Reply to "Alternator Light always on"

Here an explanation of how the idiot light works a friend got on line and sent to me which may or may not help you. He goes further into troubleshooting which I hope doesn't confuse things further.

Good luck


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Yes, the alternator light does work by a balance. When you turn on the key, 12 volts is supplied to the alternator's field windings through the idiot light. This gives an indication that the alternator's field circuit is intact, and it also provides the current to start the alternator charging. In fact, the alternator may fail to charge if that light bulb is burned out. When the engine starts and gets up to a high enough rpm for the alternator to start putting out voltage, there is a diode inside that takes some of the output voltage and featured back to the terminal that goes to the idiot light. The net result of this is that when the engine is running and up to proper rpm, the idiot light is receiving 12 volts at both ends, which means that the total voltage drop across the light is the zero volts and the light doesn't light.

The problem you're describing usually occurs when there is a break in the wire from the alternator's output terminal back to the battery, it does not have to be a complete break, a high resistance connection is quite sufficient to cause a light to come on. What is happening is that the alternator is full output voltage is getting sent to one terminal of a lightbulb, and the other end of the bulb is connected up to the battery positive terminal through the ignition switch, which is now at a lower voltage because of the voltage drop in the positive wire. This voltage drop causes the light to light, and also results in the battery charging poorly if at all. Usually when this is the case, the light will be very dim at idle and will get brighter as you rev the engine.

To diagnose the problem, put the positive wire of your volt meter on the alternator output terminal and the negative wire of your volt meter on the battery plus terminal. Put some load on the battery, such as lights or something, rev the engine, and see how much voltage is getting dropped between the alternator output and the battery terminal. If everything is good, it will be well under 1 volt. I suspect you will find you are dropping several volts in this wire, which indicates a breakage or high resistance connection somewhere along the wire. Probe between both ends of each section of the wire to find out where the high voltage drop is, then repair the problem when you find it. If you want to be half ass and not go through the trouble of tracing the wire, simply run a very heavy, about eight gauge or so wire from the alternator's output terminal back to the battery plus and your problem will probably be solved.

While you're checking things, put the negative terminal of your meter on the alternator case and the positive terminal of your meter on the battery minus, apply a load, and rev the engine. If you have much voltage drop here, start checking the voltage drop from point-to-point once again until you find the problem, then repair it. If your alternator is rubber mounted, there may be a grounding strap between the alternator case and the block which needs to be present and in good condition. Also check the grounding strap between the engine and the body or battery negative. I don't suspect you have a ground side problem here, but when I've got the meter out and I'm looking into things I always check both ends of the situation.