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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 27 Apr 2004 05:49:55 Share Post by Email
In Reply to: Alternator - the little wire to the idiot light, Oldsaabguy, Mon, 26 Apr 2004 18:59:17
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Assuming the battery isn't stone cold dead, it'll have a voltage across it. 9-10 volts sounds like a tired battery supplying a load.

In order to charge the battery, the alternator needs to be supplying a voltage ABOVE the battery voltage - that way current flows into the battery, charging it.

Your lack of battery light is most probably the issue, and if I had to guess, the thin wire from the light to the alternator (usually green or yellow) is broken. Or corroded, as they like to do.

The battery light is a much harder worker than a simple idiot light. Simplified, an alternator is a generator that uses electromagnets instead of permanent magnets. If the electromagnets don't have current in them, there is no magnetic field, and the alternator is only so much spinning metal, not producing current.

When you turn the key to ON, current flows from the battery, through the battery light, through the thin wire, and into the 'field coils' of the alternator. This current creates a magnetic fiedl, and since current is flowing through the bulb, the bulb lights up. The engine starts and spins the alternator - since there is a magnetic field there, it creates current. The voltage regulator takes this current, and uses some of it to drive the field coils itself. Since the VR is now supplying the current to the coils, the current through the BATT light pretty much drops to zero, and the bulb goes out. If the alternator stops putting out current, current will flow through the light, and it'll come on. Once the alternator is up and running, it provides all of the electrical needs of the engine, and also charges the battery to replenish what was lost starting the engine. The purpose of the battery is to provide starting power; after that, it's pretty much along for the ride.

Obviously, if the bulb is burned out or the wire broken, no current flows through the field coils, and the alternator never 'starts up'. A normal voltage supplied by the alternator is 13-14 volts.

If the battery isn't dead (and 10 volts is weak, but not dead), and when the key is turned to ON the BATT light doesn't come on, fix the batt light circuit. Most likely the wire either fell off the back of the alternator, or has corroded off.

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