Ignition
N, C, Central Point
W, P, STA, Tachograph rev counter (stator single phase tap AC output)
D Dummy
Please Check the manufacturers instructions for specific application of wiring terminal designation.
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