Automotive Alternators are true work horses...

The modern automotive alternator is a true work horse. In this blog we'll delve into the GM type vehicle alternator family, which was for many years the best designed and the basis for today's vehicle alternator technology...


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 60ís, 70ís and early 80ís, 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 todayís 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. Todayís 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 rotorís current. Varying the fieldís 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 80ís. It employs an internal regulator and only two connections are needed for the alternator to workóthe 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 90ís 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

ē 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.

ē 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 alternatorís 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.

ē 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 alternatorís 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 alternatorís charge terminal (red terminal) and ground (alternatorís 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 alternatorís 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 alternatorís 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 alternatorís 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 itís 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.


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