GM Firebird 1982-1992 Repair Guide




An alternator differs from a DC shunt generator in that the armature is stationary, and is called the stator, while the field rotates and is called the rotor. The higher current values in the alternator's stator are conducted to the external circuit through fixed leads and connections, rather than through a rotating commutator and brushes as in a DC generator. This eliminates a major point of maintenance.

The rotor assembly is supported in the drive end frame by a ball bearing and at the other end by a roller bearing. These bearings are lubricated during assembly and require no maintenance. There are six diodes in the end frame assembly. These diodes are electrical check valves that also change the alternating current developed within the stator windings to a direct (DC) current at the output (BAT) terminal. Three of these diodes are negative and are mounted flush with the end frame while the other three are positive and are mounted into a strip called a heat sink. The positive diodes are easily identified as the ones within small cavities or depressions.

The alternator charging system is a negative (-) ground system which consists of an alternator, a regulator, a charge indicator, a storage battery and wiring connecting the components, and fuse link wire.

The alternator is belt-driven from the engine. Energy is supplied from the alternator/regulator system to the rotating field through two brushes to two slip-rings. The slip-rings are mounted on the rotor shaft and are connected to the field coil. This energy supplied to the rotating field from the battery is called excitation current and is used to initially energize the field to begin the generation of electricity. Once the alternator starts to generate electricity, the excitation current comes from its own output rather than the battery.

The alternator produces power in the form of alternating current. The alternating current is rectified by 6 diodes into direct current. The direct current is used to charge the battery and power the rest of the electrical system.

When the ignition key is turned ON, current flows from the battery, through the charging system indicator light on the instrument panel, to the voltage regulator, and to the alternator. Since the alternator is not producing any current, the alternator warning light comes on. When the engine is started, the alternator begins to produce current and turns the alternator light off. As the alternator turns and produces current, the current is divided in two ways: part to the battery(to charge the battery and power the electrical components of the vehicle), and part is returned to the alternator (to enable it to increase its output). In this situation, the alternator is receiving current from the battery and from itself. A voltage regulator is wired into the current supply to the alternator to prevent it from receiving too much current which would cause it to put out too much current. Conversely, if the voltage regulator does not allow the alternator to receive enough current, the battery will not be fully charged and will eventually go dead.

The battery is connected to the alternator at all times, whether the ignition key is turned ON or not. If the battery were shorted to ground, the alternator would also be shorted. This would damage the alternator. To prevent this, a fuse link is installed in the wiring between the battery and the alternator. If the battery is shorted, the fuse link melts, protecting the alternator.

An alternator is better that a conventional, DC shunt generator because it is lighter and more compact, because it is designed to supply the battery and accessory circuits through a wide range of engine speeds, and because it eliminates the necessary maintenance of replacing brushes and servicing commutators.


To prevent serious damage to the alternator and the rest of the charging system, the following precautions must be observed:

Never reverse the battery connections.
Booster batteries for starting must be connected properly: positive-to-positive and negative-to-ground.
Disconnect the battery cables before using a fast charger; the charger has a tendency to force current through the diodes in the opposite direction for which they were designed. This burns out the diodes.
Never use a fast charger as a booster for starting the vehicle.
Never disconnect the voltage regulator while the engine is running.
Avoid long soldering times when replacing diodes or transistors. Prolonged heat is damaging to AC alternators.
Do not use test lamps of more than 12 volts (V) for checking diode continuity.
Do not short across or ground any of the terminals on the AC alternator.
The polarity of the battery, alternator, and regulator must be matched and considered before making any electrical connections within the system.
Never operate the alternator on an open circuit. make sure that all connections within the circuit are clean and tight.
Disconnect the battery terminals when performing any service on the electrical system. This will eliminate the possibility of accidental reversal of polarity.
Disconnect the battery ground cable if arc welding is to be done on any part of the car.


There are many possible ways in which the charging system can malfunction. Often the source of a problem is difficult to diagnose, requiring special equipment and a good deal of experience. However, when the charging system fails completely and causes the dash board warning light to come on or the battery to become dead the following items may be checked:

  1. The battery is known to be good and fully charged.
  3. The alternator belt is in good condition and adjusted to the proper tension.
  5. All connections in the system are clean and tight.


See Figures 1, 2 and 3

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Fig. Fig. 1: Loosening the wire connector bolt

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Fig. Fig. 2: Removing the through-bolt

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Fig. Fig. 3: Unplugging the harness

While internal alternator repairs are possible, they require specialized tools and training. Therefore, it is advisable to replace a defective alternator as an assembly, or have it repaired by a qualified shop.

  1. Disconnect the battery ground cable.
  3. Tag and disconnect the alternator wiring.
  5. Remove any drive belt(s) which may interfere with alternator removal.
  7. Support the alternator and remove the mount bolt(s). Remove the unit from the vehicle.

To install:
  1. Position the alternator into place and install the mount bolt(s) loosely.
  3. Install the drive belt(s). On V-belts, tighten the belt enough to allow approximately 1 / 2 in. (13mm) of play on the longest run between pulleys.
  5. Connect the alternator wiring.
  7. Connect the battery ground cable.