I have a alternator that tested bad. I went and took it in for service and the alternator charges now but the gen idiot light is on now. I unhook the two wires from the alternator and the light goes out. If its plugged in and I start the car and step on the gas it goes dimmer. Is it the alternator that is making the light stay on or could there be something else?

I have a alternator that tested bad. I went and took it in for service and the alternator charges now but the gen idiot light is on now. I unhook the two wires from the alternator and the light goes out. If its plugged in and I start the car and step on the gas it goes dimmer. Is it the alternator that is making the light stay on or could there be something else?

It the light is on when running is most likely not charging. What is the voltage, at the battery, with the car running at a fast idle?

I have 12.63 volts with the car off and when its on I get about 13.8 to 14.3 volts.

Still a bad alternator.

That light only comes on when something has grounded inside the alt.

Most likely a diode failure-----time to rebuild it, or get another one.

The wire for that lamp comes from the ignition buss in the fuse box>>>to the lamp>>>and to the alternator. Sometimes via a resistance wire.

On vehicles without the lamp----a resistance wire (or diode) is needed to prevent the car from running after the key has been turned off.

Old chevy's had 3-wire hook-ups .. you probably have a one wire alternator (internal resistor) .. still works just the idiot light will stay on.