Theory of Operation
The alternator is responsible for supplying the automotive
electrical system with the needed electrical power to charge the battery and
run the entire vehicle. Three different types of alternators are seen
on vehicles running today. The External regulator, internal
regulator and computer controlled type. Alternators with external
regulators were used in the 60’s, 70’s and early 80’s, but are no longer
used today. Most vehicles on the market now have either internal or computer
controlled regulation alternators. The race to build more efficient vehicles
has led to the vast complexities of today’s automotive systems. Alternators
are no exception. With the integration of more electronic and electrical
components to maximize efficiency and create comfort, the alternator is
being stressed to its maximum. Today’s alternators work with a very small
margin of power leftover. In other words, when the driver has all the
electrical loads on (A/C, radio, defogger, heated seats, etc) the alternator
is at its maximum output. Over the years newer and higher output (higher
amps) alternators have been devised to meet these high electrical demands.
Future developments like FET (Field Effect Transistor) regulators and 42
Volt systems will increase the output even more.
Alternators work by the principle of induction. Induction
is the ability to set electrons in motion (power generation) by running a
conductor (wire) through a magnetic field. Alternators have two basic
parts, the stator and the rotor. The stator is the copper wire
windings or the coils on the inside of the body of the alternator. This set
of three coils does not move and remain fixed, hence the name stator. The
stator is responsible for receiving the lines of magnetic flux, providing a
path and setting the electrons in motion. Power generation actually takes
place in the stator. The rotor is responsible for creating and rotating the
magnetic field, which cuts across the stator coils to set the electron in
motion. The magnetic field is applied by the voltage-regulator or the ECM
and is done by varying the rotor’s current. Varying the field’s current
controls the output power of the alternator (voltage and current).
Alternators also employ a series of diodes (electronic one way valves) to
rectify or convert the alternating current (switching polarity) into
direct current (one polarity). Most alternators employ three stator
coils which are out of phase by 120 degrees. The diodes rectify or convert
all the electrical power produced by the alternator into direct current to
run the vehicle and charge the battery. The GM CS series alternator (CS 121,
130, 144) was first seen in the late 80’s. It employs an internal regulator
and only two connections are needed for the alternator to work—the battery
and the L terminal connection. Terminals P, F and S are optional.
Terminal P is connected to the stator and could be connected to a speed
sensitive circuit (RPM). Terminal F is connected internally to the
field and is sometimes used for diagnostics purposes or failure indicator.
Terminal S could be connected to battery voltage. On these
alternators, the regulator switches the rotor field at about 400 Hz with a
duty-cycle signal. Typical duty cycles of 10% at low energy demands and 90%
at high electrical loads are common.
The CS alternator turns the dash-charge light on if the
charging voltage goes above or bellow specifications. The terminal L of the
alternator is connected directly to the dash-charging light or in later
systems to the ECM. The ECM, on late 90’s models, controls terminal L
directly and turns the dash-charge light on if it sees a charging system
problem. It is important to understand that on ECM controlled CS alternators
the ECM does not control the duty cycle of the alternator field. This is
controlled directly by the internal voltage regulator. The ECM only controls
whether terminal L is ON (charging) or OFF (not charging). In essence the CS
alternator is not a fully computer controlled alternator, as in late model
GMs. On these newer systems, the actual duty-cycle field voltage signal is
provided directly by the ECM. Therefore, the ECM acts as the voltage
regulator. The CS alternator also has fault detection circuitry on the L
terminal, which is monitored by the ECM. If the charge voltage goes too high
or too low the internal alternator/regulator circuitry shorts the L terminal
to ground. Since terminal L is directly connected to the ECM, it senses the
ground on terminal L and sets a charging system faulty code. Therefore,
terminal L is used as both a control and a diagnostics connection.
Conditions that Affect Operation
• The battery charge, as in any other charging system,
affects the alternator output. A fully charged battery is the most important
and the first item to check for in the charging system.
• Electrons need a good (no resistance) flow to be able to
charge the battery. It is important to always check the charge output wire
for excessive resistance, which should be about 150 to 200 mV or less. A
voltage drop of more than 300 mV is not acceptable. The alternator’s ground
is the other important electrical connection to consider. The engine should
be grounded thoroughly. A ground voltage drop of more than 100 to 200 mV
with the engine running or 300 mV with engine cranking is not acceptable.
• On non-computer controlled alternators, the charge light
on the vehicles dash is what turns the regulator on. One side of the light
bulb is connected to power (12Volts) and the other side to the L terminal.
The 300 mA of current going through the dash light bulb is in-charge of
exciting (turning on) the alternator’s voltage regulator. If the light bulb
is out, has excessive resistance, or the fuse that feeds one side of the
bulb is open then the alternator will not change. The light may even be
OK, but its filament may be defective (high resistance) preventing the right
amount of current from reaching the regulator. The voltage regulator
must see the excitation voltage coming from the dash light. On a computer
controlled CS alternator, the ECM provides that excitation voltage (12
Volts) itself. The dash light is turned on/off through another circuit also
by the ECM. In CS systems, the ECM turns the alternator off on certain
occasions, as during an acceleration period, to give more power to the
engine taking the load off of it.
Component Testing
1. After verifying that the alternator is not charging at
the battery, check the output right at the alternator. With a voltmeter,
probe right at the alternator’s charge terminal (red terminal) and ground
(alternator’s body). If low voltage is also seen here then check the L
terminal, but if normal charge voltage is seen instead, then either the
ground or the charge wire has excessive resistance.
2. With the engine running, measure the voltage (voltage
drop) across the alternator’s body and the battery ground terminal. No more
that 100 to 200 mV should be seen.
3. Also with engine running, measure the voltage across the
alternator’s charge terminal and battery positive. No more than 100 to 200
mV should be present. Any deviation from this figure points to ground or
charge wire resistance.
4. With the alternator’s L terminal disconnected,
verify that battery voltage is present by using a voltmeter. Ground
this terminal (using jumper wires) and verify that the dash charge light is
lit. This verifies the integrity of the excitation circuit or the L terminal
and works whether it’s computer controlled or not. (This procedure may
require the engine to be running on computer controlled alternators).
5. If the alternator is computer controlled, disconnect the
L terminal wire, connect a voltmeter to the L terminal wire and
command the L terminal on and off with a scan tool. The voltmeter should go
to 12 volts and then to 0 volts therefore verifying the integrity of the
charge circuit (L-terminal). A test light could be used at the L-terminal in
place of the voltmeter on non-computer controlled alternators only. This
circuit is also used for diagnostics purposes and the ECM may interpret the
test light as a circuit problem. If the L terminal circuit works fine and
the power/ ground circuits do not exhibit any excessive resistance, then the
alternator is at fault.
NOTE: A test light should
be used to test the L-terminal on non-computer controlled
alternators. By putting the test light in series with the
L-terminal, both lights (dash and test light) should light-up dimly.
On the other hand, since computer controlled alternators have the
L-terminal controlled by the ECM, a test light can not be used. The
reason for this is that the ECM provides 12 volts, but very little
amperage at the L-terminal, and not enough to light up a test light.
Also this circuit is also used for diagnostics purposes. A test
light may be seen as a faulty circuit by the ECM. In this case, a
voltmeter should be used instead of a test light.
These guidelines may also be used for other alternators as
well. Except reference to the L-terminal, which only applies to GM vehicles.
