Alternator Faults: Battery warning light doesn’t go
out. On
older cars this is normally caused by an alternator fault, although
it can also be caused by the wiring going to earth before reaching it.
To check –
simply remove the warning light lead. If the light then goes out the
fault is in
the alternator. If it doesn’t the problem is in the wiring. N.B. There
are
some exceptions to this rule as some vehicles are fitted with an item
called a
charge relay but the test applies in about 95% of cases.
Battery warning light doesn’t come
on – Remove the warning
light lead and earth it. If the light comes on, its an alternator
problem if it
doesn’t it’s a wiring/bulb fault. (The same exceptions apply as in the
previous
paragraph).
Battery light goes out but
battery does not charge: This is normally NOT a alternator
fault. The cause is frequently a slipping
fan belt (they don’t always screech) or a bad connection
either to earth or to
be positive side of the battery. To check the fan belt inspect it for
tightness,
wear, and also inspect the pulleys.
The belt should drive on it’s sides –
not its bottom. If the belt is too deep in the pulley groove inspect
closely and
suspect wrong/worn belt or pulleys. Secondly if you connect a multi-
meter
between the positive and negative terminals and then move the positive
to the
alternator outlet with the engine running the voltage should be the same
i.e.
13.5 – 14.5 volts if it is charging.
The following grid gives
some more common faults.
Component testing:
This Section deals with testing
the individual components of an alternator.
It is not the definitive list, but a guide. So don’t think that just
because
the component appears to test out without problems that it has no faults
–
unfortunately life isn't that simple.
I am going to assume the
alternator has been stripped, cleaned and that you
have read the other relevant sections of this web.
Rotor
Check for circuit through
windings with meter or test light. Typical
resistance is about 3.5 ohms for 12 v and 8 ohms for 24 v. Test this by
connecting between the two ‘slip-rings’. (The part which the brushes rub
against). Next check that the rotor windings don’t go to earth by
putting one
of the leads to the body. Visually the rotor windings should not be
burnt and
the slip-rings themselves relatively un-grooved. Although minor grooves
are acceptable if they
become to deep the slip-rings will have to be replaced. It will have
been worthwhile
putting your finger onto the slip-ring and spinning the unit prior to
dismantling to check that it is concentric. Small variations are
acceptable but
if in doubt skim in a lathe.
Stator
Test for circuit between each
of the phases with a test light or meter and
then check that it doesn’t go to earth. Ensure that the windings are
un-burnt
and covered in lacquer. N.B. many stator faults are caused by the
windings
shorting between each other and not through going to earth. This fault
is almost
impossible to detect, so the visual examination is all important.
Rectifier
This is the most difficult to
explain, test procedure so I am going to
describe how to test in only the simplest terms.
The rectifier consists of 1
positive and 1 negative diode for each phase
(wire) coming out of the stator. These are called power diodes and split
the
power into positive and negative voltage. To check a diode ensure that
electricity can flow in one direction but not in the other by checking
with your meter or test light and then reversing it (One way you should
get a
reading, and the other way you should not)
In addition, many rectifiers
also have excitation diodes these smaller
(normally) diodes are tested in exactly the same way.
A three phase alternator will
have three positive and three negative power
diodes plus usually another three excitation diodes.
I appreciate that this is not a
good description however, if you run into
trouble e-mail or phone us and we will help if possible.
Regulator
Difficult to test when it is
off the vehicle. These days they are frequently
combined with a brush box and tend to be changed on any major rebuild.
This is
the one part of an alternator that will have intermittent faults. So
depending
on what the fault is it may be worth changing it especially if no other
problems
can be found.
General
Bearings – If noisy replace
them. N.B. Always check bearing housings
for wear.
Casings
Check for cracks/stripped
threads or broken studs.
Hope this helped.
