Main.TrailIndexPage | Electrical Systems | Electrical.Alternator


This component is part of Electrical Systems.


An Alternator is an electrical generator that produces alternating current.


The Alternator is used to charge the battery and to power the electric system when the engine is running. The stronger construction of automotive alternators allows them to use a smaller pulley to turn twice as fast as the engine, improving output when the engine is idling. The availability of low-cost solid-state diodes from about 1960 onward allowed car manufacturers to substitute alternators for DC generators. Automotive alternators use a set of rectifiers (diode bridge) to convert AC to DC

When driving the car, the alternator needs to support the power consumption of all of the accessories. The battery is indeed another load on the alternator (because some current is being used to charge it).

Only when idling, or when the demands of the accessories exceeds the capacity of the alternator, will the battery start supplying current also.

The famous formula is P=VI, Power equals Voltage times Current. The alternator, if rated at 490W, outputs a voltage of some 14 volts, and can supply around 35 Amps. The battery, if original and rated at 55Ah, can supply 55 Amps for an hour... then it is dead. Some of us have 70Ah batteries installed.

If the current drain exceeds that which the battery and the alternator can provide, your battery will deplete during use and you will need to put it on a charger in between uses (a trickle charger is ideal).


Testing Alternator

The first test is to make sure the charging bulb in the dash is working. Turn on the ignition to the run position without starting the car. If the bulb is on it is working, if not you need to get the bulb working. The current through the bulb excites the alternator and gets it charging. Sometimes it is just a bad connection at the bulb or the bulb is blown. Without this bulb working your alternator wont charge.

Ignition key on, engine not started, charging bulb is illuminated ...

Voltage measurements

  1. Aim the headlights on something you can see while starting the car (e.g. garage door). Turn on your headlights with the car not running. Leave the lights on for a few minutes. Start the car, headlights should be noticeably brighter.
  2. Use a DC voltage meter:
    • Engine not running - you should see a reading of 11.5-12.5 volts.
    • Engine running - you should see a reading of 13.5 to 14.5 volts

Alternator Adjustment / Removal

Bosch 35amp Alternator Wiring

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